**Approaches to Stakeholder Engagement and Monitoring**

## **Application of Hydrologic Tools and Monitoring to Support Managed Aquifer Recharge Decision Making in the Upper San Pedro River, Arizona, USA**

### **Laurel J. Lacher, Dale S. Turner, Bruce Gungle, Brooke M. Bushman and Holly E. Richter**

**Abstract:** The San Pedro River originates in Sonora, Mexico, and flows north through Arizona, USA, to its confluence with the Gila River. The 92-km Upper San Pedro River is characterized by interrupted perennial flow, and serves as a vital wildlife corridor through this semiarid to arid region. Over the past century, groundwater pumping in this bi-national basin has depleted baseflows in the river. In 2007, the United States Geological Survey published the most recent groundwater model of the basin. This model served as the basis for predictive simulations, including maps of stream flow capture due to pumping and of stream flow restoration due to managed aquifer recharge. Simulation results show that ramping up near-stream recharge, as needed, to compensate for downward pumping-related stress on the water table, could sustain baseflows in the Upper San Pedro River at or above 2003 levels until the year 2100 with less than 4.7 million cubic meters per year (MCM/yr). Wet-dry mapping of the river over a period of 15 years developed a body of empirical evidence which, when combined with the simulation tools, provided powerful technical support to decision makers struggling to manage aquifer recharge to support baseflows in the river while also accommodating the economic needs of the basin.

Reprinted from *Water*. Cite as: Lacher, L.J.; Turner, D.S.; Gungle, B.; Bushman, B.M.; Richter, H.E. Application of Hydrologic Tools and Monitoring to Support Managed Aquifer Recharge Decision Making in the Upper San Pedro River, Arizona, USA. *Water* **2014**, *6*, 3495-3527.

### **1. Introduction**

### *1.1. Social Context*

Balancing the social and economic water needs of humans with those of the environment, frequently referred to as sustainable water management, presents a global challenge, especially in arid and semi-arid regions, such as the American Desert Southwest. Understanding that the concept of sustainable water management is subjective, particularly in areas where human groundwater extractions have long exceeded average annual natural recharge, this study focuses on the tension between human demands for groundwater and the groundwater needs of an exceedingly rare perennial river system within the same basin of Southeast Arizona.

We report here on a combination of hydrologic tools and monitoring that are supporting significant progress toward many measures of sustainability on the scale of decades at one important site. The case for developing a regional groundwater recharge network along the Upper San Pedro River is built upon a series of studies and reports funded by the Upper San Pedro Partnership (Partnership), and vetted by its Technical Committee over a period of approximately 15 years. The Partnership is a consortium of 23 agencies and organizations working together to meet the long-term water needs of the Sierra Vista Subwatershed by achieving sustainable yield of the regional aquifer to: (1) preserve the San Pedro Riparian National Conservation Area (SPRNCA); and (2) ensure the long-term viability of Fort Huachuca. The purpose of the Partnership is to coordinate and cooperate in the identification, prioritization and implementation of comprehensive policies and projects to assist in meeting water needs in the Sierra Vista Subwatershed of the Upper San Pedro River Basin. Richter and others [1] provide a more complete discussion of the purpose, methods, and vision encompassed by the Partnership. In addition, recent groundwater modeling efforts have focused stakeholders' collective understanding of the hydrologic system on actionable strategies. Together, this body of work helps identify potential spatial and temporal groundwater recharge targets for mitigating many of the negative environmental, regulatory and economic consequences that may result if groundwater inputs to the San Pedro River diminish as a result of pumping, as predicted by groundwater modeling. Using this information, three of the stakeholders including the U.S. Army Compatible Use Buffer (ACUB) Program, Cochise County, and The Nature Conservancy are collaborating to develop an aquifer protection and recharge network of sites in the San Pedro River basin to protect flows in the river from anticipated pumping-related depletions over the next 50 to 100 years. This interim solution is intended to allow time to develop other longer-term strategies for addressing issues of the accumulated groundwater deficit in the regional aquifer and climate change. Examples of such longer-term strategies could include gradual elimination of consumptive use of groundwater, limitations on new pumping, enhanced utilization of urban runoff, and importation of water from outside the basin. Most of these strategies are controversial and/or expensive, and would require significant political and legal efforts to secure physical water supplies. This approach—using the best available science to implement a relatively uncontroversial and legally available interim solution to preserve baseflows while more difficult long-term solutions are pursued—may serve as a model for other dry-land river basins.

### *1.2. Study Area*

The San Pedro River flows north 279 kilometers (km) from its headwaters in northeastern Sonora, Mexico, to its confluence with the Gila River in southeastern Arizona, U.S.A. This paper focuses on the 2460-km2 Sierra Vista Subwatershed ("subwatershed") of the Upper San Pedro basin ("basin"), north of the United States-Mexico boundary (Figure 1). This subwatershed area includes about 47 km of the river, most of it within the San Pedro Riparian National Conservation Area (SPRNCA), designated by the United States Congress in 1988 to protect and enhance the riparian area and its aquatic resources [2].

**Figure 1.** Map of the Upper San Pedro Basin showing the extent of the USGS groundwater flow model [3] and the San Pedro Riparian National Conservation Area. The Sierra Vista subwatershed is the area north of the United States—Mexico boundary.

Elevations within the Sierra Vista subwatershed range from more than 2800 meters (m) above mean sea level in the Huachuca Mountains on the western edge of the basin, to 1052 m at the Tombstone stream-flow gaging station at the north (downstream) end of the subwatershed. Precipitation across the topographically diverse basin ranges from 35 centimeters per year (cm/yr) near Tombstone in the central-north part of the subwatershed to about 76 cm/year in the highest parts of the Huachuca Mountains [4,5]. Like other areas in the desert southwest, precipitation in the study area is predictably bimodal with a summer monsoon season from July through mid-September accounting for about one half, and a winter wet season from December through March that accounts for another one third of the annual precipitation. Occasionally, tropical storms will trigger very large runoff events in October, but otherwise the fall months are typically dry. Stream flow is lowest in May and June, with peak monthly flows occurring during the summer monsoon season (Figure 2). Average monthly minimum flows range from about 1300 cubic meters per day (cu-m/d) at the Palominas stream-flow gaging station in the south (upstream) end of the subwatershed to 19,300 cu-m/d at the Charleston station, and 8100 cu-m/d at the Tombstone station on the north (downstream) end of the study area (Figure 2). Snow melt in the Huachuca Mountains often supports flow in mountain springs and intermittent streams in the spring months of March and April.

**Figure 2.** Mean monthly stream flow for the three long-term monitoring sites (shown in Figure 1) on the Upper San Pedro River in the Sierra Vista subwatershed for the period of record at each site. The y-axis is plotted on a log scale to highlight the minimum flows in May and June.

**Mean Monthly Flow for Three San Pedro River Sites**

Groundwater discharge from the regional basin-fill aquifer supports baseflows in the river and evapotranspiration by near-stream phreatophytes. Groundwater pumping from that same aquifer supplies a human population in the region that is expected to increase by 46% by the year 2050 [6]. Sierra Vista, Arizona, is the largest municipality in the basin with a population of roughly 45,300 [7]. The U.S. Department of Defense installation at Fort Huachuca borders Sierra Vista on the west and north sides and is a major economic force in the basin [8].

Because of its federal designation and its globally significant biodiversity [9], the SPRNCA has been the subject of extensive hydrological and ecological research. One major finding was that stream flow permanence explained most variance in the basal area of two keystone riparian species, Fremont cottonwood (*Populus fremontii*) and Goodding willow (*Salix gooddingii*) [10,11]. As a

result, conservation strategies have focused on maintaining or increasing stream flow through

### *1.3. Hydrological Studies in the Basin*

### 1.3.1. General Hydrologic Processes

The Upper San Pedro River is one of the most intensively studied semi-arid stream systems in the world. Estimated ages of the earliest paleo-Indian sites along the banks of the ancestral San Pedro River date back to 11,000 to 13,000 years ago [12,13]. Two Clovis Indian sites have been associated with mammoth kills in the subwatershed, providing some of the earliest dates for human hearths of this culture [13]. The basin has been grazed since the time of the first Spanish explorers in the 1500s [14], and has hosted two major metals mining operations in Tombstone (late 1800s) and Bisbee (1880s–1975), Arizona. In addition, Cananea, Mexico boasts a major copper mine that is currently in active production. The major U.S. Department of Defense Army installation at Fort Huachuca has been in active operation since 1877 and has one of the earliest water rights claims in the basin [15]. More recently, the town of Sierra Vista has grown to include over 45,000 people (including Fort Huachuca). All of these activities have had significant impacts on the landscape and water resources in the basin. Hereford [14] presents a clear description of the land use changes coupled with exceptional periods of flooding near 1900 that led to significant entrenchment and stabilization of the San Pedro River stream channel and dewatering of much of the shallow alluvium of the river channel.

reductions in groundwater pumping and managed aquifer recharge at key locations.

The Walnut Gulch Experimental Station (Figure 1) was established in the northeast area of the subwatershed by the Research Division of the Soil Conservation Service in 1951, and has been the source of continuous instrument-based hydrologic and rangeland studies since that time [16]. In 1966, Brown and others [17] evaluated the water resources of Fort Huachuca, which lies immediately west and north of the City of Sierra Vista (Figure 1). Brown and Aldridge [18] estimated San Pedro River surface discharge from the international boundary with Mexico to its confluence with the Gila River and inputs to the system from tributary inflow and from mountain-front recharge. Much of the assessment of hydrologic resources in the basin that followed came as a byproduct of the development of groundwater models, including those by Freethey [19], Vionnet and Maddock [20], Corell and others [21], Goode and Maddock [22], and Pool and Dickinson [3].

The Arizona Department of Water Resources (ADWR) evaluated the groundwater resources of the basin in 1990 [23], and then again in 2005 [24], in order to determine whether or not the basin should be classified as an Active Management Area (AMA) under the 1980 Groundwater Management Act of Arizona. The ADWR determined that the basin did not meet the statutory criteria for AMA designation [23,24], but much of the research that was conducted as part of that assessment remains highly relevant. Pool and Coes [25] described the state of the knowledge of hydrogeology of the subwatershed. As part of that work, an extensive monitoring program was initiated in the subwatershed that included geophysical surveys, ephemeral stream flow monitoring, installation or refurbishment of a number of stream gaging stations, aquifer storage monitoring using microgravity techniques, and basin-wide water level monitoring. Much of this monitoring program continues today.

In support of the conservation mission of the SPRNCA, Section 321 of the Defense Authorization Act of 2004 [26] revised how the federal Endangered Species Act applies to the Fort Huachuca Military Reservation and directed the Partnership to "...restore and maintain the sustainable yield of the regional aquifer [of the Sierra Vista Subwatershed] by and after September 30, 2011." It also required annual progress reports on these efforts, which were produced for most calendar years from 2002 through 2011 (see [27], for example).

The Partnership sponsored several research reports during this time, which provided the scientific basis for development of the U.S. Geological Survey (USGS) groundwater model by Pool and Dickinson [3]. These studies include Coes and Pool's [28] assessment of ephemeral-stream channel and basin-floor infiltration and recharge, Gungle's [29] analysis of the timing and duration of ephemeral stream flow in the subwatershed, and Leenhouts and others' [30] analysis of the hydrology, vegetation-hydrologic relationships, and evapotranspiration requirements and plant-water sources in the SPRNCA. Leenhouts and others [30] established additional streamflow and groundwater monitoring in the subwatershed, including streamflow stage and permanence data, near-stream alluvial aquifer groundwater and vertical gradient monitoring, and a continuous meteorological and eddy covariance monitoring station for measurement of evapotranspiration in the SPRNCA.

A statistical analysis of the trends in streamflow in the San Pedro River was also published in 2006 by Thomas and Pool [5]. Leake, Pool and Leenhouts [31] used Pool and Dickinson's [3] five-layer groundwater model of the basin to conduct a capture and recharge analysis that mapped the effects of pumping and recharge across the subwatershed on groundwater discharge to the SPRNCA. Kennedy and Gungle [32] analyzed baseflow discharge from the subwatershed at the USGS gaging station near Tombstone, Arizona, at the north end of the subwatershed. Most recently, Lacher [33] updated the Pool and Dickinson [3] groundwater flow model to include recent changes in pumping and artificial recharge in the subwatershed, and simulated the projected effects of population growth-driven increases in pumping on groundwater levels and baseflow through 2105.

### 1.3.2. Managed Aquifer Recharge

In 2006, Stantec [34] developed a Flood Control Urban Runoff Plan for Cochise County that evaluated the size, placement, and efficacy of 30 existing and proposed stormwater detention basins on the west side of the subwatershed functioning as de facto recharge basins. As part of this study, GeoSystems Analysis [35] used a detailed precipitation-recharge-stormwater runoff regression model, based on the U.S. Department of Agriculture's Automated Geospatial Watershed Assessment Tool [36] for the urbanized Coyote Wash watershed in Sierra Vista to estimate the cost, recharge volume, and urban-enhanced runoff for that watershed and several others on the west side of the San Pedro River within the subwatershed. GeoSystems Analysis previously developed the stormwater regression model used for the Upper San Pedro Partnership and Cochise County based on detailed AGWA stormwater modeling results for the Coyote Wash watershed located in the City of Sierra Vista [37].

Predictive groundwater modeling [33] indicates that within the next 100 years, two regional cones of depression will merge and reduce groundwater flow from the regional aquifer to the San Pedro and Babocomari (a major tributary to the San Pedro) rivers (Figure 3). These simulations incorporate pumping increases over time that reflect U.S. Census growth rates [5,6] and maintain constant natural recharge and evapotranspiration at 2003 levels (as published in the USGS groundwater model [2]) for the entire 21st century. Figure 4a maps the west-east transect A-A' for the simulated groundwater-level profiles shown in Figure 4b. This transect goes through the Sierra Vista-Fort Huachuca cone of depression, and illustrates how simulated groundwater levels at the groundwater divide between the cone of depression and the river have already declined by 17% since pre-development conditions in 1902 and are predicted to double that level of decline, to 35%, by the year 2100. These reductions in groundwater levels produce commensurate percent reductions in groundwater gradient (change in head divided by distance) as measured from the same point on the groundwater divide to the river. While gradients directly under the river are not yet showing the same degree of impacts as those farther west, the ultimate outcome of these simulated changes would be greater baseflow losses in some losing reaches, smaller gains in some gaining reaches, and possibly the conversion of some reaches from gaining to losing. The resulting groundwater-driven reduction in baseflow and shallow groundwater would likely impair the dependent riparian systems [33].

**Figure 3.** Simulated drawdown (m) in the primary regional aquifer of the basin (**a**) in 2000; (**b**) in 2050; and (**c**) in 2100 [33].

**Figure 4.** (**a**) Cross-section A-A' in the center of the Sierra Vista subwatershed (adapted from Stromberg and others [10]; (**b**) Simulated groundwater-level profiles in 1902, 2013, and 2100 along the A-A' cross-section showing change in gradient (*ǻi*) from í17% in 2013 to í35% in 2100.

**181**

One potential solution to the problem of aquifer storage depletion would entail importation of at least some water from a source outside the basin [38]. However, at this time, importation does not appear likely in the foreseeable future (see companion paper by Richter and others [1]). In lieu of a replacement water supply, water managers and stakeholders in the subwatershed recognize the need for a near- to medium-term (*i.e.*, years to decades) intervention to protect stream baseflows and the associated riparian areas.

The City of Sierra Vista has more than 10 years of monitoring data from its Environmental Operations Park (EOP) where treated effluent is recharged at a rate of roughly 9100 cubic-meters per day (m3 /d) through artificial wetlands and recharge basins between the City's pumping center and the San Pedro River. This project was designed and constructed to create a groundwater mound to sustain surface water flow and supplement alluvial groundwater levels during low-flow periods [39]. Groundwater modeling that incorporates the EOP recharge data indicates that the project is successfully contributing recharge to both the nearby cone of depression in the regional aquifer and to baseflows in the San Pedro River [33,40]. Further groundwater modeling presented in this study suggests that additional recharge projects designed to utilize enhanced urban runoff, treated effluent or other local supplies near the San Pedro and Babocomari rivers may successfully mitigate anticipated pumping-related baseflow depletions for up to 100 years.

While conservation efforts within the subwatershed have reduced consumptive use of groundwater in the basin, particularly on the Fort Huachuca Army installation, the cumulative storage loss in the regional aquifer within the subwatershed from the past half century of pumping, estimated to exceed 800 MCM (refer to explanation provided in Section 2.3.2) remains a threat to the San Pedro River and its tributaries by intercepting mountain-front recharge and by reducing the groundwater gradient that drives aquifer discharge to the rivers. Because of this storage deficit and the fact that virtually all water use in the subwatershed depends on groundwater, almost any level of continued groundwater use poses an increasing risk of stream and riparian evapotranspiration capture over time.

### 1.3.3. Streamflow Permanence Monitoring

Direct measurement of baseflow, defined as the portion of streamflow derived from groundwater, is complicated by the effects of prolonged storm and/or snowmelt runoff (which tend to exaggerate baseflow estimates) and evapotranspiration (which reduces apparent baseflow). Acknowledging these complexities, practitioners have found that inter-annual stream-flow permanence in a river system that experiences intermittent drying in some reaches is a useful indicator of both hydrologic and ecological conditions in the San Pedro basin. Using a technique called wet/dry mapping, analysis of surface water presence (strict criteria are used to ensure consistent definition of surface water versus minimal puddles) during the driest time of year (mid June in the San Pedro Basin) reveals areas with high year-to-year variation in the length of surface wetting (Figure 5). By limiting the influence of storm events as much as possible, and assuming no significant changes in the condition of the riparian forest, these variations in wetted length are believed to be the best available physical expression of local groundwater conditions and may provide early warning of ecological changes [41].

**Figure 5.** Wet/dry results from the San Pedro Riparian National Conservation Area. The heavy river line shows reaches, which were wet in June 2013. Bars on right side represent wet reaches for each year, 1999–2013. Labels on the far right identify the 10 analysis segments, each covering 8.1 km. The four green properties (parcels 1 through 4) on map were recently acquired for groundwater protection and recharge projects. Redrawn from Turner and Richter [41].

Wet/dry mapping, as applied in the San Pedro River basin, uses citizen scientists annually to map the spatial extent of surface water in a river or stream. The method provides a comprehensive snapshot of conditions for the whole river at the same date each year, allowing comparisons of year-to-year variability [41]. Beginning in 1999, staff from The Nature Conservancy and the U.S. Bureau of Land Management coordinated volunteers to map the spatial extent of surface water within the SPRNCA. The exact dates varied slightly, but mapping was conducted during the third weekend of June each year to coincide with the lowest flow before the expected start of the summer rainy season. Through the years, progressively more of the river and its tributaries have been surveyed, increasing to 231 km of the mainstem and 266 km of tributaries in 2013. This paper addresses only the 80 km of mainstem river surveyed through SPRNCA.

### **2. Methods**

### *2.1. Wet/Dry Mapping*

Surveyors walked predetermined segments of the river, recording the coordinates of beginning and end points of all surface water segments greater than or equal to 9.1 m long using paper data forms and consumer-grade Global Positioning System units. They disregarded any dry gaps less than 9.1 m long in otherwise wet reaches. The resulting point coordinates were imported to a Geographic Information System (ArcGIS, Environmental Systems Research Institute, Redlands, CA, USA), and snapped to the closest points on a linear representation of the river. To identify localized trends, organizers partitioned the SPRNCA into 10 equal segments, 8.1 km long. Segments were analyzed for probability of trend using the Mann-Kendall test. For graphic purposes, we calculated and display the Sen estimate of linear trend (detailed methods are provided by Turner and Richter [41]). Starting in 2007, maps and summary data from the wet/dry surveys have been posted each year to a web site (http://www.azconservation.org) for public distribution.

Results from wet/dry mapping in the SPRNCA (Figure 6), as a whole, show about half the river has permanent surface water with some year-to-year variation but no trends. However, analysis of the smaller segments shows considerable variation Turner and Richter [41]). The southernmost segment, Segment 1, displays a significant downward trend, while Segment 2 trends upward. Most of the other segments have either year-to-year up/down results or a stable condition without trend. The distribution of wet and dry reaches has provided a simple way to prioritize and site conservation actions aimed at reduced groundwater pumping and managed aquifer recharge, as described in Section 3 below. Wet/dry mapping data are also expected to provide a quantitative measure of conservation progress after those strategies are implemented.

**Figure 6.** Total wetted lengths for the 10 analysis segments in Figure 3. Segment numbers increase from south to north (downstream). Revised from Turner and Richter [41].

### *2.2. Capture Mapping*

### 2.2.1. Overview of Capture

The Upper San Pedro River's position near the geographic center of an alluvial basin makes it a classic example of basin-fill hydrology. The existence of a major pumping center at Sierra Vista/Fort Huachuca between the primary area of recharge for the subwatershed (the Huachuca Mountains on the western boundary) and the river (Figure 1) also makes the basin a classic case study in groundwater capture. Capture is the increase in recharge to, and (or) decrease in discharge from, a basin that eventually occurs as a result of groundwater pumping. Theis [42] first addressed the consequences of groundwater pumping from a previously undeveloped system in 1940:

*"Under natural conditions…previous to development by wells, aquifers are in a state of approximate dynamic equilibrium. Discharge by wells is thus a new discharge superimposed upon a previously stable system, and it must be balanced by an increase in the recharge of the aquifer, or by a decrease in the old natural discharge, or by loss of storage in the aquifer, or by a combination of these."*

Lohman and others [43] further clarified the definition of capture from Theis' description:

*"The decrease in discharge plus the increase in recharge is termed capture."*

This definition of capture may be written as:

$$
\mathcal{Capture} = \Delta \mathcal{R} + \Delta \mathcal{D} \tag{l}
$$

where *ǻR* and *ǻD* equal the increase in recharge and the decrease in discharge, respectively.

In the Upper San Pedro basin, as in many basins in the southwest deserts of the United States, aquifer storage is plentiful as a result of recharge over thousands of years of depositional history, but potential sources of capture are limited primarily to reductions in riparian ET, reductions of groundwater discharge to streams (baseflow), and direct capture of streamflow. The numerical partitioning of extracted groundwater into its constituent sources through a water-budget process is a common and fraught practice in many water-short areas of the United States. Bredehoft, Papodopulos, and Cooper [44] attributed this practice to "perhaps the most common misconception in groundwater hydrology" which is "that a water budget of an area determines the magnitude of possible groundwater development." Under this line of reasoning, many water managers have concluded that all water entering the system as natural recharge is available for extraction without long-term deleterious effects. Brown [45] addressed the problem with this argument through an example of a well whose cone of depression eventually expands to intersect a stream in which he demonstrated that,

*"…the rate at which the hydrologic system reaches a new steady state depends on the rate at which the natural discharge [in his example to a stream] can be captured by the cone of depression."*

Figure 7 illustrates the evolution of a groundwater system which receives natural recharge (through precipitation) at a fixed and limited rate, *R0*, and discharges to a stream as baseflow at a rate of *D0***.** In Figure 7a, the system is in equilibrium prior to any significant groundwater development. In this natural state of equilibrium (roughly prior to 1940 in the Upper San Pedro basin [3]) recharge equals discharge (Equation (2)):

$$R\_0 = \,^\bullet D\_0 \tag{2}$$

**Figure 7.** (**a**) Groundwater discharging to stream under equilibrium conditions; (**b**) Pumping at rate Q from a well intercepting groundwater that would have discharged to the stream under equilibrium conditions; Q is roughly equal to the rate of change in aquifer storage (¨S) plus reduced groundwater discharge to the stream (¨S); (**c**) Pumping at the same rate (Q) under a new equilibrium condition. Now, pumping is reducing groundwater discharge to the stream (¨D) and inducing recharge directly from the stream (¨R), and aquifer storage depletion ceases (¨S = 0). Adapted from Winter and others ([46], Box C, p. 15) and from Heath ([47], p. 33).

In Figure 7b, pumping at a rate of *Q* is superimposed on the system, producing a rapid rate of change in aquifer storage near the well (*ǻS*) and intercepting some groundwater that would otherwise have discharged to the river (*ǻD)*, but having no immediate effect on discharge to the stream (*D0*) due to the persistence of a groundwater "high" between the well and the river. At this point, pumping is roughly equivalent to the rate of change in aquifer storage plus the reduction in groundwater discharge to the stream (Equation (3)).

$$\mathbf{Q} = \Delta \mathbf{S} + \Delta \mathbf{D} \tag{3}$$

and recharge and discharge are no longer in balance (Equation (4)):

$$R\_0 \neq Q + \mathcal{D}\_0 \tag{4}$$

After some time (Figure 7c), the cone of depression intercepts the stream and reverses the gradient of the groundwater so that it now flows toward the well from all directions, directly capturing streamflow. Even though the pumping rate (*Q*) remains constant, recharge from precipitation also remains fixed at *R0*, so the only sources of water for extraction come reduced groundwater discharge to the stream, capture (increased recharge) from the stream, itself, and possibly some additional aquifer storage. A new equilibrium is achieved when aquifer storage is no longer being depleted (*ǻS =* **0***)* and pumping is balanced by the capture of stream flow and decreased discharge to the stream (Equation (5)):

$$R\_0 = \mathcal{Q} + (\mathcal{D}\_0 - \Delta \mathcal{D} - \Delta \mathcal{R}) \tag{5}$$

or,

$$\mathcal{Q} = \Delta \mathcal{R} + \Delta \mathcal{D} \tag{6}$$

Lohman [48] identified another potential source of capture not expressly described above. He referred to it as "salvaged rejected recharge from precipitation." This potential source of capture would comprise a new source of recharge (*ǻR*) as described in Equations (1) and (6). In the case of a large alluvial basin like the San Pedro, where much of the shallow alluvial aquifer is separated by a thick sequence of confining materials from the underlying regional aquifer, this source of potential capture would derive from the occasional replenishment of the shallow alluvial aquifer during runoff from one or more very large precipitation events. Runoff exceeding 25M cu-m/d occurs roughly every 5 to 6 years at the Palominas station (Figure 1), so any excess alluvial aquifer storage opened up by groundwater pumping could capture flood flows that would otherwise have been rejected and remained in the stream.

For the period 2000 to 2009, Kennedy and Gungle [32] found that alluvial aquifer storage changes that govern baseflow measured at the Tombstone stream-flow gaging station (near the far downstream end of the study area (Figure 1)) were a function of upstream riparian ET and summer precipitation. Although they did not identify groundwater pumping as a clear source of baseflow decline in that reach over that short period, they do caution that, "[c]ontinued regional groundwater pumping will, however, eventually lead to a decline in the contribution of regional groundwater to base flow." This contrast between the interannual scale of fluctuations in alluvial aquifer storage and the multi-decadal scale of changes in regional aquifer discharge is highly significant for long-term water management planning. Our study focuses on these longer-term regional aquifer changes, but acknowledges the importance in improving our understanding of the linkages between sources of capture at different temporal and spatial scales.

Groundwater simulations [3,33] indicate that the Upper San Pedro basin is transitioning between the second and third scenarios illustrated in Figure 7. Of course, streamflow is not the only potential source of capture in most groundwater basins, and may not be available at all, as in basins where the streambed has lost connectivity with the underlying aquifer. Other potential sources of capture include riparian evapotranspiration, groundwater inflow from boundary sources, and groundwater outflow from the basin. Because groundwater and surface water systems operate on such different time scales, stresses on a groundwater system may not manifest as baseflow depletions for many years. Unfortunately, "in many circumstances the dynamics of the groundwater system are such that long periods of time are necessary before any kind of an equilibrium condition can develop. In some circumstances the system response is so slow that [groundwater] mining will continue well beyond any reasonable planning period" ([44], p. 55–57). The goal of the study described in this paper is to anticipate the future impacts of 20th and 21st century pumping on the San Pedro River and to develop a strategy to mitigate those effects as they occur.

### 2.2.2. Development of Capture Map

Leake and others [31,48] developed a unique tool that utilizes the Pool and Dickinson [3] regional groundwater flow model to assess simulated pumping-induced capture of streamflow, spring discharge, and riparian ET across the model domain in response to a unit pumping stress at every location in the model, and presents the results as a map overlay with colored contours representing the amount of capture, as a fraction of the pumping rate, after pumping for a specified amount of time (Figure 8). Leake and others [49] provide detailed steps for developing a capture map using a groundwater model. Alternatively, capture for a given location can be calculated as the pumping time needed to reach a depletion-dominated supply (the time at which capture begins to provide greater than 50% of total groundwater pumped). A typical time scale for a basin in the American Southwest might be 0 to 100 years [50]. Capture map development can also be run in reverse, providing estimates of the total increase in streamflow, riparian evapotranspiration, and spring flow, as a fraction of recharge rate, after recharging at a unit rate for a specified amount of time [51]. The two are not necessarily the inverse of each other. Leake and others [31] calculated stream and riparian area capture resulting from pumping in the lower basin fill primary aquifer (layer 4 of the 5-layer groundwater model [3]) in the Upper San Pedro basin. They used the same method to calculate response in the stream/riparian area from recharge applied to the top-most layer of the model that overlies the extent of layer 4 (layers 1, 2, or 4).

### 2.2.3. Use of Capture Map in the San Pedro Basin

Richter and others [1] discuss the evolution of the capture map concept and its use in policy development within the basin. While the capture map does not replace groundwater modeling, it is a simple, intuitive tool that permits the layman to gain a better understanding of the degree of connectivity between the groundwater system and the river. Policy makers and stakeholders embraced the capture map in the San Pedro basin as a guide for various preliminary decisions on community development and ecological restoration.

**Figure 8.** Computed capture (as a percentage of pumping rate) of streamflow, riparian evapotranspiration, and spring flow that would result for withdrawal of water from model layer 4 at a constant rate for 10 years. The color at any location represents the fraction of the withdrawal rate by a well at that location that can be accounted for as changes in outflow from and or inflow to the aquifer for model boundaries representing streams, riparian vegetation, and springs. Redrawn from Leake and others [31].

### 2.2.4. Advantages/Disadvantages and Limitations of the Tool

Capture maps highlight where pumping will have the greatest impact on a water resource (such as a river) within a specified period of time, and thus make an excellent preliminary planning tool for water managers, commercial and residential developers, and conservationists who can benefit from reducing the immediate impacts of groundwater pumping. The similarly constructed recharge map provides similar planning benefits, offering the planner an initial overview of the most advantageous sites for storm-water, treated effluent, or other recharge facilities.

Despite the value of capture maps for communicating some complex hydrologic concepts to lay audiences, details about specific volumes and rates of capture for specific sources/sinks in the basin cannot be extrapolated directly from this tool. Capture maps, based on linear superposition, reflect the assumption of constant hydrologic properties of the aquifer, assuming that pumping causes no non-linear behavior in the hydrologic system. This limitation, and the fact that each cell of the model is stressed in isolation to make the map, means that capture maps cannot replace the use of a full groundwater model for examining the cumulative impacts of various stresses and sinks that change over time or that cause nonlinearities in hydrologic properties or boundary conditions.

### *2.3. Near-Stream Recharge Simulations*

### 2.3.1. Description of Model

Simulations in this study used the most recent and most sophisticated groundwater flow model of the Upper San Pedro Basin available [3]. Although the model area includes all of the 4500-km2 basin of which 40% is in Mexico, this study focuses on the Sierra Vista subwatershed within the United States (Figure 1). This MODFLOW-2000 [52] model is based on a uniform 250 m × 250 m grid spacing oriented north-south in alignment with the basin. The stream is represented by the Stream Package [53], hydraulic flow is modeled with the Layer Property Flow (LPF) package, and riparian evapotranspiration is modeled with the Evapotranspiration (EVT) Package of MODFLOW-2000. The two-season model reflects the seasonal significance of evapotranspiration in the riparian zones along the San Pedro and its tributaries. The cool season extends from mid October to mid March, and the warm season runs from mid March through mid October [3].

As with all groundwater models, this model has several limitations. Most significantly, it does not simulate flood flows, which are known to contribute significant recharge to the alluvial aquifer near the center of the basin as well as along some ephemeral tributaries to the mainstem of the San Pedro River. This seasonal "topping off" of the shallow alluvial aquifer may support baseflows in the river through one or more dry seasons, and is an important component of the riparian system. While this shortcoming means that the model tends to underestimate true baseflow (regional aquifer plus shallow alluvial aquifer contributions), it does not preclude the model as a useful tool for analyzing the effects of pumping on the regional aquifer's contribution to baseflow. For this reason, the term "baseflow" in this study refers strictly to that component of total baseflow that derives from the regional aquifer where most of the pumping in the basin occurs.

Figure 9 illustrates the conceptualized hydrogeologic cross section (upstream view) of the subwatershed and shows how the model layers correspond to that conceptualization. The model structure includes five layers in a stacked-bowl configuration representing sediment accumulation in the structural depression between two bounding mountain ranges, as is typical of the Basin and Range province of the western United States [3]. Only model layer 5 is found throughout the entire region (Figure 10).

**Figure 9.** Conceptualized cross section of basin showing model layers. Adapted from Figure 3 in Pool and Dickinson [3]).

**Figure 10.** Model layers [2] in plan view with San Pedro River intersecting various layers. Light blue line represents river location but does not necessarily signify perennial flow.

### 2.3.2. Use of Model in Water Resources Assessments

Initial groundwater modeling efforts by Pool and Dickinson [3] simulated transient groundwater levels and baseflows in the basin resulting from 20th-century (1902–2003) pumping and changes in riparian evapotranspiration associated with climate and evolving geomorphology of the stream channel [14]. These simulations reflected the development of a major cone of depression under the Sierra Vista/Fort Huachuca population center on the west side of the river, and a smaller area of groundwater depletion very close to the river near the communities of Palominas and Hereford, where agricultural pumping occurred for many decades (Figure 3a). The effects of groundwater use in Mexico also manifest as an irregular cone of depression progressing northward from the south edge of the regional aquifer.

Simulations by Lacher [33] built on the work of Pool and Dickinson [3] and projected pumping through the 21st century based on population projections developed by the Arizona Department of Commerce [54] and TischlerBise [55], and calculating the total drawdown (Figure 3) and change in baseflow over the 1902–2100 period (Figure 11). These simulations predicted widespread increases in aquifer storage depletion across the western side of the basin during the 21st century (Figure 3b,c). They also quantified projected declines in baseflow in the basin over the next 100 years due to pumping and evapotranspiration, as well projected increases in baseflow until about 2050 resulting from the recharge facility at the City of Sierra Vista's Environmental Operations Park (EOP) (Figure 11b).

**Figure 11.** Simulated baseflow capture from 1902 to: (**a**) 2000; (**b**) 2050; and (**c**) 2100. Capture is cumulative [33] and measured in cu-m/d. EOP indicates location of Sierra Vista's Environmental Operations Park wastewater recharge facility.

Water budget calculations and groundwater simulations suggest that cumulative total groundwater depletion is presently on the order of 800 MCM in the subwatershed with annual net storage loss over the last decade on the order of 5 to 7 MCM [56,57]. Policy makers in the subwatershed have considered the feasibility of importing water at a maximum rate of roughly 37 MCM/yr [38]. However, even that most optimistic rate of importation would require nearly 25 years to bring the subwatershed's cumulative water budget back into balance. In the meantime, ongoing pumping-induced baseflow capture would continue to depress the groundwater gradient between the pumping centers and the river (Figure 4) further reducing baseflows.

### 2.3.3. Near-Stream Simulated Recharge Site Selection

Although artificial recharge of urban-enhanced runoff through detention basins has long been considered a viable option for mitigating impacts of groundwater pumping, the basins have previously been designed to target the active cone of depression in the Sierra Vista/Fort Huachuca area [34] rather than the river. The concept of simulating targeted near-stream recharge arose from the process of quantifying the pumping-induced capture of baseflow (*i.e.*, a reduction of groundwater discharge from the regional aquifer to the river) over time using the regional groundwater flow model [33]. With simulated current baseflow capture for the entire basin and projections of aquifer storage and baseflow capture trends over the next century, near-stream recharge was identified as a potential mechanism for addressing projected baseflow "deficits" (declines from 2003 (end of transient model calibration period) baseline values) in targeted areas of the basin.

Taken together, the suite of hydrologic tools pointed to sections of the San Pedro River where greater protection was warranted and where meaningful impacts—such as converting an intermittent stream reach into perennial reach—could be made. Simulations with the groundwater model [58] suggested that developing a distributed, strategically located network of recharge projects near select river reaches might result in groundwater mounding that would effectively compensate for baseflow capture, thus, protecting baseflow and the riparian vegetation community from the anticipated effects of pumping for several decades or more.

### 2.3.4. Application of Recharge Models

A small core team of technical experts within the Upper San Pedro Partnership Technical Committee selected three trial sites for simulating hypothetical recharge near the river: (1) Palominas; (2) Garden Canyon; and (3) Babocomari (Figure 12). The site selection process was informed by both simple geography (upper, middle, and lower part of the subwatershed) and current and projected baseflow capture in the river system. Simulated recharge at each trial site consisted of surface recharge over four 250 m × 250 m model cells (0.25 km2 ) for the period 2012–2111. For each of the three trial sites shown in Figure 12, the recharge simulation investigation involved increasing recharge, as needed, to prevent any decline in baseflow below baseline (2003) levels downstream of the site over the simulation period while also preventing simulated surface flooding at the trial site.

Starting with 0.62 MCM/yr, simulated recharge at each site was incrementally increased, and baseflow response tested, until baseflow downstream of each site remained at or above 2003 levels for the 100-year simulation period. Figure 13 shows simulated change in the cool season (October–March) baseflow from 2003 (end of the transient calibration period) conditions in the years 2030, 2050, 2070, and 2111. As the decreasing cool (blue and green) colors in the northern half of the subwatershed over time indicate, baseflows are predicted to fall below 2003 levels in all of the mainstem San Pedro River and on the Babocomari by 2111. Recharge at the Sierra Vista EOP successfully maintains simulated baseflows in the mainstem above 2003 levels until at least 2070, but the impacts of pumping (deepening and widening cone of depression) overwhelm the recharge benefits by about the turn of the century.

**Figure 12.** Simulated trial recharge sites (Babocomari, Garden Canyon, and Palominas) in the Sierra Vista subbasin. Riparian condition class reaches delineated within the SPRNCA [10]. Adapted from Figure 42 in Stromberg and others [10].

**Figure 13.** Simulated difference in baseflow (cu-m/d) in basin streams from 2003 conditions in: (**a**) 2030; (**b**) 2050; (**c**) 2070; and (**d**) 2111.

### 2.3.5. Simulation Results

Hydraulic conductivity, antecedent depth to groundwater, and projected aquifer storage depletion over time controlled the simulated baseflow response to recharge at each of the three trial sites. Since simulated recharge applied at each of the three trial sites was tailored to meet the anticipated baseflow deficit downstream, each site demanded a unique recharge distribution and exhibited a unique response to the simulated recharge. Figure 14 illustrates the recharge rates determined by trial and error as necessary to sustain simulated baseflows at or above 2003 levels in the groundwater model [3] through 2111. While each of the three test sites exhibited a unique response to the simulated recharge rates shown in Figure 14, the Babocomari site exhibited a much higher demand for recharge and a much more pronounced response in the underlying groundwater than the other two sites [58]. For the purpose of illustration, only the Babocomari test site recharge results will be discussed in detail here.

**Figure 14.** Simulated recharge rates for the Babocomari, Palominas, and Garden Canyon test sites.

**Simulated Recharge Rates for Individual Test Sites**

Figure 15 illustrates the groundwater response to recharge at the Babocomari site. The black line represents depth to groundwater (DTW) under the stream adjacent to the recharge site in the baseline model with no simulated recharge at the Babocomari site. The orange line shows DTW in response to a constant rate of 0.62 MCM/yr. recharge, and the blue line shows DTW in the same location under the stream for the varying-recharge scenario illustrated by the green "Variable Recharge Rate" line. In the absence of any intervention (black DTW curve), simulated heads at this site are projected to drop by more than 10 m over the 21st century in response to pumping. However, these simulations suggest that incrementally increasing recharge from an initial rate of 0.74 MCM/yr in 2012 to 3.2 MCM/yr by 2100 would successfully maintain groundwater levels under the river at or slightly above the 2003 level.

The oscillation in the blue DTW curve reflects the fact that the simulated variable recharge increased groundwater levels under the river to a depth between the top of the evapotranspiration (ET) surface (1.5 m below top of aquifer) and the ET extinction depth at 6 m below the top of the aquifer. Thus, groundwater is more accessible to riparian vegetation in the varying-rate recharge scenario than in the baseline case, but baseflows still remain at or above 2003 levels in the area of the stream downstream where baseflow declines are projected under baseline conditions.

**Figure 15.** Simulated variable recharge rate at the Babocomari site and depth to water (DTW) under stream adjacent to the Babocomari recharge site from 2012 to 2111 for three scenarios: (**a**) no recharge; (**b**) 0.62 MCM constant-rate recharge; and (**c**) variable-rate recharge scenarios [58]. Evapotranspiration (ET) zone occurs between 1.5 and 6 m [3].

**Simulated Depth to Groundwater Under Stream Near Babocomari Site and Simulated Variable Recharge Rate Applied, 2012-2111**

### 2.3.6. Optimization of Recharge Rates

Section 2.3.5 presented the results of simulated varying-rate recharge at each of three trial sites individually along the San Pedro and Babocomari Rivers. One additional simulation combining the three trial recharge sites was run in order to answer the question of whether hydrologic efficiencies might be gained with simultaneous recharge at all three sites [58]. Because the reach of the San Pedro affected by simulated recharge at the Garden Canyon site is downstream of both the Babocomari and Palominas sites, some reduction in the required recharge at the Garden Canyon site was achieved by combining all three trial recharge sites in a single simulation. Figure 16 illustrates the reduced recharge required at the Garden Canyon site to maintain simulated baseflows downstream of that site at 2003 levels when recharge is simulated at the Palominas and Babocomari trial sites concurrently.

Total recharge required to maintain simulated baseflows at 2003 levels downstream of each of the three test sites is shown in Figure 17. The blue curve shows the total recharge required when the three sites are simulated independently of each other, and the green curve shows the total recharge requirement when all three test sites are simulated concurrently. The difference between the curves illustrates the efficiency gained by combining the three recharge test sites. The maximum total recharge rate in the independent simulations is 4.93 MCM/yr (3.35 MCM/yr average over the 2012–2111 period), but that value drops to 4.63 MCM/yr (2.91 MCM/yr average) for the concurrent recharge simulations. The average recharge saved by operating all three sites concurrently is 0.45 MCM/yr. Figure 18 shows the final simulated change in baseflow from 2003 conditions when recharge at the three test sites is optimized for concurrent simulation of the three sites.

**Figure 16.** Simulated variable recharge rates for the Babocomari, Palominas and Garden Canyon test sites. Simulating recharge at all three sites concurrently allowed a reduction in Garden Canyon recharge relative to the rates for the independent recharge simulations.

**Simulated Recharge Rates Optimized for Combined Test Sites**

**Figure 17.** Total simulated recharge at the Babocomari, Palominas, and Garden Canyon test sites required to maintain baseflows downstream of each site at or above 2003 levels when recharge is simulated at each site independently and when all three sites are simulated concurrently.

**Figure 18.** Simulated difference in March baseflow (cu-m/d) from 2003 conditions with optimized variable-rate recharge at three trial recharge sites in: (**a**) 2030; (**b**) 2050; (**c**) 2070; and (**d**) 2111.

2.3.7. Advantages/Disadvantages and Evolution of the Tool

Any perceived bias in the construction of the model, or simple disagreements among stakeholders with the technical modeling approach, can be problematic in terms of how modeling results will be used, if at all. Engagement of stakeholders throughout the model development process is essential for it to be embraced as a useful tool for decision making among varied interests. In this case, the groundwater model [3] was developed in response to wide dissatisfaction with some precursor models of the subwatershed. Richter and others [1] describe the process by which stakeholders were involved in the development of the Pool and Dickinson model [3] and the evolution of trust in the model among technical water resources experts and politicians in the basin.

Several years of experience with the Pool and Dickinson model [3] in developing and communicating results of baseline projections helped pave the way for using it as a tool to evaluate the prospects of near-stream recharge. The model made experimenting with various recharge rates and tracking the resultant changes in baseflow a fairly quick and inexpensive undertaking. While we feel that these simulation efforts successfully conveyed the general concept and potential merits of near-stream recharge to the public and decision makers, significant criticism arose from our choice of an arbitrary initial recharge rate of 0.62 MCM/yr for the hypothetical recharge sites. We viewed the initial near-stream recharge simulations as an exploratory mission to determine how much water would be required to produce the desired effect on baseflows in the San Pedro River, irrespective of the potential feasibility of attaining that quantity of water or distributing it in the locations of interest. As our initial simulation results were presented to stakeholders, many of them questioned the value in simulating recharge with water that is not available and for which no plans to develop were pending. We saw the near-stream modeling process as a "proof-of-concept" effort, but others quickly made the leap to the real difficulties in securing water for recharge. Significant time and energy were expended in efforts to bridge this conceptual gap, and in hindsight, more early effort to clarify the purpose and strategy behind the simulations would have been helpful.

### 2.3.8. Application of Recharge Simulations to Upper San Pedro Parcels

The primary value in the groundwater modeling efforts undertaken for the three "test sites" described above was in proving the potential benefit of multiple recharge sites near the river. In recent years, the U.S. Army Compatible Use Buffer program, Cochise County, and The Nature Conservancy collaboratively acquired and set aside for conservation purposes four parcels on the west side of the San Pedro River totaling 2226 hectares within the subwatershed (Figure 5). Collectively, these properties make up the physical sites currently under consideration for development of a network of near-stream recharge projects. The groundwater modeling process detailed above for the three hypothetical "test sites" was employed to some extent in the preliminary planning process for parcels 1 and 3 in Figure 5. As projects move from the conceptual phase to the physical site investigation stage, use of the groundwater model is being adapted to suit the project planning needs for the individual sites. While development of a recharge project on parcel 1 was constrained by several factors (parcel size, flood-control objectives, location of high-permeability soils off site), the current planning process for parcel 3 is relatively unconstrained. Modifications to model structure to reflect observed field conditions and refinement of the model grid to allow for more detailed simulation of potential recharge are two of the anticipated outcomes of the next phase of investigation at parcel 3.

### **3. Results and Discussion**

The suite of analytical tools discussed here is being used to inform key decisions necessary to balance groundwater use and maintain San Pedro River flows and associated riparian area ecological health. An extensive collection of hydrological studies and a robust, long-term monitoring program in the San Pedro basin have provided policy makers and stakeholders with important information about the complex relationships between groundwater condition, streamflow, and the ecological integrity of the riparian system within and near the SPRNCA. As a result, most of the stakeholders in the subwatershed understand that much of the San Pedro's riparian vegetation uses groundwater from the stream alluvium, and that this alluvial aquifer stores water from flood flows, receives groundwater from the regional aquifer, and contributes baseflow to the river during low-flow periods.

The groundwater model used in this study does not incorporate the complex interactions between flood-driven recharge of the shallow alluvium which influences baseflow to varying degrees from year to year, and pumping-induced depletions of the regional aquifer, which take decades to centuries to alter baseflow. While the model's authors made every effort to exclude storm-flow influences from the baseflow measurements they used in model calibration [3], it is virtually impossible to ensure that any given measured baseflow value represents only regional aquifer groundwater. The implication of this limitation is that the model may slightly overestimate the regional aquifer's contribution to baseflow, which would manifest as overestimated aquifer transmissivity and/or streambed conductance. That outcome, in turn, means that the model would also overestimate the simulated impact of pumping on baseflow until capture reaches its maximum (equilibrium) value. For the purpose of this study, which is to develop strategies for mitigating pumping-related impacts on baseflow, that response would be conservative and acceptable.

The Pool and Dickinson [3] model represents the culmination of years of study and is the best tool currently available for the study area. As discussed by Richter and others [1], using a groundwater model that is accepted by the vast majority of decision makers to perform predictive groundwater modeling has been essential for beginning to make management and project decisions from a common starting point. The modeling has demonstrated that within the next 100 years, two regional cones of depression will enlarge and likely change the nature of the hydrologic connection between the San Pedro River and the regional aquifer, reducing baseflow and impacting the dependent riparian system and thus wildlife populations.

Awareness and acceptance of an impending problem, however, is only the first step in finding a solution. The additional tools of the wet/dry maps and groundwater capture/recharge maps helped to focus management attention on finding both the most vulnerable areas of the system and the most beneficial locations for mitigation efforts. Analysis of wet/dry maps showing surface water presence during the driest time of year and areas with high year-to-year variation in wetted length may be the first physical evidence of changes in local groundwater conditions at the river. While wetted length is not solely controlled by groundwater conditions (it may also be affected by climate), different trends in various reaches of the river may help identify areas at higher risk of future ecological changes. Aligning these low-flow river reaches with the groundwater capture maps provided a rough indication of the rate that recharge in those areas of the aquifer might respond, expressing itself as baseflow in the San Pedro River. The capture maps also suggested the suitability of various locations for recharge that may communicate with the alluvial aquifer and/or the San Pedro River. On-site investigation of actual hydrogeologic conditions and suitability for recharge at a particular location is the first step toward refining a preliminary conceptual model derived from the tools described in this paper.

In the San Pedro basin, the use of these complementary approaches informed the purchase of the most hydrologically sensitive lands near or adjacent to SPRNCA (Figure 5) in order to both defer residential and/or agricultural development and provide the opportunity for near-stream recharge project development. The concept of a strategically located network of recharge projects near these river reaches evolved, in part, from the success of more than a decade of managed aquifer recharge at the City of Sierra Vista Environmental Operations Park in supporting both baseflow and replenishing the deep regional aquifer. We anticipate that recharging urban-enhanced runoff, storm water, and treated effluent near at-risk reaches shown would create groundwater mounds to sustain surface water flow and supplement alluvial groundwater levels during low-flow periods, effectively compensating for the otherwise deleterious impacts of encroaching cones of depression in the regional aquifer on baseflows for the next several decades or more.

### **4. Conclusions**

The application of the study methods presented in this paper and the development of system-specific techniques appear to have great promise for protecting dry-land riparian systems from the impacts of groundwater extraction, surface water diversions, and the extremes of climate change for up to several decades. The ecological, cultural, and economic significance of the San Pedro River has made it one of the most well-studied and understood river systems in the world. The tremendous volume of data and hydrologic tools already developed for this specific system coupled with many years of collaborative partnerships that have matured with the science make the San Pedro basin a very unique policy environment. There is likely no other river system with an identical set of social, political, and ecological circumstances, but the hydrologic analysis tools described in this paper can be used anywhere. The strength of collaborative partnerships and knowledge of which tools they jointly support is key to building a common understanding of the history, goals, and resources at hand to make real progress.

### **Acknowledgments**

The authors would like to acknowledge the Walton Family Foundation for its support in funding much of the science and policy development in the Upper San Pedro Basin in recent years. They would also like to express gratitude to the reviewers whose collective input substantially improved this paper.

### **Author Contributions**

The text of this article was written by Laurel J. Lacher, Dale S. Turner, Bruce Gungle, and Brooke M. Bushman, with contributions by Holly E. Richter. Laurel Lacher conducted background research on capture and urban-enhanced runoff, and performed the simulations of near-stream recharge. Bruce Gungle compiled and synthesized water budget data for the Upper San Pedro Partnership and provided the literature review. Dale Turner compiled and synthesized wet/dry mapping data. Brooke Bushman provided coordination of the research efforts in her role as the Upper San Pedro Basin Program Coordinator for The Nature Conservancy. Holly Richter provided content review and helped shape the presentation of our findings.

### **Conflicts of Interest**

The authors declare no conflict of interest.

### **References**

## **Development of a Shared Vision for Groundwater Management to Protect and Sustain Baseflows of the Upper San Pedro River, Arizona, USA**

### **Holly E. Richter, Bruce Gungle, Laurel J. Lacher, Dale S. Turner and Brooke M. Bushman**

**Abstract:** Groundwater pumping along portions of the binational San Pedro River has depleted aquifer storage that supports baseflow in the San Pedro River. A consortium of 23 agencies, business interests, and non-governmental organizations pooled their collective resources to develop the scientific understanding and technical tools required to optimize the management of this complex, interconnected groundwater-surface water system. A paradigm shift occurred as stakeholders first collaboratively developed, and then later applied, several key hydrologic simulation and monitoring tools. Water resources planning and management transitioned from a traditional water budget-based approach to a more strategic and spatially-explicit optimization process. After groundwater modeling results suggested that strategic near-stream recharge could reasonably sustain baseflows at or above 2003 levels until the year 2100, even in the presence of continued groundwater development, a group of collaborators worked for four years to acquire 2250 hectares of land in key locations along 34 kilometers of the river specifically for this purpose. These actions reflect an evolved common vision that considers the multiple water demands of both humans and the riparian ecosystem associated with the San Pedro River.

Reprinted from *Water*. Cite as: Richter, H.E.; Gungle, B.; Lacher, L.J.; Turner, D.S.; Bushman, B.M. Development of a Shared Vision for Groundwater Management to Protect and Sustain Baseflows of the Upper San Pedro River, Arizona, USA. *Water* **2014**, *6*, 2519-2538.

### **1. Introduction**

Many aquifers within the United States contain an essential—yet shrinking—supply of water for both people and natural systems. Groundwater resources support the irrigation of crops, drinking water supplies, and industry. Declining groundwater levels strongly affect riparian ecosystems in the semi-arid southwestern United States, where many aquifer systems are characterized by a large volume of water in storage, but a relatively small rate of natural annual recharge and discharge [1]. Because groundwater also supports natural systems such as wetlands, riparian systems, lakes, streams, and rivers, it has become increasingly difficult for water managers in this region to meet both increasing human water demands and the water needs of natural systems under persistent drought conditions [1,2]. In Arizona, perennial streamflows have significantly declined across the state—at least seven river systems could be dewatered over time, and an additional four will experience degradation if actions are not taken to reverse these trends [2]. In other words, it is increasingly difficult to manage groundwater supplies sustainably in either short or long time frames.

Widespread acceptance/adoption of "sustainable yield," which acknowledges long-term impacts of human pumping but tries to balance those impacts with environmental flow needs, represents a paradigm shift in groundwater management from the more common "safe yield" management paradigm that assumes it is acceptable for consumptive human uses of water to equal groundwater inflows. The name "safe yield" implies some level of security in terms of water availability, which by the very definition of the term is not afforded to water dependent natural systems if they are downstream of human water uses. Sustainable yield, on the other hand, more broadly addresses social, economic and environmental aspects of water availability. The methods for estimating sustainable yield, however, remain largely subjective and poorly understood by the general public, decision makers, and even water resources professionals.

This paper provides a regional case study of the Upper San Pedro River Basin of southeastern Arizona where groundwater management has focused for over a decade on the goal of sustainable groundwater yield, and proposes a generic framework for stakeholder engagement in this process, as well as lessons learned. While several questions and challenges persist, and the implementation of key strategies is ongoing, we present the tools and processes that have proven effective to date there. In particular, we offer a clear definition of sustainable use of groundwater, a conceptual framework for collaborative regional efforts to work toward attaining it along with an example of how the framework was applied in the basin, and examples of specific policies and projects that were developed to foster sustainable use there.

### **2. The Upper San Pedro Basin**

The Upper San Pedro Basin lies within the Basin and Range Province of the southwestern United States and is roughly bisected by the international boundary between Mexico and the United States (Figure 1). The basin is bounded on the east, west, and south by mountains that drain to the river near the center of the alluvial valley. The basin contains up to 520 meters of fill accumulated during the late Tertiary and early Pleistocene [3]. Runoff from the mountains recharged the basin fill over millennia, creating a vast aquifer underlying the San Pedro River. Today, dry-season flows in the San Pedro River depend almost entirely on groundwater discharge. In recent years, concern over potential pumping-related depletions of fragile surface water supplies has lent urgency to efforts to integrate the management of these two connected resources.

**Figure 1.** Map of the Upper San Pedro Basin showing the location of the San Pedro Riparian National Conservation Area managed by the U.S. Bureau of Land Management and the U.S. Army installation at Fort Huachuca within the Sierra Vista Subwatershed, just north of the United States—Mexico international boundary. From [4] (Figure 1).

Despite the fact that Arizona law generally does not recognize the hydrologic connection between groundwater and surface water, collaboration aimed at integrated groundwater-surface water management in the Upper San Pedro basin has been ongoing for decades, both within the United States and, to a lesser extent, between the United States and Mexico. The State of Arizona is in the process of delineating the "subflow zone" of river alluvium adjacent to the San Pedro River in order to protect senior surface water rights. Management, monitoring and modeling efforts focused on groundwater-surface water interactions in the Sierra Vista Subwatershed (Subwatershed) have supported vital scientific understanding of the physical basin. However, building a shared vision toward such an integrated water management approach along the binational San Pedro River is challenging for many reasons, including: differences in the political structure, economic development, cultural norms and values, water law, and language on either side of the border combined with a highly variable and complex physical system. Browning-Aiken *et al.* [5] laid out some of the processes used for collaborative watershed management of the San Pedro based on the principles of collective action theory, dispute resolution, adaptive management, power dynamics, and sustainability. The complex binational legal constraints pertinent to San Pedro water issues were also described by Browning-Aiken *et al.* [6]. This paper, however, focuses only on activities on the United States side of the border.

Within the United States, Congress created the San Pedro Riparian National Conservation Area (SPRNCA) in 1988 [7], the first Riparian National Conservation Area of its kind in the nation, and charged the U.S. Bureau of Land Management, to manage it "…in a manner that conserves, protects, and enhances the riparian area…" and other resources. This streamside riparian habitat, composed of Fremont cottonwood, Goodding willow, mesquite bosques, and sacaton floodplain grasslands, supports high levels of biodiversity and functions as a migratory bird corridor of hemispheric importance [8]. It includes approximately 64 km of the 279-km river that flows north to eventually join the Gila River, itself a tributary river to the larger Colorado River (Figure 1).

Several miles away from the SPRNCA another national asset, the U.S. Army installation at Fort Huachuca, had its own needs for groundwater to sustain its military mission associated with national security including communications testing. Fort Huachuca represents a major driver for southern Arizona's economy as the largest employer in the region and contributes approximately \$2 billion (U.S.) annually to the state's economy [9]. Located between these two federal interests, the residents of the City of Sierra Vista and Cochise County depend upon the same limited groundwater resources as the National Conservation Area and Fort Huachuca.

In terms of the legal and regulatory context, there are no state restrictions on groundwater extraction along the San Pedro River except for pumping from the zone of subflow, typically a narrow band along the river corridor corresponding to fluvially deposited alluvium. In Arizona, the legal priority of surface water rights is governed by the claim filing date: the earlier the filing date, the more senior and defensible the water right. However, a comprehensive adjudication of water rights on the Gila River system has been ongoing for decades, including federal and other water rights claims along the San Pedro, therefore, considerable uncertainty regarding the nature of surface water rights continues to exist. However, there is a clear legal distinction between surface water rights, which can be defended against more junior competing surface claims, and groundwater use, which is almost wholly unregulated in the state outside of specifically designated Active Management Areas.

Arizona law prevents placing any use limitations—or even requiring a water meter—on wells with a maximum pump capacity of 132 liters/min or less [10], even within the state's Active Management Areas. While the Upper San Pedro River Basin is outside of any state groundwater management area, Cochise County is one of only two counties in Arizona that have adopted requirements that subdivisions in the County must obtain a Designation of Water Adequacy. This program, administered by the Arizona Department of Water Resources (ADWR) requires water companies or subdivisions to show proof of a 100-year water supply before development is permitted. A total of twenty-seven privately owned local water utilities that depend upon groundwater supplies are regulated at the state level by the Arizona Corporation Commission and Arizona Department of Environmental Quality and operate in the area. In addition, three public water supply providers operate municipal water utilities.

### **3. History of Collaborative Water Management in the Basin**

A consortium named the Upper San Pedro Partnership (Partnership) was created through a Memorandum of Understanding (MOU) in 1998 in response to the Arizona Department of Water Resources Rural Watershed Initiative. This collaboration also developed, at least partially, in response to a situation where "dueling hydrologists" hired by different factions provided widely varying opinions about the fate of groundwater and the San Pedro River. The Partnership provided a vehicle for local jurisdictions to work together alongside a range of federal and state agencies, as well as with non-governmental organizations and business interests. The organization's purpose is to meet the long-term water needs of both the SPRNCA and the area's residents [11]. According to the Partnerships mission statement, this goal is to be accomplished through the identification, prioritization, and implementation of policies and projects related to groundwater conservation and (or) enhancement [12].

One of the first objectives for the Partnership was to create a collaboratively-developed regional groundwater model on which all interests could agree and then utilize it for decision making. The model, developed by the USGS, was funded through multiple federal agency budgets, with additional supporting studies funded by other some of the other Partnership members. Over the course of the five years it took to build, USGS hydrologists provided a high level of transparency about the structure of the model and the empirical data sources used to calibrate it [8]. Ultimately, this collaborative model building process served to establish a clear context and common understanding of the complexities of the hydrogeology, surface and groundwater systems, human water demands, and riparian vegetation trends and water needs. During this time, the Partnership was also recognized (in 2003) by the U.S. Congress via Public Law 108-136, (Section 321) [13], which charged the Partnership with achieving sustainable yield of the Sierra Vista Subwatershed regional aquifer by 30 September 2011.

The Section 321 legislation also required the U.S. Secretary of the Interior to deliver nine annual reports to Congress on the water management and conservation measures necessary to restore and maintain the sustainable yield of the regional aquifer by and after 30 September 2011. Future federal appropriations to the Partnership were to depend on the Partnership's ability to meet its annual goals for groundwater deficit reduction. On behalf of the Secretary and following Partnership decisions about methods and content, the reports were compiled and written by USGS staff of the Arizona Water Science Center with the assistance of other Partnership members. What the 321 legislation did not provide was a Congressional definition of the term "sustainable yield of the regional aquifer."

The Partnership chose a definition of sustainable yield based on the competing objectives view of sustainability [14]

"…managing [groundwater] in a way that can be maintained for an indefinite period of time, without causing unacceptable environmental, economic, or social consequences" [15].

This was operationalized to mean, "…a sustainable level of groundwater pumping for the Sierra Vista subwatershed could be an amount between zero and a level that arrests storage depletion, with the understanding that to call a level of use sustainable (other than zero) will entail some consequences at some point in the future" [16].

Figure 2 summarizes the progress of the 23 member agencies in their collaborative efforts to reduce the groundwater deficit through water conservation, recharge and reuse programs after the Section 321 legislation was enacted.

**Figure 2.** Estimated actual Sierra Vista subwatershed annual storage deficit and projected annual storage deficit that would have occurred had no management, conservation, or incidental yields due to human activity taken place. Incidental yields include increased recharge of runoff due to urbanization. The projected annual storage deficit is based on 2002 pumping rates and actual (not projected) population data from the State of Arizona and the U.S. Census through 2012. Modified from [17].

### **4. Development of a Shared Vision for Sustainability**

Based on the approach used along the Upper San Pedro River, we developed a generic conceptual model (Figure 3) consisting of six components for developing a shared vision for sustainable groundwater management among diverse stakeholders, and for the subsequent implementation of measures to test and refine strategies over time.

**Figure 3.** Conceptual Model for the Development of a Shared Vision of Sustainability for Integrated Water Management: The process of developing a shared vision of sustainability for regional groundwater management first requires an initial investment in *building a common understanding* of: the context of the water management challenge among stakeholders, the specific criteria for meeting environmental, social and economic needs, the theory of what needs to change to meet these criteria, and lastly, the strategies that will result in the desired outcomes. The subsequent *implementation of projects or policies* will have a better chance of providing multiple benefits and avoiding conflict when preceded by these steps.

Given the physical, economic, and social/legal/political scope and complexity of managing groundwater and surface water at the regional scale, various water managers and stakeholders typically have differing assumptions and opinions regarding management priorities, strategies, and potential outcomes. The development of a shared understanding of these multi-faceted complexities provides an essential foundation upon which to build a more collaborative approach and more robust solutions. This can be critical, especially given that the decisions of certain water managers and/or stakeholders may directly impact, either for benefit or detriment, the interests of others in terms of water availability. However, given the urgency, timing, and often political or legal sensitivities associated with some of these regional water management challenges, the initial investment in building a common understanding among various affected interests is not always made before the execution of plans, or implementation of projects or policies. Other authors have described that the "co-evolution of preferences" takes place through developing shared values and that people ultimately change their demands out of a motivation not just of helping others meet their needs, but because their perceptions and understanding of the issue have also fundamentally changed [18].

Therefore, a process that builds a common understanding of the specific criteria for meeting environmental, social, and environmental water management needs, as well as agreed upon strategies to address these criteria, is crucial to not only avoid subsequent conflict between interests but to build the most effective and robust solutions. In addition, specific desired outcomes for sustainability should be accurately defined, as well as the theory of what specifically needs to change to reach these outcomes with specific timeframes in mind. The strategies and theory of change can subsequently be tested through the collective implementation of projects and/or policies only if adequate monitoring programs are in place to do so at the appropriate spatial and temporal scales (Figure 3).

### *4.1. Develop a Clear Context*

In our experience, the physical complexity of groundwater systems alone can be tremendous, and the simultaneous consideration of social and economic factors can seem insurmountable to stakeholders working together to identify shared solutions for regional water management. One of the key lessons learned from collaboration along the San Pedro was the pivotal step of directly engaging stakeholders early in the process to participate in defining the scope of technical investigations from their own perspectives as decision makers. However, this approach is not intuitive for scientists, who have been trained to approach problems from a purely technical perspective. Decision makers need specific types of information for making high-risk policy, finance, and political decisions. Even if risks are inherent or unavoidable, the ability of scientists to quantify the probability of certain outcomes can be very useful for decision makers to choose between various alternatives. Enabling scientists to understand the specific information most needed by decision makers early in collaborative planning processes is imperative. The subsequent steps in developing a shared vision of sustainability all depend upon getting this initial step of the process right [19].

Developing social and economic criteria related to groundwater management is sometimes hard to definitively quantify or even anticipate in a qualitative sense into the future. However, in the San Pedro example, the fact that the core interests of some of the stakeholders were conceptually defined through the development of even qualitative criteria (such as "Fort Huachuca remains operational unless for reasons unrelated to water") helped to build a shared understanding and advanced discussions toward a possible solution set. One approach taken by the Partnership was to develop a decision support system (DSS) model based on the USGS groundwater model to help decision makers understand the impacts and cost-effectiveness of different combinations of water-conservation measures and management policies [20,21].

The primary technical tools used along the San Pedro River to explore the physical aspects of regional water management options and to aid in their development are discussed in detail by Lacher *et al.* [22]. While various research, data collection, and monitoring efforts were conducted from 1998 to 2014, the development of a groundwater model acceptable to all stakeholders was the overarching process that united stakeholders around a common understanding of the physical system. As stakeholders began deconstructing the complexities of the system by discussing the individual assumptions that went into that physical model, they recognized the need for improved information on which to base the model, and additional technical studies and/or predictive modeling tools were developed, such as stormwater/runoff models, evapotranspiration models, and riparian health inventories to provide better context regarding pivotal aspects of physical systems. As these types of additional studies strengthened the development of the regional groundwater model over time, it also had a secondary, but very important direct benefit for stakeholders—it improved their own common understanding of the physical system, and the eventual modeling results at the regional scale became more and more intuitive to them as well [20].

A common understanding of the legal, social and economic context of regional groundwater management issues emerged over the years from monthly Partnership meetings, multiple public opinion surveys conducted by various groups with an interest in regional water issues, and contracted studies, as well as through annual production of the "Legal Impediments" portion of the Section 321 reports to Congress. In addition, building an understanding of the relative costs for enhancing water supplies through a detailed assessment of a wide range of strategies proved essential for decision makers. The Partnership conducted a cost/yield study of 74 water management alternatives [23]. This process helped clarify the universe of all stakeholders' preferences and ideas about possible water management solutions and put all these alternatives in a common currency of relative cost and benefit. It also reinforced the concept that no one, or even several, projects could address the existing short- and longer-term water challenges. Instead, based on an increased understanding of the physical system, stakeholders came to realize that an array of long-term demand-reduction measures would be needed along with more immediate aquifer recharge and stream flow protection measures.

### *4.2. Define Specific Criteria for Meeting Environmental, Social and Economic Needs*

For the cost/yield study of water management activities, the Partnership defined seven environmental criteria for sustainability, including two groundwater, three surface water, and two ecological criteria, through a facilitated consensus-driven process (Table 1).

**Table 1.** The suite of criteria developed by the Upper San Pedro Partnership for sustainable yield.

### 4.2.1. Environmental Criteria

In general, some of the defining environmental criteria commonly associated with sustainable yield include: (1) avoid excessive depletion of surface water and excessive reduction of groundwater discharge to springs, rivers, wetlands, and riparian vegetation (defined as capture); (2) prevent the intrusion of contaminated water to the groundwater system during induced recharge; and (3) avoid irreparable impact to any groundwater-dependent ecosystems, and prevent land subsidence from groundwater withdrawals [24].

Along the San Pedro River, the groundwater-dependent riparian habitat composed of native Fremont cottonwood and Goodding willow forest would experience increased mortality and declining recruitment and give way to invasive, non-native tamarisk if groundwater depths were to fall and persist beyond about 3 m below land surface within the riparian area [25,26]. Loss of surface flow to capture would likewise also result in the loss of wetland herbaceous plants such as rushes, sedges, and bulrush, dependent on continuously moist soils [26]. As the number and length of reaches with perennial surface flow decrease, the number and diversity of aquatic species would decrease as well [27].

Since the environmental consequences of falling groundwater elevations in near-stream locations would include the degradation of the current riparian and aquatic habitats along the San Pedro River [26] affecting species dependent on those habitats as well, *maintenance of groundwater elevations* was clearly a key criterion. However, in consideration of longer term time scales and larger spatial scales, the increase in *storage of the surrounding regional aquifer* was also considered a meaningful criterion for inclusion as well, given its connection and influence on the near-stream alluvial aquifer. *Surface water availability, water quality, and riparian health considerations* were also included as key criteria for environmental sustainability.

### 4.2.2. Social Criteria and Consequences

Typically, when the social consequences of sustainable groundwater development are discussed, it is in reference to a physical shortage of available and (or) uncontaminated groundwater supply for human use. In general, access to good quality potable groundwater supplies should be equally available to all residents; down-gradient users should have a water right equal to up-gradient users; and groundwater pumping should not damage the existing water rights to spring and surface waters [24].

For the San Pedro, not only was the physical *availability of water to meet human demands* one of the social criteria identified by the Upper San Pedro Partnership, but for them, the ability of the *local communities to influence and control their own destiny in water management decisions* was also a clear priority. The eventual establishment of a regional network of sites owned and operated by County and/or municipal governments for managed aquifer recharge purposes clearly met those criteria.

The recharge of treated wastewater effluent to sustain groundwater is a human health concern expressed by some along the San Pedro River, and *sustaining water quality* was identified as one of the social criteria for overall sustainability. Since 2003, the City of Sierra Vista has recharged approximately 3.1 million cubic meters per year (MCM/yr) of its treated wastewater with the aim of mitigating the effects of long-term groundwater pumping in the Sierra Vista Subwatershed. Water quality monitoring of spring discharge has been conducted near this recharge site and it was found that no constituent concentrations had exceeded any federal standards as of 2009 [28].

### 4.2.3. Economic Criteria and Consequences

In the United States desert southwest, most water users expect groundwater development to fulfill the water demands for agricultural irrigation, industrial uses, and residential development while maintaining an economically feasible depth to water with regard to pumping and well construction costs [29,30]. For the San Pedro, Fort Huachuca's water use is constrained by federal law, specifically the Endangered Species Act (ESA). Two federally listed endangered species, the Huachuca water umbel (*Lilaeopsis schaffneriana* var*. recurva*), a small semi-aquatic vascular plant that grows in moist soils along the San Pedro River, and the southwestern willow flycatcher (*Empidonax traillii extimus*), a songbird generally associated with permanent water, rely on the surface flow and riparian system of the San Pedro River corridor for habitat. Fort Huachuca and the U.S. Fish and Wildlife Service completed a Biological Opinion in March 2014 addressing these issues for the next 10 years [31]. From the perspective of sustainable groundwater yield, then, the use of groundwater for economic development (*i.e.*, to support the Fort's mission) is constrained by endangered species such as the Huachuca water umbel and southwest willow flycatcher, as reflected in the ESA.

This tension between regulatory mechanisms and economic drivers reverses the normal relationship between water use and economic need. In this case, Fort Huachuca must minimize its water use in order not to cause unacceptable adverse economic impacts to the Subwatershed. Therefore the set of environmental criteria—specifically hydrological—as listed in Table 1 are also direct measures of the reduction of social and economic risks. Thus, the issue of balancing sustainable groundwater use in the Sierra Vista Subwatershed revolves around the environmental needs of the San Pedro Riparian National Conservation Area's aquatic and riparian communities, which are inextricably linked to local economies and the federal military installation that acts as an economic engine across all of southern Arizona.

### *4.3. Define What Specifically Needs to Change through Strategies and Desired Outcomes*

It became clear to San Pedro decision-makers and stakeholders through the process of developing the numerical groundwater model with the USGS, development of a spatially explicit Decision Support System (DSS) model based in the USGS groundwater model, and later working with consulting hydrologists who ran various simulations [22,32,33], that balancing human demands with flows in the river required not only the quantification of current withdrawal rates, but the management of impacts expressing themselves today due to water uses of the past century. In addition, predictive simulations of anticipated changes in groundwater trends over the coming century was essential to inform the decisions we make about current groundwater management. This was a much more complex and multi-dimensional view of the problem and its possible solutions, both spatially and temporally, than simply attempting to balance the current year's groundwater budget deficit. And yet, understanding these complex relationships actually clarified and simplified the necessary strategies and outcomes by setting more realistic expectations about what could be realized in the short-term, as opposed to longer timeframes. For example, a balanced groundwater budget within the Subwatershed might not be accomplishable within the time frames of years to decades, nor would it necessarily ensure that flows would be protected. However, over longer timeframes of centuries a balanced budget will be essential, and there are actions we can begin today that will contribute toward these longer term goals.

Once the regional groundwater model was developed, specialized applications of it were also possible, including development of a regional groundwater capture map (Figure 4) to provide a comprehensive spatial view of pumping and recharge impacts or benefits at any location in the subwatershed [34]. This more-intuitive representation of the system's physical dynamics resonated with decision makers and the public alike, and began to clearly highlight that near-stream locations had higher importance than locations closer to the regional cone of depression in terms of anticipated depletions of the river from pumping.

This understanding helped stakeholders move toward near-stream solutions that could most effectively benefit flows. While not eliminating the need to balance the overall groundwater budget throughout the subwatershed over longer time frames, strategies to sustain and enhance river flows in the short-term needed to center around near-stream locations to have the most impact. Once partners began to focus on the concept of an optimized suite of sites for both aquifer protection and recharge along the river corridor, the model was used to assess sites that could protect the most vulnerable sections of the river. This information was used to identify specific parcels of land that were feasible for acquisition. Some of those were later acquired, and subsequent groundwater modeling efforts used higher-resolution, local-area models to assess specific site and reach recharge characteristics and to simulate more specific recharge scenarios [22].

### *4.4. Implement Specific Projects and Policies*

Member agencies of the Partnership have been implementing a wide array of projects and policies targeted at their collective goal of "the identification, prioritization, and implementation of policies and projects related to groundwater conservation and (or) enhancement" for approximately 15 years. The establishment of a dedicated fiscal agent (the City of Sierra Vista) and ongoing collaborative budget approval processes gave partners an effective way to pool resources and apply funding swiftly to key science and monitoring needs as they developed. These projects included water conservation outreach programs, residential water audits, water fixture rebate programs, construction of stormwater detention basins, and effluent reuse and recharge facilities.

However, as more predictive model simulations were run, an increased focus developed on projects that had the most immediate benefits for flows in the river: those that increased near-stream aquifer recharge. This included land acquisition and conservation easements specifically aimed at the permanent retirement of high-volume pumping over the last two decades. However, given a better understanding of the temporal and spatial dynamics of the groundwater system, Cochise County, The Nature Conservancy and Fort Huachuca recently added a new strategy to near-stream groundwater protection efforts. The addition of multiple aquifer recharge locations became a priority to complement the existing City of Sierra Vista effluent recharge facility that went into operation in 2002. The Nature Conservancy identified available land in areas believed to be the most productive for near-stream recharge, based on the groundwater capture map (Figure 4), wet-dry mapping of surface flows (described below) and other tools [22]. Thanks to funding for land acquisition made available from the U.S. Department of Defense Army Compatible Use Buffer Program, and in combination with drastically-reduced property values due to depressed market conditions in the past several years, the opportunity to acquire land previously slated for development arose at near-stream locations. Between 2011 and 2014, The Nature Conservancy purchased 2056 hectares and Cochise County purchased another 194 hectares of hydrologically sensitive land (Figures 4–6). This network of four properties, totaling over 2250 hectares, and spread along 34 km of the river, far exceeds the amount of land originally envisioned as attainable for managed aquifer recharge purposes.

**Figure 4.** Groundwater capture mapping shows where managed aquifer recharge offers the greatest benefits for the riparian system within a 50-year timeframe. Dots indicate existing recharge projects. Historically most were constructed as detention basins for downstream flood control with secondary recharge benefits to the larger regional aquifer (over the warmer colors), and more recently to more directly benefit flows in near-stream locations (over the cooler colors). Outlined and numbered near-stream recharge sites are locations where aquifer recharge projects are currently under construction or being investigated as future project locations. Redrawn from [34].

**Figure 5.** One of a series of in-channel infiltration basins recently constructed at Recharge Site #1 near the San Pedro River where on-site monitoring (e.g., soil-moisture probes, pressure transducers) will be used to quantify the relative performance of the individual structures, within this constructed channel that receives surface run-off from upstream residential areas.

**Figure 6.** The in-channel basins under construction include infiltration trenches and drywells at Recharge Site #1, within the constructed channel. The channel is perpendicular to the river, and the river's riparian corridor is visible in the background.

After the acquisition of this recharge network, the collaborating partners are conducting site assessments that include hydrogeologic sampling, more-detailed stormwater modeling simulations, and potential source water locations. Ongoing planning by the County and local municipalities will now have additional options for managing both stormwater and effluent, at the places with the most regional benefit for river flows.

### *4.5. Monitor Progress toward Desired Outcomes*

The member agencies of the Upper San Pedro Partnership remain committed to securing continued funding for a broad suite of monitoring activities to evaluate the response of the regional groundwater system to their ongoing project and policy development. The USGS and USDA Agricultural Research Service monitor regional and alluvial aquifer water levels, main-stem, tributary, and low-flow mountain stream gaging, storage change monitoring using micro-gravity methods, and streamflow permanence. The Nature Conservancy, in cooperation with the U.S. Bureau of Land Management, also leads an annual monitoring effort using GPS mapping of surface flows, a technique called wet/dry mapping, to determine the absence or presence of surface flows during the driest time of year (mid June in the San Pedro Basin). This 16-year dataset is used to track year-to-year variability of the length of surface flows, and used to infer changes in alluvial groundwater conditions that may provide early warning of ecological changes [22]. The USGS also continues to monitor water quality at the Charleston gaging station on the San Pedro River as part of the National Water-Quality Assessment program.

The Partnership has recently committed significant funding to a comprehensive, multiple-year analysis of progress toward sustainable groundwater use in the Subwatershed. While that analysis is not yet complete, their previous annual reports to Congress included a suite of eight indicators to measure progress toward sustainable yield, as shown in Table 2. It is important to note how strongly this suite of indicators aligns with the previously defined environmental criteria for sustainability described in Table 1.

**Table 2.** A suite of eight indicators was used to describe progress toward sustainable yield in the Section 321 reports to Congress that were prepared by the USGS (e.g., [17]). They relate directly to the environmental criteria for sustainability developed by the Partnership.

### **5. Results and Conclusions**

Based on the San Pedro experience, approaches such as the Partnership that directly engage affected policy makers, stakeholder organizations, regulatory agencies, and the scientific community can more effectively implement the necessary projects or policies, than if the partners were addressing the same challenge as individual interests. The involved partners more deeply understand the need for management measures, but are engaged in the actual exploration and development of possible alternatives, and witness the results and progress toward specific desired outcomes through adaptive management over time [35]. This was certainly the case for the Partnership as they first quantified the annual yield from a wide array of member agency water conservation, reuse, and recharge projects, then implemented dozens of them since 1998 [6] (Figure 2).

However, the Upper San Pedro Basin is unique in many ways. The presence of an important federal military installation and a federally protected riparian corridor within the Sierra Vista Subwatershed have brought a level of interest and involvement absent from many other basins with similar hydrogeologic conditions. The federal nexus in water issues has also resulted in significant funding to assess groundwater pumping impacts and to help mitigate those effects. Without Congressional funding for the Partnership and much of the federally-sponsored scientific research that supported development of the groundwater model, the state of the science would likely not have advanced to its current level.

Despite these unique socio-political aspects, the San Pedro Basin represents one of the best examples of riparian corridors remaining in the desert Southwest. The Gila River that drains more than 60% of the state no longer has any undammed perennially flowing segments, and is dry over most of its length. Many of the state's once-flowing, now-dry rivers reflect the impacts of long-term groundwater pumping in the mid to late 20th century. They provide a stark reminder of how directly connected groundwater and surface water resources are for our desert rivers.

### *5.1. Lessons Learned*

### **Lesson 1:** *Engage decision makers and key stakeholders early in the process to define the science and technical tools needed for an integrated water management approach.*

These needs should be tailored explicitly to the existing conceptual models of key stakeholders, and the gaps in understanding, disagreements and/or misperceptions that they hold. This approach strengthens the foundation for shaping meaningful criteria for success, the formation of effective strategies, and the definition of meaningful desired outcomes. The Upper San Pedro provides an example of a stakeholder-driven process where project implementation was driven by an evolving science-based understanding of the system, and additional financial resources and political support were generated over time in response to an enhanced understanding and appreciation of the challenges and opportunities. As stakeholders progressively learned more about the system, they were also in a better position to make the case for generating additional public and private funding to support their efforts.

### **Lesson 2:** *Collaboratively define desired outcomes as specifically as possible both temporally and spatially.*

The process of defining "sustainable yield" for the San Pedro is still underway more than 11 years after Congress mandated its implementation in the Subwatershed. By some measures, such as per-capita water consumption rates and managed aquifer recharge, efforts to mitigate the effects of groundwater pumping have been very successful. However, developing the predictive models to more specifically understand the response of the physical system allowed decision makers to recognize that, while their previous efforts would aid in slowing overall aquifer storage depletion, they would not necessarily protect the river from pumping-induced capture in shorter time frames (years to decades). Later efforts to initiate near-stream recharge arose from a better understanding of both the spatial and temporal aspects of the system, and strengthened the recognition that both short-term and long-term actions and effects were important.

**Lesson 3:** *Stakeholders with varied interests are more likely to work successfully toward a common goal if they feel that their individual interests are represented, and can actually benefit from the process.*

Challenging economic and legal contexts should not prevent diverse parties from working toward a solution if they perceive that their interests are represented in, and perhaps even benefit from a shared vision with other interests. Even though some objectives may seem to be competing (e.g., preserving reasonable depths to groundwater for water supply wells AND preserving baseflows in the river), finding a common thread among the parties—such as preservation of a vital economic driver for the region—can lead diverse parties to define and accept a mutually beneficial outcome. Once stakeholders recognize what outcomes of a solution might look like (such as baseflow supported by near-stream recharge), they may better reach consensus about how to achieve that proposed solution. For the San Pedro, the acknowledgement of all three aspects of sustainability—economic, social and environmental—helped to build trust, agreement and eventually support among interests. In addition, it opened conversations to the consideration of more specific objectives aimed at both the short- and long-term. The parties acknowledged that preserving flow in the river was the most immediate short-term concern, while also recognizing the need for longer-term efforts to maintain supplies at municipal pumping centers.

**Lesson 4:** *The importance of effective communication and two-way learning between scientists and decision makers cannot be overstated.*

While scientists and subject experts may recognize specific physical trends and processes in respect to hydrologic systems, other stakeholders may not agree on the nature or even the existence of them. Conversely, water managers and decision makers function within an operating environment that includes many dynamic political, financial, and legal factors that are not clear to scientists. Developing a shared understanding of these challenges as they relate to key water management decisions may take years. How do we help decision makers with little or no technical knowledge of complex groundwater hydrology understand that the pumping of half a century ago will manifest as declines in baseflow over the next half century? Even more problematic is trying to convince them to invest in expensive solutions to a crisis that—if the solution works—will never materialize. Accepting these hydrologic "mysteries" that are taking place in an invisible underground system they will never see requires a considerable leap of faith.

The burden lies with both the scientific community and decision makers to invest the required time and effort communicating and learning about the environmental, social, and economic aspects of regional water management to be able to develop meaningful collaborative strategies together. The development of a set of specific criteria for meeting environmental, social, and economic needs as part of a shared vision of sustainable groundwater management is an essential first step toward the development of that understanding.

### **Acknowledgments**

The authors would like to recognize the support of the Upper San Pedro Partnership, and its respective 23 member agencies, over the past 15 years in developing the science and fostering the collaboration required for the progress that has been made toward groundwater sustainability in the Upper San Pedro Basin. The USGS has also made significant contributions toward not only the development of the groundwater model in the basin, but also toward the conceptual understanding of sustainable yield on a broader basis. Lacher Hydrological Consulting has played a pivotal role in running various groundwater model simulations. The Department of Defense Army Compatible Use Buffer (ACUB) Program provided significant funding for the acquisition of land for aquifer recharge sites, and the Walton Family Foundation generously provided private funding for the subsequent design and engineering of initial aquifer recharge facilities. The authors thank David Goodrich and two anonymous reviewers for substantially improving this article.

### **Author Contributions**

The text of this article was written by Holly E. Richter, Bruce Gungle, Laurel J. Lacher, Dale S. Turner and Brooke M. Bushman. Holly E. Richter and Bruce Gungle wrote the bulk of the history of the partnership and development of the shared vision content. Laurel Lacher helped formulate the basic concept of the paper, provided input on the concept of sustainable groundwater use, and assisted with organizing the paper's structure. Dale S. Turner conducted background research on the technical tools, helped hone the sustainable yield concepts, provided content review, and also served as our primary internal editor. Brooke Bushman added current project implementation content, developed several figures and maps, and also provided content review.

### **Conflicts of Interest**

The authors declare no conflict of interest.

### **References**

## **The Role of Transdisciplinary Approach and Community Participation in Village Scale Groundwater Management: Insights from Gujarat and Rajasthan, India**

**Basant Maheshwari, Maria Varua, John Ward, Roger Packham, Pennan Chinnasamy, Yogita Dashora, Seema Dave, Prahlad Soni, Peter Dillon, Ramesh Purohit, Hakimuddin, Tushaar Shah, Sachin Oza, Pradeep Singh, Sanmugam Prathapar, Ashish Patel, Yogesh Jadeja, Brijen Thaker, Rai Kookana, Harsharn Grewal, Kamal Yadav, Hemant Mittal, Michael Chew and Pratap Rao**

**Abstract:** Sustainable use of groundwater is becoming critical in India and requires effective participation from local communities along with technical, social, economic, policy and political inputs. Access to groundwater for farming communities is also an emotional and complex issue as their livelihood and survival depends on it. In this article, we report on transdisciplinary approaches to understanding the issues, challenges and options for improving sustainability of groundwater use in States of Gujarat and Rajasthan, India. In this project, called Managed Aquifer Recharge through Village level Intervention (MARVI), the research is focused on developing a suitable participatory approach and methodology with associated tools that will assist in improving supply and demand management of groundwater. The study was conducted in the Meghraj watershed in Aravalli district, Gujarat, and the Dharta watershed in Udaipur district, Rajasthan, India. The study involved the collection of hydrologic, agronomic and socio-economic data and engagement of local village and school communities through their role in groundwater monitoring, field trials, photovoice activities and education campaigns. The study revealed that availability of relevant and reliable data related to the various aspects of groundwater and developing trust and support between local communities, NGOs and government agencies are the key to moving towards a dialogue to decide on what to do to achieve sustainable use of groundwater. The analysis of long-term water table data indicated considerable fluctuation in groundwater levels from year to year or a net lowering of the water table, but the levels tend to recover during wet years. This provides hope that by improving management of recharge structures and groundwater pumping, we can assist in stabilizing the local water table. Our interventions through *Bhujal Jankaar*s (BJs), (a Hindi word meaning "groundwater informed" volunteers), schools, photovoice workshops and newsletters have resulted in dialogue within the communities about the seriousness of the groundwater issue and ways to explore options for situation improvement. The BJs are now trained to understand how local recharge and discharge patterns are influenced by local rainfall patterns and pumping patterns and they are now becoming local champions of groundwater and an important link between farmers and project team. This study has further strengthened the belief that traditional research approaches to improve the groundwater situation are unlikely to be suitable for complex groundwater issues in the study areas. The experience from the study indicates that a transdisciplinary approach is likely to be more effective in enabling farmers, other village community members and NGOs to work together with researchers and government agencies to understand the groundwater situation and design

interventions that are holistic and have wider ownership. Also, such an approach is expected to deliver longer-term sustainability of groundwater at a regional level.

Reprinted from *Water*. Cite as: Maheshwari, B.; Varua, M.; Ward, J.; Packham, R.; Chinnasamy, P.; Dashora, Y.; Dave, S.; Soni, P.; Dillon, P.; Purohit, R.; Hakimuddin; Shah, T.; Oza, S.; Singh, P.; Prathapar, S.; Patel, A.; Jadeja, Y.; Thaker, B.; Kookana, R.; Grewal, H.; Yadav, K.; Mittal, H.; Chew, M.; Rao, P. The Role of Transdisciplinary Approach and Community Participation in Village Scale Groundwater Management: Insights from Gujarat and Rajasthan, India. *Water* **2014**, *6*, 3386-3408.

### **1. Introduction**

India is the largest user of groundwater in the world with an estimated usage of 230 km3 per year [1]. Globally, areas under groundwater irrigation are the highest in India (39 million ha), followed by China (19 million ha) and the USA (17 million ha), and at present 204 km3 y<sup>í</sup><sup>1</sup> of groundwater is pumped annually in India [2]. Several reasons may be attributed to this phenomenon. Access to groundwater increased since the 1970s, when diesel and electric pumps became affordable to most small landholders. The causes of increased groundwater use are also rooted in population growth and economic expansion, and as result the annual groundwater use now probably exceeds the annual rainfall recharge. The notion of groundwater as a private resource, the rights of which are associated with land rights, has led to an exploitative extraction regime [3].

Farmers in semi-arid parts of India use groundwater to save rainfed crops from failure and to increase yields. As it is a relatively cheap and easily accessible water resource for individual farmers, irrespective of their farm size, groundwater is often extracted beyond its natural recharging capacity. With increased use of groundwater, the depth to the water table in many parts are fluctuating considerably during the year and the use of groundwater has risen to a level that groundwater from shallow aquifers is not adequate to meet the rising demand. Hence, groundwater from deeper aquifers is being pumped by the drilling of tube wells. There are also instances where fresh groundwater at shallow depths has been depleted, rendering marginal quality water from deeper layers of the aquifer [4]. The extensive use of groundwater resources by farmers all over the country pumping out water in an unregulated manner creates its own sets of complex management and sustainability issues.

The use of groundwater in agriculture is important in India, as it has enabled farmers to manage deficiencies in monsoonal rainfall, allowed dry-season irrigation, thus contributing to poverty alleviation. For this reason, a range of on-ground works to recharge groundwater are being implemented at the village scale throughout India as a part of the Government of India's "Mahatma Gandhi National Rural Employment Guarantee Act" (MNREGA) to enhance livelihood opportunities while developing a durable asset base. A significant part of the investment through MNREGA is for enhancing long-term, local water security by on-ground structures such as check dams, percolation tanks, surface spreading basins, pits and recharge shafts [5]. The development of on-ground structures to enhance groundwater recharge in India is called "watershed development". It is a long running program of Government of India and has significant hydrologic consequences, in particular, altering the runoff regime in downstream regions and groundwater recharge at local and regional scales.

In spite of all the efforts in the past to improve the sustainability groundwater in India, the problem of groundwater management is still severe, particularly in Rajasthan and Gujarat. In this project, called **M**anaged **A**quifer **R**echarge through **V**illage level **I**ntervention (**MARVI**), the research is focused on developing a suitable participatory approach and methodology with associated tools that will assist in improving supply and demand management of groundwater. Another important aspect of the project is education of and engagement with village communities, local NGO and government agencies to facilitate them working together to achieve sustainable groundwater management.

In this article, we report on some key findings from the MARVI project with two main objectives: (i) to show how basic hydrologic information collected by farmers and supplemented with hydrologic, agronomic and socio-economic data collected by the project team is leading to an assessment and understanding of the groundwater storage changes; and (ii) to reveal how this information and engagement activities can be used to empower village communities and other stakeholders to develop and assess their own viable options for groundwater management, including managed aquifer recharge and measures to reduce water demand while sustaining livelihoods.

### **2. The Study Watersheds**

The work reported here was conducted in the Meghraj watershed in Aravalli district, Gujarat, and the Dharta watershed in Udaipur district, Rajasthan, India (Figure 1). Both watersheds have a semi-arid climate, with the average annual rainfall in excess of 600 mm, but more than 90% of this rainfall is received during the monsoon months of June to September. Most farmers in the two watersheds grow maize, black gram, mungbean, guar, soybeans (recently introduced) and vegetables as *Kharif* crops during the rainy season. Wheat, gram and mustard are the main *Rabi* crops grown during the winter season. Farmers who have access to groundwater (and in some instances canal water) grow two crops a year and those who have access to water supplies throughout the year also grow some summer crops such as vegetables and fodder.

**Figure 1.** The Meghraj and Dharta watersheds. The inset map shows the location of the watersheds in the states of Gujarat and Rajasthan in India.

The occurrence and distribution of rainfall in both the Meghraj and Dharta watersheds are highly uneven in both time and space. *Kharif* crops are mainly dependent on the vagaries of the monsoon and are often at risk of either complete or partial crop failure due to inadequate rainfall, or rainfall not occurring at a critical stage of crop growth. Therefore, the uneven and erratic distribution of rainfall provides a major challenge to growing crops successfully and to sustaining a decent livelihood. When rainfall does not occur at the right time or in the required amount, some supplementary irrigation, also called "life saving irrigation", using rainwater stored on the surface or drawn from the underground aquifer systems can make a huge difference in avoiding crop failure.

A number of *in situ* conservation measures, including farm ponds, percolation ponds and check dams have been constructed in the two watersheds under both the Integrated Watershed Management (IWM) programs and MNREGA. The State Governments of Gujarat and Rajasthan, along with the Central Government have invested significant amounts in these two watersheds in the past, and continue to do so by constructing more of these structures. However, it is not clear how effective these programs are, and what impacts these investments are having on groundwater security. Figures 2 and 3 show the MAR structures in the Meghraj and Dharta watersheds.

**Figure 2.** Location of MARs in Meghraj.

**Figure 3.** Location of MARs in the Dharta watershed.

It is important to note that both watersheds are in hard rock aquifer areas. It well known that hard rock aquifers have low porosity and low connectivity and the movement of groundwater occurs through faults, fissures and fractures. Hence they store limited volumes, and when stored water is withdrawn by pumps, the emptied pores are not immediately filled by flows from adjacent areas. As result of low rain-recharge, and low porosity and low connectivity, the depth to water table fluctuates considerably during the year and significant water scarcity is often experienced during summer months or drier years.

Most farmers in the Meghraj watershed belong to a tribal community, while those in the Dharta watershed are from mainstream groups. The farming practices in the two watersheds have not advanced adequately to cope with declining water supplies. For this reason, the physical and socioeconomic conditions in the two watersheds provide a diversity of transdisciplinary research opportunities and engagement issues around groundwater recharge and management.

### **3. Study Approach**

The study approach in the MARVI project is underpinned by transdisciplinary research with a main focus at the "village scale" to understand the complex interrelations between rainfall, aquifer recharge, groundwater pumping and livelihood opportunities. We define transdisciplinary research as one in which both researchers from different unrelated disciplines and non-academic participants, such as farmers and other villagers, work together for a common goal and create new knowledge and theory to improve a complex situation. Thus, in this project we recognized the importance of involving local villagers and other stakeholders through this approach during the research process and engaged them in participatory groundwater monitoring and education to explore options for groundwater sustainability. Figure 4 illustrates the application of relevant social and natural sciences research and engagement to improve the field situation.

**Figure 4.** Study approach in MARVI project.

With an active engagement of local villagers, the project team collected a range of hydrologic, agronomic, economic, social and cultural data at selected clusters of villages over a two-year period. The engagement of villagers and data collected are then employed to understand the current situation and develop bio-physical and socio-economic insights to evaluate the current issues, identify options and strategies, provides a scientific and evidence-based input to enhance watershed development policies.

### **4. Field Research and Data Analysis**

### *4.1. Participatory Groundwater Monitoring*

A desired outcome of the MARVI project would be collective action at village level that is mutually beneficial to all the villagers, and from which other communities could learn. To achieve this, [6] have shown the need to develop Social Capital. This project used participatory approaches to help to develop social capital competences, with training programs aimed at supporting cognitive aspects of this social capital competence. In addition, the project used participatory monitoring for some data collection to also support this development.