**Combination of Ecological Engineering Procedures Applied to Morphological Stabilization of Estuarine Banks after Dredging**

**Luís Filipe Sanches Fernandes 1,\* , António Augusto Sampaio Pinto 2 , Daniela Patrícia Salgado Terêncio 1 , Fernando António Leal Pacheco 3 and Rui Manuel Vitor Cortes 4**

Received: 27 November 2019; Accepted: 28 January 2020; Published: 1 February 2020

**Abstract:** Gravel extraction and upstream damming caused profound effects on the estuary of the Lima river (NW Portugal) which was reflected by the collapse of banks, leading further to the destruction of riparian vegetation. This led to consequences such as a progressive negative impact on the preservation of salt marshes over several decades of this protected area, which continued even after the cessation of extraction activities. In this work, we present a restoration project combining civil engineering with soft soil engineering procedures and revegetation, along with two distinct segments, and follow the recovery process. The main intention of the study is to promote hydraulic roughness in order to dissipate energy from peak flows and tides, increasing accretion and indirectly the stimulation of plant succession and salt marsh recovery. We are able to observe that the built structures (an interconnected system of groynes, deflectors and rip-rap/gabion mattress) allowed the erosion process to be detained. However, they did not allow as much sediment as expected to be trapped. The colonization of species (plants) in brackish and tidal water was a difficulty posed by this project. A more extensive restoration of all estuarine areas and river mouths, namely to overcome the sediment deficit, will require proper land-use management at the catchment scale instead of local actions.

**Keywords:** riverbank erosion; restoration; bank stabilisation; vegetation revetment

### **1. Introduction**

Coastal salt marshes are ecosystems of great ecological and economic value since they provide habitats and breeding areas for many animal species. They play a crucial role in the food chain, in the quality control of the environment and in the sedimentation dynamics in estuarine systems [1,2]. However, around the world, erosive processes related to dredging activity for navigation or gravel extraction represent essential factors for the loss of salt marshes through erosion under the living root

zones caused by flowing drainage water. This leads to the overhang of marsh vegetation growing over the banks [3,4]. In addition to the positive benefits of accreting sediment, vegetated marshes effectively dissipate wave energy [5], decreasing the impact of turbulence, which can be crucial in entraining sediment, as shown in [6,7]. The eroding process resulting from the degradation of these ecosystems was described by Castillo's group [8] in other marshes of the Iberian Peninsula, leading to the formation of vertical slopes (usually concave in their lower part), the appearance of mass-movement phenomena and the detachment of blocks of the substrate. The horizontal erosion of these slopes typically begins with the undermining of the lower part, just below the zone of live roots. This is followed by the detachment of substrate blocks from the upper part of the slope, detaching the plants from their roots. Of course, navigational conditions in this estuary (mainly for tourism and fisheries) induce another disruptive factor in acceleration erosion because of wave energy [8,9], but also because channel incisions close to the banks are also observed in order to overcome unfavorable depths.

Ecological engineering has been increasingly used in order to stabilize river banks [10–16], but this is still very uncommon in estuarine areas in Portugal. Some other studies supported the use of hydrodynamic models to predict bank stability under flow conditions [17–20]; in addition, these are essential for the identification of adequate vegetation to improve restoration processes [21–24]. In Portugal, there has also been increasing attention paid to soil engineering techniques for the control of fluvial erosion and for the settlement of riparian galleries in physically disturbed streams [25–28].

The Lima Estuary is subject to many detrimental human impacts. It is the recipient of point pollution originating from the town of Viana do Castelo, an important pulp-mill factory and non-point pollution from agriculture. An input of persistent organic pollutants to the estuarine water and nutrients has led to eutrophication processes in the lower part of the estuary [29]. We also call for attention to be paid to the disturbance by boat navigation and the inherent introduction of fuel and paint residuals into the estuarine system. In spite of the multiple human impacts, the main purpose of this study is to stabilize the river banks along the estuary of the Lima river, as a result of decades of unregulated gravel extraction, causing profound effects on the bank morphology and the destruction of riparian vegetation, either due to tidal action or situations of peak flow. Over the past three decades, there has been a progressive bank cutting leading to substantial marsh losses, also affecting recreational activities. Moreover, the present work shows the implementation of a restoration project aimed at the natural reposition of salt marshes. This is an innovative procedure that has not been used before in Portugal and attempts to provide the necessary conditions that may drive the restoration of the lost wetlands (and the protection of the existing ones) by reverting and supporting river banks that surround previous salt marsh areas, which were washed away after the collapse of the protecting banks. The basis of this design was a combination of civil engineering with soil engineering procedures that not only secure river banks against further erosion but may also increase the patterns of sediment deposition. The biophysical recovery and protection processes were carried out along two distinct segments in the right bank (the nearby Cardielos and Portuzelo villages), both in the estuarine zone of the Lima river. We must emphasize that the downstream part of this river, including the estuary, is included in a protected area of Nature 2000, associated with the preservation of wetlands and riparian layers. Therefore, this work is crucial to defining the procedures for more extensive action. We hypothesize that, by increasing the hydraulic roughness along the banks, we may increase the sedimentation rate along the banks and aid the recolonization process.

### **2. Materials and Methods**

### *2.1. Study Area*

The Lima river catchment is shared between Portugal and Spain, and the run-off average flowing into the estuary is 3298 hm3 , whereas 1598 hm3 corresponds to the Portuguese part (which includes a near 35 km length, with an average slope of 0.1%). The downstream part of this river represents a transition between a narrow and steep valley towards a progressive gentle slope (0.024%) along with a shallow-vee valley form and finally a large floodplain. The average annual precipitation in this hydrographic basin is high (1444 mm) but averages 2745 mm in some sub-basins. It has a very humid climate and is a hydrographic basin with an excess of water availability throughout the year, with water shortages in the summer months [30,31]. These conditions favor the occurrence of frequent floods in the downstream areas of the main catchment. Impacts on water quality are relatively low since we observe a dominant land use of forest stands (eucalyptus and pine trees) and, in the lower parts of the valley, extensive agriculture characterized by small patches of vineyards, orchards and grasslands with cattle breeding. The areas prioritized for the rehabilitation projects of Cardielos and Portuzelo, both on the right bank, were a consequence of the demands of the municipalities due to the loss of recreational grounds and the increasing pressure on multiple infrastructures (marginal roads, sports and leisure equipment, etc.), but also to protect a layer of marshes in the neighborhood.

### *2.2. Disturbance Factors*

The intense dredging related to gravel extraction over nearly three decades in the lower segment of the Lima river has led to a complete change in the morphological character of the river mouth, with the main current flow and thalweg being relocated towards the right margin derived from the intense withdrawal of sediments. Moreover, the sedimentary supply dramatically decreased after 1992 due to river regulation (two tail-race dams were built upstream, Alto Lindoso and Touvedo; the former is the second-most important hydropower system in Portugal, with a dam of 110 m in height and a reservoir capacity with 347.8 hm3 , with a maximum area of 1072 ha, whereas the Touvedo dam, 7 km below Alto Lindoso, has a height of 43 m, and the reservoir covers 172 ha. Bank instability is the direct consequence of the deepening of the river channel and the exceedance of the critical height of the river banks, which led to its subsequent collapse. We can see in Figure 1 that the comparison of the studied area (upper part of the estuary) between 1965 and 2010 shows the intense sediment loss in this period and the transformation of a braided channel into a progressive linearization of the river banks, resulting in a river with a significantly higher stream power. Damming is known to affect the entire downstream segments by trapping sediments and reducing the sediment transport capacity because of the strong reduction of peak flows. The downstream geomorphic and ecological effects of dams are largely determined by the relative changes in the sediment transport regime, with consequences on channel incision and bank instability [32–34].

The detailed studies conducted by INAG (Portuguese water institute) [31] concerning the decision of whether or not to authorize the extraction activities allowed intense ecological impacts to be conclusively determined, demonstrating that the lowering of the estuary bed exceeded 7 m in depth in some points. Such studies also demonstrated that gravel mining, which ceased effectively in 1992, was environmentally unsustainable, and no further authorizations were processed, except for maintaining navigation conditions in the harbor. However, the morphological adjustments necessarily continued dramatically, which can also be explained by the fact that all 19 identified exploitation extractions exceeded the legal limits, driving estimated total values reaching 600,000 m3 /year [31]. In addition, the constant pressure on the marshes and on the banks of the lower segment of the river led to the loss of important habitats for conservation.

The survey carried out in situ and by aerial photography led us to consider that interventions with long-term purposes could not be limited to the consolidation of banks, but that they also should modify hydrodynamics because of the continued process of excavation of the lower layers of the bank. These display a complex structure, with a less cohesive layer at the base, affecting the stability of the entire bank. These aspects are similar, both in Cardielos (Figure 2, left) and Portuzelo (Figure 2, right), where the progressive erosion has led further to the collapse of the riparian layer.

This part of the estuary, downstream of the previous section, displays a higher vulnerable condition, which is due to the fact that the lowering of the estuarine bed caused by dredging reached a considerably higher depth close to the banks (the channel deepening reached here is 4–6 m).

**3. Results**

*3.1. Cardielos Section*

gently sloping.

*Water* **2020**, *11*, x FOR PEER REVIEW 4 of 14

**Figure 1.** Aerial photographs of a part of the R. Lima estuary with the localization of the two considered priority segments for intervention: the comparison of the year 1965 with meander characteristics and the year 2010 without sediment in the river is shown. The yellow line intervention areas represent Portuzelo and red represents Cardielos. **Figure 1.** Aerial photographs of a part of the R. Lima estuary with the localization of the two considered priority segments for intervention: the comparison of the year 1965 with meander characteristics and the year 2010 without sediment in the river is shown. The yellow line intervention areas represent Portuzelo and red represents Cardielos. *Water* **2020**, *11*, x FOR PEER REVIEW 5 of 14

**Figure 2***.* Cardielos (left panel) and Portuzelo (right panel) sections in 2010, previous to the bank reinforcement, where an apparent bank collapse is visible. **Figure 2.** Cardielos (left panel) and Portuzelo (right panel) sections in 2010, previous to the bank reinforcement, where an apparent bank collapse is visible.

This part of the estuary, downstream of the previous section, displays a higher vulnerable condition, which is due to the fact that the lowering of the estuarine bed caused by dredging reached a considerably higher depth close to the banks (the channel deepening reached here is 4–6 m).

There were multiple purposes of this project besides bank protection. We are aware that some of the civil engineering techniques used may have negative aesthetic implications, so there was concern about visual mitigation; besides, the rehabilitation should improve the riverine habitats, allowing a revegetation process towards a riparian gallery. Therefore, in this rehabilitation, we integrated different concepts of bank protection and hydrodynamics processes through the selection of convenient engineering techniques, with the additional purpose of stopping the irretrievable loss of an area of high biodiversity (wetlands/marshes) and marginal leisure infrastructures. The promotion of hydraulic roughness to progressively increase accretion (and, indirectly, salt marsh

The project was developed along with two temporal phases carried out between 2011 and 2013. It was defined as the first set of six triangular groynes, which were located in the most eroded segment. Afterwards, we built a second layer of groynes, closer to the water level, with a smaller dimension, which was further disposed along the same segment to complement and strengthen the first layer. This field of two lines of groynes, implanted along approximately 1 km, then formed a group of structures that acted together with the objective of causing the water to flow some distance from the riverbank and to increase hydraulic roughness. A groyne increases the roughness of the bank on which it is constructed and, in doing so, creates a zone of lower flow velocity in which the tendency for erosion is less and the deposition greater. Typically, eddy currents form in the pools between groynes where the water flows upstream along the bank [35]. These are wall-like structures, perpendicular to the flow direction and pointed towards the edges where the nose of the groyne is

Both sets of layers were built with rip-rap material, whereas the second line removes their visual impact since it is below the waterline at high tide. This group of structures (Figure 3 and Figure 4) include granite rocks of 0.5–0.8 m in diameter packed in a layer thickness between 1.5–1.9 m and creates structures ranging in length from 13–29 m, depending on the topographical conditions where

2, right), where the progressive erosion has led further to the collapse of the riparian layer.

recovery) was inherent to the conception of the project.

### **3. Results**

There were multiple purposes of this project besides bank protection. We are aware that some of the civil engineering techniques used may have negative aesthetic implications, so there was concern about visual mitigation; besides, the rehabilitation should improve the riverine habitats, allowing a revegetation process towards a riparian gallery. Therefore, in this rehabilitation, we integrated different concepts of bank protection and hydrodynamics processes through the selection of convenient engineering techniques, with the additional purpose of stopping the irretrievable loss of an area of high biodiversity (wetlands/marshes) and marginal leisure infrastructures. The promotion of hydraulic roughness to progressively increase accretion (and, indirectly, salt marsh recovery) was inherent to the conception of the project.

### *3.1. Cardielos Section*

The project was developed along with two temporal phases carried out between 2011 and 2013. It was defined as the first set of six triangular groynes, which were located in the most eroded segment. Afterwards, we built a second layer of groynes, closer to the water level, with a smaller dimension, which was further disposed along the same segment to complement and strengthen the first layer. This field of two lines of groynes, implanted along approximately 1 km, then formed a group of structures that acted together with the objective of causing the water to flow some distance from the riverbank and to increase hydraulic roughness. A groyne increases the roughness of the bank on which it is constructed and, in doing so, creates a zone of lower flow velocity in which the tendency for erosion is less and the deposition greater. Typically, eddy currents form in the pools between groynes where the water flows upstream along the bank [35]. These are wall-like structures, perpendicular to the flow direction and pointed towards the edges where the nose of the groyne is gently sloping.

Both sets of layers were built with rip-rap material, whereas the second line removes their visual impact since it is below the waterline at high tide. This group of structures (Figures 3 and 4) include granite rocks of 0.5–0.8 m in diameter packed in a layer thickness between 1.5–1.9 m and creates structures ranging in length from 13–29 m, depending on the topographical conditions where they are implanted. The second set (closer to the water) was composed of material with similar diameters, but packed around an axis of material with a small grain size (20–30 cm) and placed over a synthetic mat.

Because this set was placed in a plane which was more exposed to tidal and river flow energy, it was planned that the foundation of the structure should be place at a level close to the depth of the expected scour; this level was indicated by a careful observation along this section of the river. The defined layout (straight in plan and perpendicular orientations) allows the set of groynes to trap a moderate amount of sediment upstream and downstream, keeping the current more or less parallel to the bank and offering a medium potential for scour at the head. Both sets of groynes were rooted successively in a set of structures, which are listed in order from water level as follows: a) rip-rap between 3–5 m long; b) gabion mattress (placed on a gravel layer after shaping the bank), which is characterized by a wire basket filled with rock (covered with 20 cm of soil for planting and a wire mesh to decrease tidal washing); c) vegetation roll and willow fascine; and d) a gravel layer with soil (40 cm) covered by tridimensional geomats after bank reprofiling (Figure 4).

Finally, the described structures were vegetated with autochthonous hygrophytic and salinity-resistant herbaceous and woody species, such as reeds and rushes, combined with semi-halophyl macrophytes and salinity-resistant shrubs (*Juncus maritimus*, *J. acutus* or *J. e*ff*usus*, *Typha angustifolia*, *Phalaris arundinacea*, *Agrostis stolonifera*, *Scirpus maritimus*, *Festuca arundinácea*, *Phragmites* spp., *Tamaryx tamaryx*, *Carex* sp. or *Najas* spp.). In the vegetation roll and willow fascine layer, over the severely eroded area, we also conducted hydroseeding since the area is a space which is intensely used by visitors. Table 1 shows the techniques involved at the Cardielos site, including the floristic composition associated with the specified structures.

a synthetic mat.

diameters, but packed around an axis of material with a small grain size (20–30 cm) and placed over

*Water* **2020**, *11*, x FOR PEER REVIEW 6 of 14

they are implanted. The second set (closer to the water) was composed of material with similar diameters, but packed around an axis of material with a small grain size (20–30 cm) and placed over

**Figure 3.** General view of the intervention area at the Cardielos site. **Figure 3.** General view of the intervention area at the Cardielos site. mesh to decrease tidal washing); c) vegetation roll and willow fascine; and d) a gravel layer with soil (40 cm) covered by tridimensional geomats after bank reprofiling (Figure 4).

**Figure 4.** Planned techniques and vegetation species along the bank profile designed for the Cardielos site. **Figure 4.** Planned techniques and vegetation species along the bank profile designed for the Cardielos site.

### *3.2. Portuzelo Section*

section.

**4. Discussion**

First layer (close to the water)

The techniques designed and implemented for this area, with lengths of approximately 150 m, are schematized in the profile shown in Figure 5 and Table 2. Again, the objectives, besides bank stabilization, allowed for the settlement of vegetation and increased the roughness on the submerged bank in order to trap sediments and to dissipate the energy from river flow and tidal dynamics, contributing to long-term sustainability. Besides, this bank constitutes a barrier that protects a large salt marsh. Therefore, it is a crucial aspect of this defense system for the preservation of this sensitive environment. As Figure 5 shows, from the base to the top of the bank different layers, we successively used a) rip-rap with large boulders (0.6–0.8 m) in a foundation frame of wood piles, with stakes driven into the riverbed (since the river depth was higher when compared to the previous section) in order to promote roughness (groynes were not considered because of the water depth); b) bank reprofiling to smooth the slope, which was covered with a layer of geogrids, filled in the lower part with gravel (for adequate infiltration) followed by soil and further vegetated, where reeds were placed near the base and woody vegetation (mainly *Tamaryx* sp.) was planted in the upper layer, and finally a wire mesh was used to decrease the potential tidal washing; and c) top lining and plantation with willow species as well as a row with broadleaf trees to improve the landscape attractively for visitors and to increase the overall bank consolidation. Besides this, we installed drains to allow the water to flow between the estuary and marshland. *Water* **2020**, *11*, x FOR PEER REVIEW 8 of 14

**Figure 5***.* Planned techniques and vegetation species along the bank profile designed for the Portuzelo **Figure 5.** Planned techniques and vegetation species along the bank profile designed for the Portuzelo site.

site. **Table 2.** Planned techniques and vegetation species along the bank profile designed for the Portuzelo section.

**Bank Profile Techniques Vegetation Species**  Bank base Rip-rap (60/80 cm) - Geocells - Following this, a hydraulic study was conducted (using the HEC-RAS model) to compare the hydraulic conditions of the bank before and after the projected intervention, in order to estimate the energy dissipation of the flow energy. The hydraulic modelling enabled us to simulate various scenarios; in particular, the effect of increasing the hydraulic roughness to provide sedimentation

Second layer Planting *Salix atrocinerea; Salix*

This study includes local mitigation actions which have been applied to solve the most dramatic erosion problems in specific estuarine area sections. Of course, it would be more convenient to adopt a global management plan for the restoration of the entire estuary considering the stressing agents. However, the design presented here in the two considered areas represents a process to defend the salt marshes in this protected area, which we assume that will act as a motivation for extension to the different impacted areas of this estuary. Besides this, the inherent value of salt marshes for biodiversity, in the R. Lima estuary is that they also act as filters trapping or accumulating heavy metals, especially in the more densely vegetated high marsh layers [36], which play a significant role in dealing with the industrial effluents discharged upstream. More holistic approaches take

organic) -

spp

*salviifolia*

Bidimensional geomats (synthetic and

and the inherent stabilization of the bank toes. Consequently, these actions contributed indirectly to creating the conditions for the natural colonization and resettlement of marginal vegetation.

### **4. Discussion**

This study includes local mitigation actions which have been applied to solve the most dramatic erosion problems in specific estuarine area sections. Of course, it would be more convenient to adopt a global management plan for the restoration of the entire estuary considering the stressing agents. However, the design presented here in the two considered areas represents a process to defend the salt marshes in this protected area, which we assume that will act as a motivation for extension to the different impacted areas of this estuary. Besides this, the inherent value of salt marshes for biodiversity, in the R. Lima estuary is that they also act as filters trapping or accumulating heavy metals, especially in the more densely vegetated high marsh layers [36], which play a significant role in dealing with the industrial effluents discharged upstream. More holistic approaches take advantage of conceptual models such as the one presented by Bergh's team in [37], where the dysfunctional patterns in habitat and community structures were traced back to anthropogenic changes in the physical and chemical processes, with the identification of key parameters and distinct rehabilitation proposals.

In these specific areas, we adopted soft engineering solutions to coastal flooding, namely by incorporating the planting of marsh vegetation in the intertidal zone for the purpose of promoting sedimentation and dissipating wave energy. We also followed the principles of Morris [38], for whom a successful design would employ plant species with varying degrees of tolerance to flooding, maximum drag, broad vertical ranges within the intertidal zone and which form a successional series. However, each rehabilitation method has to be observed under its specific conditions: if we provided the conditions for accretion because of a sediment deficit, other situations may require an inverse approach. This was the case for Garcia-Novo's team [39], which projected a hydraulic scheme favoring sand deposition upriver, avoiding its transfer to the Donãna marshes (South Spain) in order to prevent the excess of silting during flood events, which caused an unstable substrate with a lack of vegetation.

Thus, to estimate the hydraulic differences in order to analyze the ability to dissipate the energy created by the introduced structures, we computed the shear stress and current velocity for different recurrence periods, between the initial situation and considering the disposed of sets of groynes (Table 3).

**Table 3.** Values obtained by simulation with the HEC-RAS model to compare hydraulic parameters before and after the intervention.

The hydraulic simulation was conducted to evaluate the results of flow magnitudes corresponding to two frequent events (2.33 and 5 years) and one extreme event (100 years). The hydraulic model developed adopted a range of manning values, n (Table 3), in order to calibrate the model based on the reference data and the conditions observed in situ.

wetland (Figure 7).

is reflected in Figure 8.

We may conclude that there was a significative reduction of shear stress, which reached about 65%, corresponding also to the estimated lower current velocities, as a consequence of increasing the resistance to flow (displayed by Manning coefficients), which may also act as a sediment trap, protecting the base of the bank. However, as a result of the type of solutions implemented in the Cardielos area, there was an increase in speed; nevertheless, there was no risk of bank collapse.

Following the appropriate post-appraisal of the implemented project in the target areas of Cardielos and Portuzelo, we may draw some conclusions and recommendations. In the first 2 years after the project's conclusion, we could observe that, in Cardielos, all the structures showed a convenient resistance to critical environmental conditions. This is the case for the two rows of groynes, as well as the rip-rap or the gabion mattress (Figure 6), representing, therefore, a convenient solution since the erosion impact also decreased substantially and created the required barrier for bank protection preserving the built leisure structures. Besides this, the subsequent field surveys allowed us to observe that no more obvious scour holes were formed around the groyne layers. However, we also must accept that not much sediment deposition was observed between these structures, in contrast to our expectations, which retarded the natural re-vegetation process. The less successful results were observed in the layer affected by to the tidal movement, where we noticed a low success of woody vegetation development as the stake rooting was deficient, probably because of the small size of this biological material (less than 30 cm in length). Another cause was the lack of protection in relation to trampling (people and animals). In the case of the layer in the upper bank, other than the influence of the tides, we could observe better results, with higher plant survival and floristic diversity. With regard to the area of Portuzelo, the robustness of the rock base protection was evident, as well as the stability of the plateau following the installation of the geogrid wall. However, the planting success was only relative, such as the natural colonization by macrophytes or herbs, but the viability rate was more intense with the plantations of shrubs based on tamarisk. At low tide, it was possible to check for the proper functioning of the installed drains which were integrated into the created protection structure, which allowed the water to flow into the marshland, keeping a constant water level in this ecosystem, which essentially contributes to the sustainability of this wetland (Figure 7). *Water* **2020**, *11*, x FOR PEER REVIEW 10 of 14 allowed us to observe that no more obvious scour holes were formed around the groyne layers. However, we also must accept that not much sediment deposition was observed between these structures, in contrast to our expectations, which retarded the natural re-vegetation process. The less successful results were observed in the layer affected by to the tidal movement, where we noticed a low success of woody vegetation development as the stake rooting was deficient, probably because of the small size of this biological material (less than 30 cm in length). Another cause was the lack of protection in relation to trampling (people and animals). In the case of the layer in the upper bank, other than the influence of the tides, we could observe better results, with higher plant survival and floristic diversity. With regard to the area of Portuzelo, the robustness of the rock base protection was evident, as well as the stability of the plateau following the installation of the geogrid wall. However, the planting success was only relative, such as the natural colonization by macrophytes or herbs, but the viability rate was more intense with the plantations of shrubs based on tamarisk. At low tide, it was possible to check for the proper functioning of the installed drains which were integrated into the created protection structure, which allowed the water to flow into the marshland, keeping a constant water level in this ecosystem, which essentially contributes to the sustainability of this

**Figure 6.** Rip-rap (left) and first row of small groynes (right) in Cardielos, after the implementation of the project in Cardielos in 2014. **Figure 6.** Rip-rap (left) and first row of small groynes (right) in Cardielos, after the implementation of the project in Cardielos in 2014.

Of course, this action was focused in a specific part of an overall degraded estuarine environment. Immediately upstream and downstream of the rehabilitated sections, there is still a constant progression of the pressure on the banks and the consequent set-back of the bank line, which

**Figure 7.** Rip-rap (left) and geogrid disposal (right) in the Portuzelo site (2014).

is reflected in Figure 8.

the project in Cardielos in 2014.

wetland (Figure 7).

**Figure 6.** Rip-rap (left) and first row of small groynes (right) in Cardielos, after the implementation of

Of course, this action was focused in a specific part of an overall degraded estuarine

allowed us to observe that no more obvious scour holes were formed around the groyne layers. However, we also must accept that not much sediment deposition was observed between these structures, in contrast to our expectations, which retarded the natural re-vegetation process. The less successful results were observed in the layer affected by to the tidal movement, where we noticed a low success of woody vegetation development as the stake rooting was deficient, probably because of the small size of this biological material (less than 30 cm in length). Another cause was the lack of protection in relation to trampling (people and animals). In the case of the layer in the upper bank, other than the influence of the tides, we could observe better results, with higher plant survival and floristic diversity. With regard to the area of Portuzelo, the robustness of the rock base protection was evident, as well as the stability of the plateau following the installation of the geogrid wall. However, the planting success was only relative, such as the natural colonization by macrophytes or herbs, but the viability rate was more intense with the plantations of shrubs based on tamarisk. At low tide, it was possible to check for the proper functioning of the installed drains which were integrated into the created protection structure, which allowed the water to flow into the marshland, keeping a constant water level in this ecosystem, which essentially contributes to the sustainability of this

**Figure 7.** Rip-rap (left) and geogrid disposal (right) in the Portuzelo site (2014). **Figure 7.** Rip-rap (left) and geogrid disposal (right) in the Portuzelo site (2014). allowing us to conclude that the extra cost of building heterogeneous habitats in the intense intervention bore no relation to results. Ecological engineering is a very promising approach to

Of course, this action was focused in a specific part of an overall degraded estuarine environment. Immediately upstream and downstream of the rehabilitated sections, there is still a constant progression of the pressure on the banks and the consequent set-back of the bank line, which is reflected in Figure 8. maintaining intertidal marshes in equilibrium, but we believe that it has to incorporate a larger area of the estuary. This is also the concept of Danielsen's group [5] and Morris [38], who adopted the extensive planting of marsh vegetation in the intertidal zone to promote sedimentation and dissipate wave and river energy, with the additional positive benefits of accreting sediment.

**Figure 8.** Two segments of the estuary banks, adjacent to the rehabilitation project in Cardielos (2014), showing that the severe erosion still progresses along the un-revetted banks, either in the upper section (left) or in the lower section (right), which requires an extension of the techniques already **Figure 8.** Two segments of the estuary banks, adjacent to the rehabilitation project in Cardielos (2014), showing that the severe erosion still progresses along the un-revetted banks, either in the upper section (left) or in the lower section (right), which requires an extension of the techniques already implemented.

implemented. Much research has focused on the importance of vegetation floodplains to create transient storage for channel sediments, becoming efficient traps—also for pollutants—and avoiding streambank retreat [42,43] (see Curran and Hession [44] for a compilation of the vegetative impacts on hydraulics and sediments). The sediment trapping ability of the vegetation allows for more growth and consequently further deposition; in [45], it was observed that plant succession could lead even to softwood forest establishment. We must point out that if gravel extraction were to seriously impact the upper part of the estuary, reducing the salt marsh area—an activity that is now forbidden, in the most downstream part—a 3 km navigational channel would be maintained by regular dredging activities, which now will also cause the destruction of the wetlands and changes in sediment composition (the enrichment of fine sediments with high organic matter content), where typical floristic communities have been washed away [46]. We intend that the partial rehabilitation techniques presented here may constitute a stimulus to a more global management action aimed at the protection of salt marshes in this important hot-spot; however, more consistent restoration requires another scale and the coordination Finally, we must stress that this is only a mitigation action, even if integrative; it will require a more complete study at a larger regional scale in the future, including proper actions in the entire estuary and even at the catchment level, in order to include the appropriate management actions that may contribute to overcoming the deficit of sediments in the estuarine area. For instance, Jacobs' workgroup [40], to restore tidal marshes on sites with low elevation, used a technique of controlled reduced tide (CRT), restricting the tidal regime with neap and spring tides by using high inlet culverts and low outlet valves, allowing the restoration of typical tidal freshwater vegetation. The choice between the advantages of intensive versus extensive ecological restoration should always be considered, and an interesting contribution to this subject was analyzed in terms of community biodiversity, successional changes, and costs by Gallego Fernandez and García Novo [41] in the restoration of a tidal marsh in SW Spain. Here, they compared high and low-intensity interventions, allowing us to conclude that the extra cost of building heterogeneous habitats in the intense intervention bore no relation to results. Ecological engineering is a very promising approach to maintaining intertidal marshes in equilibrium, but we believe that it has to incorporate a larger area of the estuary. This is also the concept of Danielsen's group [5] and Morris [38], who adopted the extensive planting of marsh vegetation in the intertidal

of different river authorities. This is indeed a critical overview of this project. For instance, the

zone to promote sedimentation and dissipate wave and river energy, with the additional positive benefits of accreting sediment.

Much research has focused on the importance of vegetation floodplains to create transient storage for channel sediments, becoming efficient traps—also for pollutants—and avoiding streambank retreat [42,43] (see Curran and Hession [44] for a compilation of the vegetative impacts on hydraulics and sediments). The sediment trapping ability of the vegetation allows for more growth and consequently further deposition; in [45], it was observed that plant succession could lead even to softwood forest establishment.

We must point out that if gravel extraction were to seriously impact the upper part of the estuary, reducing the salt marsh area—an activity that is now forbidden, in the most downstream part—a 3 km navigational channel would be maintained by regular dredging activities, which now will also cause the destruction of the wetlands and changes in sediment composition (the enrichment of fine sediments with high organic matter content), where typical floristic communities have been washed away [46]. We intend that the partial rehabilitation techniques presented here may constitute a stimulus to a more global management action aimed at the protection of salt marshes in this important hot-spot; however, more consistent restoration requires another scale and the coordination of different river authorities. This is indeed a critical overview of this project. For instance, the dredged material from the lower estuary (the mentioned channel for navigation) could be moved into the eroded river bank to mitigate incision, where the built groynes and deflectors (particularly at Cardielos) could promote the sedimentation and stability of the inserted gravel. The monitoring of this bedload transport, namely by particle tracking via radio telemetry [47], could allow us to obtain transport paths and increase the efficiency of this procedure.

### **5. Final Remarks**

Finally, we share the opinion of González del Tanágo's team [32], arguing for a more holistic approach to water resources and land-use management at the catchment scale in order to understand the synergistic effects of dams, sediment supply and vegetation growth to implement the appropriate management and rehabilitation actions. The authors stress very different processes and geomorphic consequences in Iberian rivers, namely gravel-bed systems as in the case of the Lima river, in which sediment deficit downstream of the dams has triggered channel incision, and other Mediterranean streams where river regulation, in contrast, resulted in channel narrowing. Here, long-term photographic registrations allowed us to conclude that there was an increase of aggradation processes and vegetation encroachment, because of the reduction of the geomorphic discharges which are able to transport fine sediment downstream from the dams and the high sediment delivery of the catchment promoted by agricultural development. These aspects, finally, show the necessity of adopting specific rehabilitation processes adapted to each catchment, according to soil use patterns, flow changes and geological and physiographic features and the important of avoiding generic solutions.

**Author Contributions:** Conceptualization, L.F.S.F., A.A.S.P. and R.M.V.C.; Methodology, A.A.S.P.; L.F.S.F. and R.M.V.C.; Validation, L.F.S.F. and R.M.V.C.; Supervision, L.F.S.F. and R.M.V.C.; Project administration, R.M.V.C.; Funding acquisition L.F.S.F., F.A.L.P. and R.M.V.C.; Software implementation, A.A.S.P. Data curation and processing, A.A.S.P. and D.P.S.T.; Writing—original draft preparation, A.A.S.P.; Writing—review and editing, D.P.S.T. and F.A.L.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** For authors integrated with the CITAB Research Centre, this work was further financed by the FEDER/COMPETE/POCI—Operational Competitiveness and Internationalization Program, under Project POCI-01-0145-FEDER-006958, and by the National Funds of FCT—Portuguese Foundation for Science and Technology, under the project UID/AGR/04033/2019. For the author integrated in the CQVR, the research was additionally supported by the National Funds of FCT—Portuguese Foundation for Science and Technology, under the project UID/QUI/00616/2019.

**Acknowledgments:** The authors are grateful to the Viana do Castelo Council who supported the project's conception and implementation and the office "Formas & Conceitos" who collaborated in all the steps of the project appraisal, from baseline surveys to option evaluation and technical design and project documentation preparation. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References and Notes**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Review* **Water-Saving Agricultural Technologies: Regional Hydrology Outcomes and Knowledge Gaps in the Eastern Gangetic Plains—A Review**

**Mohammad A. Mojid 1,\* and Mohammed Mainuddin 2**

**Abstract:** Increasing food demand has exerted tremendous stress on agricultural water usages worldwide, often with a threat to sustainability in agricultural production and, hence, food security. Various resource-conservation technologies like conservation agriculture (CA) and water-saving measures are being increasingly adopted to overcome these problems. While these technologies provide some short- and long-term benefits of reduced labor costs, stabilized or increased crop yield, increased water productivity, and improved soil health at farm scale, their overall impacts on hydrology outcomes remain unclear at larger temporal and spatial scales. Although directly linked to the regional hydrological cycle, irrigation remains a less understood component. The ecological conditions arising from the hydrology outcomes of resource-conservation technologies are associated with sustainability in agricultural production. In this paper, the philosophies and benefits of resource-conservation technologies and expert perceptions on their impacts on temporal and spatial scales have been reviewed comprehensively focusing on regional hydrology outcomes in the Eastern Gangetic Plain (EGP). Due to data inadequacy and lack of knowledge-sharing among disciplines, little is yet known about actual water saving by these resource-conservation technologies and the level of their contribution in groundwater and surface water storage over large temporal and spatial scales. Inadequate knowledge of the hydrological effects of water applied in the agricultural field leads to the implementation of water management policy based on local perspectives only, often with the possibility of deteriorating the water-scarcity situation. Therefore, multidisciplinary future research should quantify regional hydrology outcomes by measuring the components of regional water balance in order to develop a proper water management policy for sustainable agricultural production.

**Keywords:** irrigation management; rice; percolation; scale effects; hydrologic cycle

### **1. Introduction**

The global demand for food, energy and water by the ever-growing population has been forecasted to increase by 50%, 50% and 30%, respectively, in 2030 compared to 2012 [1]; in the same base period, food demand will increase by 70% to 100% by 2050 [2]. The Indo-Gangetic Plains (IGP) comprising more than 250 Mha of area across Bangladesh, India, Pakistan and southern Nepal have over 100 Mha of agricultural land and host over 750 million people [3]. The Lower Gangetic Plain, called the Eastern Gangetic Plain (EGP), comprises the adjoining states of Bihar and northern West Bengal in North-eastern India, the North-West of Bangladesh and the Terai plains of Nepal (Figure 1). The EGP is characterized by the world's highest density of rural poor, persistent yield gaps, low agricultural productivity, limited crop diversification, ample water resources [4,5], and highly fertile lands [6,7] of agricultural importance [8]. The region is therefore a global priority for sustainably increasing food production [9].

**Citation:** Mojid, M.A.; Mainuddin, M. Water-Saving Agricultural Technologies: Regional Hydrology Outcomes and Knowledge Gaps in the Eastern Gangetic Plains—A Review. *Water* **2021**, *13*, 636. https://doi.org/10.3390/w13050636

Academic Editor: Fernando António Leal Pacheco

Received: 6 January 2021 Accepted: 19 February 2021 Published: 27 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Location and area map of the Eastern Gangetic Plain (EGP) region.

Agricultural productivity is critically dependent on the availability of water. Adequate water supply significantly increases crop productivity [10,11] by introducing high yielding crop varieties, a better cropping pattern, and increasing cropping intensity [12]. Compared to rain-fed agriculture, irrigated agriculture produces two to four times more crop yields [13]. This contribution of irrigation increased global irrigated land by 76% between 1970 and 2012 [14]; the reliance of agricultural production on irrigation is expected to further increase in the future [15]. Farmers' capacity to access and use water is a major driving factor in obtaining the best yield and hence is an important variable for the food security index [16]. However, the growing competition for water by various sectors will affect farmers' ability to produce food [17,18]. So, making food production sustainable, while conserving diminishing water supplies, will be a great challenge in the future [19].

The Ganges basin has a tropical climate, with a distinct wet monsoon (June–September) and a dry winter (November–February); the summer is characteristically hot and humid. Except for the East and North-East hilly regions of the basin where annual rainfall often exceeds 4000 mm, the average annual rainfall in most other parts is 1500 mm. The rainfall is mostly concentrated in the monsoon season and the winter is almost rainless [20] but the main cropping season. In many parts of the IGP, agricultural drought and other climatic shocks severely affect crop production, thus, necessitating an adequate water supply to stabilize agricultural production [21,22]. Surface water is inadequate in the dry season, but groundwater plays a vital role in sustaining agricultural productivity. In India, 60% of the agricultural water requirement is satisfied from groundwater, covering over 50% of the irrigated area [23]; in Bangladesh, the corresponding quantities are 79% and 85% [24]. Of the many factors now threatening sustainability in agricultural productivity, water is the most crucial [25–33] since, without further improvement in water productivity, the amount of water needed for crop agriculture is predicted to increase by 70–90% by 2050 [34].

Several resource-conservation technologies like minimum tillage, no/zero-tillage, direct-seeding, bed-planting, laser land-leveling and residue retention [35–37], and watersaving technologies like alternate wetting and drying (AWD) and deficit irrigation methods have been developed over the past three decades and are being practiced in many parts of the world, including the EGP. In addition to the benefits from the conserved resources, these technologies can also change crop-water use and the regional water cycle [38] with negative impact on groundwater dynamics [39]. They save water by reducing water application in the fields, with resulting lower percolation and groundwater recharge. Large-scale adoption of these technologies can therefore lead to significant decline in groundwater levels [40–42], with possible degradation of soil quality and damage of vegetation [43]. In

many parts of the EGP, groundwater level has declined significantly, and is now threatening sustainable water supply for irrigation and drinking [44–49] with resulting negative impacts on the economy, society and environment [50–53]. Although less than one-third of the IGP has experienced declining groundwater levels [54] the situations in high-population centers (e.g., Dhaka city) and other stressed areas (e.g., the Barind area) are potentially alarming [49].

Agriculture in the IGP is mostly dominated by irrigated rice–wheat systems, which cover 13.5 Mha and play a crucial role in the food security and livelihoods of millions of people [37,55,56]. In Bangladesh and West Bengal, rice is produced on 6.05 Mha and 5.5 Mha, respectively [57]. Both mechanized and tillage-based traditional agriculture and transplanted rice cultivation with flood irrigation requiring a huge quantity of water [58–60] are a major challenge in agriculture, in order to maintain or increase rice production. Shifting current agriculture to water-efficient ones [61–65] would conserve water from being wasted through unintended purposes and make considerable water savings [66–69] to face the challenge. Conversion of conventional agriculture to resource-conservation methods [70–72] using resource-conservation technologies and water-saving measures has been demonstrated as of particular interest in this regard [29,73–76].

When water is applied in a crop field, not all of it is consumed as illustrated in Figure 2. The local surface and sub-surface hydrological systems retain a considerable portion of the applied water, which might be reusable later by other users. Consequently, irrigation has a direct link to the regional hydrological cycle, especially in areas with shallow groundwater [54]. A large part of the applied irrigation water infiltrates below the root zone and is stored in the underlying aquifer [7,43] or in downstream surface water bodies. Figure 3 conceptualizes the flow paths of the components of water from a rice field under conventional flood irrigation with pumped groundwater. The percolated water is perceived as lost by the farmers and irrigation practitioners [77] but is a gain to the local surface and sub-surface hydrological systems. The efficiency of water usage at any separate component (e.g., crop fields, ponds) within the hydrological system may be low, but the overall efficiency of the entire system can be much higher than in the individual components. So, the general concept of water use efficiency undervalues the real efficiency of the whole hydrological system. Water recycling must be integrated into the concept of water-use efficiency to develop new realistic concepts [78]. The water flux exchanging between the aquifer and vadoze zone greatly controls the dynamics of the groundwater table [39] thus raising a valid question of how the currently advocated water-saving measures impact on the hydrological cycle of a groundwater basin. Do these water-saving measures assure proper utilization of groundwater reserves? In situations where downstream aquifers and surface water bodies are fed from upstream aquifers, what will be the effects of the water-saving measures on these downstream water resources (Figure 3)? These important issues have not yet been investigated critically on the system level; only some field-scale studies have investigated the possibilities, which are also contrasting in nature. A summary of the major previous studies assessing the impacts of various agricultural water-saving technologies on local and regional hydrology is presented in Table 1. In light of this short-coming, this paper comprehensively reviewed the available literature to evaluate the present state of knowledge and emerging knowledge-gaps on this subject so as to guide future research on this topic. Note that since rice-based cropping systems dominate the agricultural landscape of the EGP [56], this study focuses on the exchange of water flux between irrigated rice fields and the underlying aquifers. The paper is structured into five major sections in addition to an introduction and a concluding section. The benefits and impacts of conservation agriculture have been reviewed in the second section. The third section highlights the complementary and contemporary meanings of water saving while the fourth section addresses the impacts of agricultural water-saving methods on regional hydrology outcomes (i.e., links between various components of the regional hydrological cycle). The next section identifies current knowledge gaps in the

key water-saving issues, including scale-effects and policy, before an overall summary and concluding section on water-saving measures and regional hydrology outcomes.

**Figure 2.** Utilization and fate of applied water to crop fields and hydrological links to groundwater resources.

**Figure 3.** The pathways of the components of water from a rice field under conventional irrigation with groundwater.

### **2. Conservation Agriculture**

### *2.1. Philosophies and Benefits*

Conservation agriculture (CA) has been developed as a response to concerns about sustainability in agriculture [55,79–83] with basic principles of rebuilding soil, optimizing crop production inputs (resource and energy), enhancing food production and optimizing profits [84–87]. It comprises application of three inter-linked principles: (i) no or minimum mechanical soil disturbance through conservation tillage (e.g., minimum or zero-tillage), (ii) biomass mulch soil cover (e.g., crop residues), and (iii) crop diversification, as well as other practices of integrated crop management [88]. Under conservation tillage, approximately 30% of the soil surface is kept covered with crop residues, which reduces erosion of surface soil by overland flow [89,90]; a crop is planted directly into a seedbed without any tillage operation in the zero-tillage system. Cultivation of wheat under zero-tillage in the ricewheat cropping system is an emerging CA-based technology in the IGP [91]. A CA-based sustainable intensification program was started in 2014–15 in two districts each of Nepal, Bangladesh, and Bihar and West Bengal in India [92]. Globally, the cropland under CA increased at 5.3 Mha annually since 1990 and reached 106 Mha in 2008/2009 [93] and 180 Mha in 2015/2016; 78 countries in the world have adopted CA practices.

**Table 1.** Summary of major previous studies assessing the impacts of agricultural water-saving technologies on local and regional hydrology. The studies are grouped by apparent and actual water saving, impacts of water-saving measures on water usage and regional water balance, gaps in current knowledge in certainty and scale-effect of water saving, and policy formulation for water resources management.

Resource-conservation technologies have revealed some promising immediate [135–137] and long-term benefits [138–140]. They reduce field-scale irrigation, fertilizer applications, labor shortages, energy use, greenhouse gas emission, and erosion of field soil; while they increase soil organic matter and biotic activity, crop diversification, yields, and farm incomes by improving resource-use efficiency [36,37,55,75,83,91,141–146]. Tillage accelerates oxidation of soil organic matter to CO2 and loss to the atmosphere, but CA reduces the oxidation rate [147,148]. Increased crop residues under CA and root exudation of carbon compounds into the soil cause a reversal of soil carbon from net loss to a net gain [86,149–151]. In spite of these multiple benefits [152–154] the farmers' prime interest in CA-based agriculture is mostly the monetary gain [155]. Nonetheless, CA is now emerging as a major component of farming systems for ensuring food security in South Asia [85,87].

### *2.2. Impacts on Soil and Water Use*

The effects of conservation agriculture on soil properties vary depending on the type of chosen system, soil-type, climatic conditions, cropping history, etc. [156–158]. Soil becomes more stable and less susceptible to erosion under zero-tillage compared to conventional tillage [158,159] and provides more satisfactory physical properties for crop production [160]. Soil organic carbon increases [92,161,162] and pH decreases [163] under zero-tillage compared to a conventional tillage system over time [164,165]. Organic matter improves soil aggregation, alters pore-size distribution, reduces soil bulk density, and increases both total and effective porosities within 0–5 cm soil profile [166,167]. The increased number of 0.5–50 µm pores augments soil-water storage and 50–500 µm pores enhance water movement through the soil [92,168]. Conventional tillage creates a surface crust of high bulk density, while long-term (e.g., 8–10 years) zero-tillage helps in forming many continuous pores extending from the soil surface to the deeper layers causing significant increase in infiltration [161,166,169–171]. Zero-tillage thus increases the saturated and unsaturated hydraulic conductivity of soils [159,162,172,173]. Conservation tillage can increase the capture of rainfall and reduce runoff due to stable aggregates and increased porosity in the surface soil [174] and water-holding capacity due to increased organic matter [159] with resulting reduction in surface evaporation. The magnitudes of water-, labor- and energy-saving of some CA practices are listed in Table 2. However, generalization about such gains in water saving for all hydrological situations can provide a wrong message in many regions. In the dry season, there is not enough water on the soil surface to increase its capture in the soil within the EGP. There are only occasional relatively ample rainfall events in some areas of the EGP, in which cases CA can make more water available for plants' use and increase the precipitation-use efficiency of the production system [166]. However, water is almost always in excess of soil's saturation capacity in the wet season, thus leaving no scope for further capturing of rainfall into the soil. The important controlling factors in conserving water in the wet season are the infiltration capacity and hydraulic conductivity of the soil. However, this likelihood has not yet been investigated.

**Table 2.** Degree of benefits of conservation agricultural (CA) practices.

### **3. Agricultural Water–Saving**

### *3.1. Water-Saving Measures*

Water-saving irrigation, groundwater regulation, shifts to rain-fed agriculture, artificial recharge to groundwater, rainwater preservation, virtual water imports and indirect approaches like energy pricing and regulation are the currently available measures to reduce regional water use [134,180]. However, appropriate water-accounting is essential to identify the scope of these water-saving practices [181]. Based on the approach of reducing evaporation, runoff losses, and the extent of free water on the soil surface [182] irrigation strategies like shallow water depth associated with wetting and drying [183,184], alternate wetting and drying, AWD [108,124,185,186], semi-drying [187], aerobic rice cultivation [188,189], partial root-zone drying [190], and non-flooded mulching [191] are being practiced in different rice-growing regions. The AWD technique allows the soil to dry for a certain pre-determined number of days after depletion of the standing water in the field before the next irrigation [192]. The multiple-shallow irrigation method (1–3 cm irrigation applied frequently) can efficiently utilize rainfall and reduce percolation and surface runoff [94]. In the aerobic cultivation method, rice is grown in well-drained dry soils with supplementary irrigation, as with upland crops [188]. Furrow irrigation with raised beds, mulching, conservation tillage, deficit irrigation [193–195] and improved weed control can also achieve substantial water-saving.

### *3.2. Apparent and Actual Water-Saving*

The impact of efficiency of water consumption and water productivity on watersaving has been investigated at field scale on several occasions e.g., [196–200]. Any effort toward improving irrigation efficiency is valuable [201], but the commonly used concepts of water-use efficiency underestimate the system-level's actual efficiency [78]. The actual fraction of the applied water that is used efficiently at a regional scale has not yet been quantified; current measurement methods are inadequate for such quantification.

All the water applied in the crop/rice fields ends up at any of, or a combination of, consumptive use, non-consumptive use, non-recoverable flow (Figure 2), and change in storage [95]. These water use-terms allow a clearer definition of various issues and options for water usage in irrigated agriculture. Water-saving through a resource-conservation technology refers to a narrow local perspective of water application by reducing percolation rates, as conceptualized in Figure 4. This water-saving does not account for return flows from the irrigated field that may be either non-recoverable outflow (e.g., to saline or otherwise polluted groundwater or surface water as schematized in Figure 5) or recoverable outflow, where it ends up in rivers or as useable groundwater source [94,95]. The return flow may be a significant contributor to groundwater recharge [131,202–204].

**Figure 4.** Conceptualizing of impacts of water-saving measures on regional surface and groundwater sources when irrigation uses groundwater.

**Figure 5.** Water loss and water saving issues under conventional and water-saving irrigation from surface water sources when underground aquifer contains polluted water (e.g., saline).

Due to various natural calamities (e.g., seasonal storms, hailstorms, cyclonic storms, heavy rainfall and floods), dry season is the main and safe cropping season in the EGP, which has an annually renewable groundwater system. Here irrigation is predominantly done with groundwater; 79% of total irrigation in Bangladesh and more than 90% of irrigation in North-West India uses groundwater. An individual farmer considers the combined outflow of water by evapotranspiration, seepage and percolation as water usage by his/her rice field and hence actual water loss in the field. However, when considering a large spatial scale, achieving water-saving by one user may be a loss to another since the seepage and percolation from one's field enter the underlying aquifer or nearby surface water sources, from where others can reuse the water [75,103] causing no net loss to the system [205,206]. The real water-saving occurs only when the non-recoverable non-usable water losses (Figure 2) are eliminated or reduced. Avoidance of peak evaporative demand, use of short-duration varieties, cultivating less water-demanding crops, and changing from ponded to non-ponded rice culture are the potential technologies for reducing evapotranspiration [205–207]. The practicability and effects of technologies on crop yields must, however, be investigated before their large-scale field adoption.

Modifications of the water balance components by resource-conservation technologies, the fate of water saved through reduced application, and hydrologic interactions across spatial scales determine whether any reduction in water application leads to actual watersaving and reduces water usage [75]. Farmers always intend to achieve maximum output from the water resource, leading them to utilize as much water as they can have access to. Society, on the other hand, prefers utilizing scarce water to maximize profits by shifting water from agriculture to high-value economic sectors. The goals of the two entities in utilizing the scarce water are clearly opposing, and therefore appropriate terminology to describe real water-saving remains a central issue of debate [95].

Interactions between non-agricultural and agricultural water usages are scale-depen dent and play a major role in water-saving [208]. At basin scale, the main interest is to reduce water usage in irrigated agriculture and transfer water to other higher-valued usages. This again implies that actual water-saving can be achieved only by reducing evaporation and water-flows to non-recoverable sinks [107]. The basin approach, instead of paying attention to individual water usage, assesses return flows, estimates wateruse efficiencies at field- and basin-scales and differentiates consumptive water-saving from non-consumptive saving (Figure 2) while accounting for water and analyzing wateruse efficiencies [209–213]. Despite many complexities in perceptions of water-saving, its ultimate objectives are clear and undisputable: to stop unsustainable exploitation of the available water resources and to increase the quantity of water for other essential and more beneficial usages. It is therefore essential to understand the scale-effects of water usage clearly to improve water-savings and water productivity [124,210,214–216].

### *3.3. Impacts on Water Use*

AWD effect: Irrigation management through alternate wetting and drying is widely practiced in many countries/regions like the Philippines, Vietnam, China and EGP [217–220]. Under AWD, the percolation rate decreases leading to water-saving; the reduction in evapotranspiration plays only a minimal role [221]. Compared to the continuous standing water rice system, the levels of water-saving by the AWD method are listed in Table 3. Percolation from the crop fields controls the transport of nitrate [94], heavy metals [222], salts [223], nutrients [224], and pesticides [225] to groundwater. So, with reduced percolation the quality of groundwater remains under safeguard. The AWD method also reduces greenhouse gas emission [226,227], uptake of arsenic in rice grain [228,229], the cost of pumping water [230,231], and concentration of methyl mercury in field soil [232].

**Table 3.** Levels of water-saving by alternate wetting and drying (AWD) method compared to the continuous standing water rice system.

Bund effect: An unsaturated zone beneath standing water and a higher hydraulic conductivity zone beneath the bunds in rice fields are developed. This causes the applied irrigation water to move through the bunds and recharges the underlying aquifer [234]. The destinations of the applied irrigation in the rice fields were measured on several occasions e.g., [205,235–238] and a significant portion was reported to percolate through the field boundaries. This type of lateral seepage flow field is horizontal first and then vertical below the bunds [239]. Often rice fields of irregular shape are transformed into regular rice fields in order to improve irrigation efficiency, keeping part of the previously generated plow pan beneath the bunds of the reformed rice field [234]. Consequently, the dominant movement of water is in the horizontal direction through the bund. The seepage flux is, however, much less than the deep percolation rate [239–241] except when rice is cultivated on terraced fields, where the seepage water moves to the downstream plots through the bunds [239]. In flat rice fields, the infiltration rate below the bunds remains close to the average infiltration rate for the crop field with plow pan beneath the bunds, but may double or more without plow pan beneath the bunds [205,239]. [234] demonstrated 50% of water lost through the bunds, 25% through evapotranspiration, and 25% equally through infiltration providing an estimated annual water loss of 41 km3 through percolation underneath the bunds of rice fields in Bangladesh. Based on this field scale estimate, sealing of bunds (e.g., by puddling) can reduce seasonal water use by 52 ± 17%. Much greater savings (~90%) can be achieved in fields with larger perimeter-to-area ratio.

Puddling effect: Puddling eliminates large pores and alters the field soils to stratified layers: a top puddled layer, muddy layer and plow pan overlying a lower layer [242,243]. A low-permeable layer, formed above the puddled layer, comprises a finer fraction of the soils in suspension [244,245]. Puddling creates a 5 to 10-cm layer of plow pan, of low hydraulic conductivity, 20–25 cm below the ground surface. The hydraulic properties of plow pan regulate the water regime in the irrigated field [236–247]. Water flow occurs under unsaturated conditions below the plow pan [243]. The percolation rate varies widely with soil texture, 3–17 mm/day for clay and 13–30 mm/day for sandy loam [245,248]. The intensity [249] and depth of puddling [250], soil-type and post-puddling time period [251], and ponding water depth [252] regulate reduction of the percolation rate in the puddled soils. The percolation rate is high during the early growth period but decreases by 35–45% with the advance of the growth stage [253–255].

Re-bound effect: The re-bound effect, a less-known proposition, suggests that when efficiency of using a resource increases, its consumption rate also increases simultaneously [113]. Jevon's contradiction/paradox in economics advocates that any technologies aimed at saving energy actually end up by achieving the contrary of what they were supposed to do. Although the re-bound effect is quite well-known in energy usage [256], it is less known in the irrigation literature. Any intervention to modernize irrigation systems will improve efficiency, reliability and flexibility of the system, with a consequent increase in demand and consumption of water, especially by progressive farmers. The re-bound effect is therefore a potential problem in water resource management as recognized by [117].

Water-saving technologies are promoted based on the supposition that a reduction in water inputs per unit of output makes a comparable water-saving. However, this assumption may not be factual for two reasons. First, whether the quantity of water spared by reducing input transforms into real water-saving depends on the destination of the saved water. A significant part of the applied irrigation water percolates to the underlying aquifer, which can be pumped by the same or other farmers for reuse (Figure 1) and hence is not lost or wasted [212]. So, there is a risk of focusing on local efficiency alone and ignoring the return flows [126]. Secondly, based on economic theory [257], water-saving technologies, by adding more value to water, may encourage farmers to use more water as observed by [114] in Pakistan and Yemen where the overall water usage increased significantly [127,258]. Contrasting evidence is also found in the central United States where new technologies reduced water usage [74].

It is crucial to quantify water extracted and water consumed separately in order to effectively investigate the re-bound effect in irrigation. The usage of extracted water can comprise a consumed part and a non-consumed part. The consumed part may comprise both beneficial and non-beneficial evapotranspiration and runoff or percolation loss that are not recoverable. The non-consumed part comprises parts of the runoff and percolation that are recoverable for further use [213,259]. So, efficiency improvements do not always reduce overall water use; these actually reduce the effective cost of net irrigation encouraging the farmers to achieve more benefit by increasing net irrigation [115,260–262].

### **4. Regional Hydrology Outcomes**

Irrigation water is an important but as yet less characterized component of the hydrological cycle in regions with intensive agricultural irrigation, due to complexity in monitoring [263]. Appropriate differentiation of the natural inter-connection between the surface and groundwater resources is an impending problem [121]. In a highly connected hydrologic system (e.g., EGP), separate management of surface and groundwater will cause conflict in water resource allocation between various sectors (e.g., irrigation, households, industry and fisheries) and exert stress on groundwater-dependent ecosystems [121–123]. Groundwater is mostly a renewable resource in the IGP because of its recharge and depletion mechanisms associated with the regional hydrologic cycle. Water extracted from the aquifers can follow a number of pathways in the hydrologic cycle (Figures 3 and 4), with some travel only over a short distance, and may not join the aquifer [264,265]. Recharge to the aquifers occurs through rainfall, seepage and percolation from rivers and canals, and irrigation return flow [99], with rainfall and irrigation return flow remaining as the major contributors for many groundwater basins ([97,98,102]). So irrigation return flow that depends on soil hydraulic properties and irrigation management practices [266] is an important outcome of irrigated rice fields [96,100,267].

Abstraction of groundwater lowers the water table in aquifers with resulting reduction in groundwater pressure head that induces groundwater recharge by drawing down water from surface sources into aquifers [268,269]. Most rivers in the Bengal Basin, having direct hydraulic contact with aquifer systems [119,120] recharge the aquifers during March to November and receive water from the aquifers during December to February. These water exchange behaviors imply that groundwater tables can be deliberately lowered to more extent in the dry season to accommodate more recharge during the monsoon. This

intervention, first put forward in the 1970s [270] and then re-examined occasionally [271], will increase groundwater reserve for irrigation during the dry season and also help control flooding during the monsoon.

Percolation from irrigated rice fields is important to the economy, environment and water resource conservation in irrigated rice-dominated South Asian countries like Bangladesh, India and Taiwan. Flooded rice fields are comparable to wetlands [101,272] and play an important role in raising groundwater level [273]. The recharge potential of rice fields is 69.2 cm for sandy loam and 37.2 cm for clay loam in India [274], between 1–2 mm/day and 7.5 mm/day in Bangladesh [275], and 21.2–23.4% of the applied irrigation water from the terraced rice fields in northern Taiwan [239]. The groundwater-dominated irrigation in Bangladesh has changed the nature of aquifer recharge and the flow patterns of groundwater with a resulting reduction in residence time of water in the aquifer, especially in the shallow aquifers [276]. Recharge from the irrigation fields can be significantly modified by changes in irrigation management practices [77,118].

Adoption of agricultural water-saving technologies at the farm level changes cropwater use and regional hydrology [38] by reducing groundwater recharge. In many groundwater irrigated areas of the EGP (e.g., the North-West region of Bangladesh) the aquifers are not currently recharged fully from other sources (e.g., rainfall and interflow from adjacent aquifers). Consequently, water-saving technologies cause decreased opportunities for groundwater irrigation. There are other factors (e.g., canal lining, reduced water diversion, leveling undulating lands) that also reduce recharge by restricting percolation with eventual decline of groundwater tables. Some countries (e.g., China) widely use mulched-drip irrigation system, which significantly modifies the dynamics of regional groundwater by changing water exchange flux between the irrigation fields and underlying aquifer [39]. The exchange flux at the groundwater table during drip irrigation period is downward and remarkably reduces after adoption of water-saving technologies [39]. Adoption of efficient water-saving measures at regional scales would significantly restrict groundwater recharge with a consequent decline in groundwater levels [40]. This will exert negative impacts on regional hydrology and ecology by degrading soil quality and deforesting, particularly in arid regions [43]. With decades of large-scale groundwater withdrawal and reduced recharge opportunity due to increasing urbanization and decreasing wetlands, water tables have already declined significantly and are continuously declining in many large urban areas (e.g., Dhaka city in Bangladesh) over time [3]. There is, however, evidence of induced groundwater recharge due to the creation of significant vertical head gradients by increasing pumping in areas with shallow water tables and permeable upper soil formation [277]. This implies that dry season abstraction of groundwater can create storage space in the aquifer that can be utilized for harvest in the monsoon. Such intervention would exert a positive contribution on overall water availability in the area [131]. The main threat in the IGP Basin is not considered to be the diminished quantity of groundwater, but the degraded water quality resulting from high arsenic and salt contents [54].

### **5. Gaps in Current Knowledge**

### *5.1. Uncertainty in Water-Saving*

The reported impacts of conservation agriculture on water-saving are yet to be ascertained and evaluated more rigorously [278–282]. Water moves through very complex pathways and the impacts of conservation agriculture are so far understandable only at field-scale, but not at the larger scale [75]. Puddling forms plow pan and also creates soil cracks in addition to preferential flow paths. Consequently, increasing percolation, instead of commonly reported decreasing percolation, has been also reported [283]. In groundwater-based irrigation systems, improved irrigation efficiency and consequent water-saving achieved by reducing irrigation applications with water-saving technologies are clearly understood at the field-scale [115,116]. However, due to the lack of measurement of the water balance components, these are poorly understood at a larger spatial scale [75,106,116,125]. When farmers in a region reduce percolation substantially, which

would ultimately recharge a usable aquifer or join to a usable surface water body on the one hand but may also increase the irrigated area with the saved water on the other (Figure 4), the overall impact may be unintended. Instead of saving water, it can actually increase water consumption and reduce water availability for other users [95,116].

The growth period of rice with high evaporative demand can be avoided by shifting planting time. Adoption of short-duration varieties will also reduce evapotranspiration and percolation loss of water. The effects of these alternative crop technologies on water losses and crop yield have not been investigated adequately yet. If field-level estimates of water-saving are extrapolated to larger spatial scales in rice-based cropping systems that utilize recycled water or surface and groundwater conjunctively, there is a possibility of underestimating the real water-saving [284]. The concept of classical irrigation efficiency for an entire basin becomes erroneous and misleading when irrigation management is considered for the water resources of a region as a whole. The discrepancy arises since the water losses with respect to which the classical irrigation efficiency is calculated are not the actual water losses when considering the whole system. It is not possible to clearly know the extent of water-savings until the destination of the lost water is correctly known [95]. It is not yet clear how the water-saving technologies alter the dynamics of overall water balance. Whether application of water-saving technologies can maintain sustainable development and what else needs to be done for this in future are still major questions [39].

### *5.2. Limited Knowledge of Recharge–Discharge Interaction*

Groundwater recharge occurs from several sources (e.g., rainfall, flood water, irrigation return flow, inter-basin transfer, etc.) through several processes, the complexity of which varies widely. In an inefficient surface water irrigation system, a large fraction of the applied irrigation water percolates to the underlying aquifer, causing a significant loss of water when considering irrigation efficiency. However, this irrigation system appears as one of the most efficient methods of recharging groundwater, as occurs in most parts of Bangladesh, India, Pakistan and elsewhere [54,99]. So, the common perception of more efficient irrigation systems that can reduce seepage and percolation losses must be thought about with great caution.

A reliable quantification of groundwater recharge from irrigation fields, although essential in order to know its impending impacts on the dynamics and quality of groundwater, is difficult and remains unresolved in regions with confined aquifers. The groundwater table is confounded with both recharge from irrigation fields and extraction by irrigation wells. Many factors like soil type and surface condition, vegetation, depth to groundwater level, and chemical quality of soil and irrigation water control groundwater recharge. Although groundwater flow and recharge from rice fields have been examined on many occasions e.g., [101,246,285–287], the effects of land use conditions on recharge and groundwater level are not yet clear [288]. When groundwater is abstracted from an aquifer, recharge from surface sources occurs under transient conditions. The knowledge of soilwater flux in the vadoze zone that can help understanding the transient recharge [289] is still limited [290]. Therefore, a major pre-requisite for sustainable groundwater management is to reduce the uncertainty in aquifer recharge from rice fields.

### *5.3. Uncertain Causes of Groundwater Decline*

Large-scale withdrawal of groundwater, increased Boro rice cultivation, dry season reduction in river flow, reduction in wetland areas, declining annual rainfall, low recharge potentiality of soils, and lack of recharging of aquifers through artificial methods are regarded as the major barriers to sustainable groundwater use in the IGP basin [291]. These factors, in their various combinations, are causing decline in groundwater level in some regions in the EGP (e.g., North-West region of Bangladesh; [49]). In a groundwater irrigation system, reduced application of irrigation may be an effective way to check groundwater level depletion [292], although contrasting results were also reported [293–295]. These

contrasting opinions and observations raise valid questions of how far irrigation return flow contributes to groundwater recharge.

Field-level water-savings can make water use more profitable by increasing crop-water productivity and may lead to greater total water use in the basin [75,116]. Mere adoption of resource-conservation technologies cannot guarantee overall water-saving unless the usage of saved water can be controlled by proper policies and regulations. However, regionalscale study is still scarce for the evaluation of impacts of water-saving on evaporation and groundwater levels [296]. A proper policy to achieve stabilized groundwater levels must not consider only the adoption of technology and management of users' demand; recharging the aquifer artificially and finding alternative water sources, i.e., supply side management, is also necessary in some situations [64]. To establish sustainable levels of groundwater usage and achieve maximum benefit therefrom, investigation of the feasibility of combination of demand management, recharge improvement and alternative water supplies are crucial [297].

### *5.4. Inadequate Understanding of Scale-Effects*

Improved irrigation methods and conveyance systems are essential to increase efficiency of water use. However, water loss through deep percolation has the possibility of reuse in another region and the quality of percolated water may undergo changes during transmission through the hydrological units. It is therefore essential to account for the usages of surface water and groundwater, losses of water while being used, and interactions of various water components at the field scale and basin scale by adopting a system approach [67]. The common system approach of water accounting requires that, in closed basins, all lost water is presumed to be re-used somewhere downstream and hence any intervention to increase efficiency of water use would not make significant water-savings. So, there is hardly any scope for water-scarce regions to reduce water stress, especially through improvement in efficiency of water use. This approach has three major faults [298]. This disregards a major element of unproductive water use, values only new water without sufficiently considering water productivity in a broader aspect, and fails to account for several co-benefits arising from increasing efficiency of water use (e.g., upgraded water quality, increased reliability and less energy demand). Because of the complexity of the impacts of water-saving technologies at large scales, good approaches must integrate the conceivable spatial and temporal effects. Often a three-dimensional surface-groundwater interaction approach [299] is considered for this; but the problem remains as yet unexplored.

### *5.5. Weakness in Policy*

In the past, agricultural water management generally concentrated attention on irrigation options and water withdrawals from rivers and aquifers. Now it dedicates more attention to managing rainwater, evapotranspiration and water reuse, and views land-use decisions as water-use decisions [103]. In current perceptions of water management, considerable water-savings can be realized if the water-saving options are assessed in terms of technical, economic and institutional aspects and selections are made based on their efficacy [67]. Although technologies play a vital role in reducing water applications per unit of crop production, the re-bound effect is always a problem. If the increase in cultivated area of a certain crop, or even the irrigated area due to the re-bound effect, can be adequately known, the regional impacts of water-saving measures could also be scientifically explainable. However, restricting the demand of water is a challenging issue [75,127] with weak institutional arrangements. In the IGP, instability in the market price of agricultural products often guides the farmers to choose crops irrespective of the set policy. The performances of water-saving technologies contrast, and their adoption is a widely debated issue. Nonetheless, promoting water-saving technologies is a popular policy for governing groundwater in many countries (e.g., Bangladesh, India, China, Spain, Mexico, and the USA). Lack of attention, proper legislation, and ineffective or less-effective institutions are the main difficulties in governing groundwater in many least-developed and developing

countries [128]. In cases when aquifers extend across more than one independent country, groundwater governance becomes extremely complex [131].

When the groundwater table is very close to the surface (within capillary rise) the declining groundwater table can increase percolation rates by increasing the hydraulic gradient that would not have happened with a deeper groundwater table. It is speculated that this will offset the gains, at least to some extent, that the adopted water-saving technologies can offer. The recharge of shallow aquifers is therefore an important mechanism that needs to be well-understood for effective management of aquifers [300]. As the scale of water use extends, water loss increases, with resulting decrease in traditional irrigation efficiency. In contrast, water recycling increases with extending scale of water use, with eventual increase in net efficiency except when recycling is not feasible at the system level. This scenario of water usage suggests that the term 'irrigation efficiency' can lead policy planners to miscommunication and misunderstanding. While the problems of groundwater are clearly intuitive, the solutions are not. Enactment of wrong, flawed or misemployed concepts of efficiency in water-resource strategy and management can bring about many unexpected problems [78]. An example is the assumption that the rate of natural groundwater recharge is the safe yield of an aquifer [301]. This water budget myth ignores the factual possibility of increasing recharge and/or decreasing discharge from the aquifer due to groundwater extraction [199]. Our knowledge of the nature of interconnection between surface and groundwater systems over a large spatial scale is not yet adequate. Consequently, many water managers have been suffering in formulating strategy and establishments separately, rather than based on the linked inter-connection of surface water and groundwater. It is important that groundwater systems are treated as complex systems, which respond dynamically to abstraction-induced perturbation. A correct account of the vadoze zone in irrigation fields [302] can enable assessment of the impacts of change and of interventions to be prioritized [77].

Effective governance, although lacking in many countries, is a prerequisite for sound water resource management [129]. Because of existing political structures and systems, adopting a policy of restricting tube wells to reduce groundwater extraction in the IGP basin seems unrealistic. Several states in India have adopted regulations to prevent/minimize groundwater mining but could not implement these regulations totally [303,304]. In Bangladesh, reliable and detailed information on water reserves, safe yield, water withdrawal patterns and groundwater quality dynamics of aquifers is lacking [130]. These knowledge gaps have raised serious concerns about sustainable use of groundwater for irrigation, especially in the North-West region of the country [305]. Recently, emphasis has been placed on increasing dry season Boro rice production in the southern zone to reduce stress on groundwater use in the North-West region [306]. However, the viability of this approach remains to be cross-examined. The potential major restrictive factors are salinity problems of soil and water, weakness in synchronized water governance and the likely effects of climate change in the southern region [130,307,308]. In Bangladesh, there are specific problems in governing groundwater usage. The number of groundwater users is very large, most water users are resource-poor, and the institutional settings are mostly ineffective to ensure execution of laws and regulations. Under such a situation, enforcement of water rights and controlling access to groundwater by permit systems are probably not feasible options. A well-conceived rational and persistent strategy is appropriate for groundwater governance. Some prospective drivers of success may be engagement of users, refinements in water pricing structures, inspiring farmers to move from high to less water-demanding crops [53], in situ rainwater conservation, deficit irrigation, modifying rice–wheat areas [309], extensive investments in technology, and advancement of proactive policies and decision-making systems. Certainly, all these options will not be equally effective at all times and places since groundwater dynamics are localized; local countermeasures, such as managed aquifer recharge, can be implemented [9]. The best option(s) for governing groundwater at specific times and locations must be, however, identified through policy research [130].

Artificial recharge to aquifers through natural drains, canals and topographical depressions is a technically feasible and economically viable option [310] in the EGP. However, this option needs to be within a proper policy framework for its implementation. If groundwater-irrigated areas are not further increased, groundwater levels are expected either not to decline further or decline at much smaller rates than currently. With checked groundwater-irrigated areas, the other possibility is that groundwater levels will attain a new equilibrium that will be lower than at the current level. This proposition, yet to be considered in national policy, implies that the existing abstraction rates of groundwater can be continued and the presumed lower groundwater levels will not hamper the environment and economic and social developments [311]. However, these suggested potentials are only propositions and because of widely variable hydro-climatic, political and socioeconomic conditions among the affected regions no single solution will be adequate for groundwater management. The most logical strategy would be to select, from among the available options, regionally-suited strategies and establish strong regulation and policy for management of regional water resources [131–133]. Therefore, sustainable long-term strategies that are appropriate and adaptable for individual regions need to be recognized and exchange of knowledge and actions between regions must be established. Thus, establishing region-specific strategy and communication systems [134] will be important topics for future research in the IGP basin.

### **6. Summary and Conclusions**

Manifold attempts have been made in different regions of the world to increase food production for the rapidly growing population since the early 1960s. There has been great success in increasing food production globally but with a tremendous resulting pressure on the production-linked resources, specifically water and soil. The accelerating stress on these vital resources in the EGP raises sustainability concerns regarding agricultural production systems. Researchers and practitioners have been facing these challenges, both locally and regionally, over the last few decades. They have developed resourceconservation technologies as a response to concerns about agricultural sustainability, with basic principles of rebuilding the soil, optimizing inputs for crop production, increasing food production, and optimizing profits [84,86,87]. This review study has summarized the benefits of these technologies, and the scale-dependency and uncertainty of some of the benefits. Also identified are the gaps in current knowledge regarding the conceptual aspects of these technologies to make agriculture sustainable over a large regional scale so as to guide the future research in proper directions.

Of these resource-conservation technologies, conservation agriculture and water-saving measures are being practiced in many regions of the world, including the EGP [85,87]. Some benefits of these technologies, such as reduced energy and nutrients usage and reduced agrochemical leaching, are scale-invariant and intuitively clear [37,83]. However, the issue of water-saving remains uncertain at the system level since it is both a temporal and spatial scale-dependent element and linked to the regional hydrologic cycle [94,95]. Water saved at the farm level could otherwise join the groundwater or surface water systems to be used later by the same or other users [75,103]. Consequently, whether water-saving achieved at the farm level makes any real saving when considering the entire groundwater or river basin has not yet been adequately investigated. Furthermore, there is evidence of increasing demand for water after adding more value by technological interventions, such as increasing irrigation efficiency by adopting water-saving measures [114]; however, contrasting evidence has also been observed [74]. Whether or not the reduced extraction of groundwater, as well as reduced recharge, under resource-conservation technologies raise groundwater storage/groundwater level or reduce it remains unresolved [306]. Apparently, the reduced extraction of groundwater is expected to increase groundwater storage, but this likelihood is also uncertain since most aquifers in the Gangetic basin discharge to the rivers as base flow in the dry season. Thus, the current level of understanding of the complexity of the hydrological link to field-applied water is inadequate due to lack of

measured data on the components of regional water balance. Lack of shared knowledge on the impacts of resource-conservation technologies on regional water balance among the pertinent disciplines, such as agricultural production practitioners (e.g., agronomists, economists, irrigation engineers) and hydrologists (e.g., groundwater hydrologists, surface water hydrologists), is another drawback in planning and implementing holistic approach to investigate regional hydrology outcomes. This inadequate knowledge of inter-linked water systems may lead to the implementation of wrong policy [121–123] merely based on local perspectives with eventual worsening of the water-scarcity situation. Therefore, all pertinent disciplines should adopt integrated research approaches to measure the components of local and regional water balance and quantify regional hydrology outcomes over a large temporal scale. Only then proper water management policy can be planned and implemented for sustainable agricultural production.

**Author Contributions:** Conceptualization, M.A.M.; funding acquisition, M.M.; writing—original draft, M.A.M.; writing—review and editing, M.A.M. Both authors have read and agreed to the published version of the manuscript.

**Funding:** The study was funded by the Australian Centre for International Agricultural Research (ACIAR) under the project 'The regional hydrological impact of farm-scale water saving measures in the eastern Gangetic plains'.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors gratefully acknowledge the contributions of the authorities of Bangladesh Agricultural University (BAU) in Bangladesh and Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia in conducting this study. The BAU authority granted a 2-week leave to the principal author to visit CSIRO, which provided access to their electronic academic resources to facilitate conducting this study.

**Conflicts of Interest:** The authors declare to have no conflicts of interest.

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