Annual drought flow and groundwater storage trends in the eastern half of the United States during the past two-third century
Low flow drainage from a river system, in the absence of precipitation or snowmelt, derives directly from the water stored in the upstream aquifers in the basin; therefore, observations of the trends of the annual lowest flows can serve to deduce quantitative estimates of the evolution of the basin-scale groundwater storage over the period of the streamflow record. Application of this method has allowed for the first time to determine the magnitudes of the trends in groundwater storage over the past two-third century in some 41 large prototypical basins in the United States east of the Rocky Mountains. It was found that during the period 1940–2007 groundwater storage has generally been increasing in most areas; these positive trends were especially pronounced in the Ohio and Upper Mississippi Water Resources Regions, but they were weaker in most other regions. Notable exceptions are the northern New England and especially the South Atlantic-Gulf regions, which saw prolonged declines in groundwater levels over this nearly 70-year long period. These observed long-term trends are generally in agreement with previous studies regarding trends of other components of the water cycle, such as precipitation, total runoff, and terrestrial evaporation. Over the most recent 20 years, from 1988 through 2007, except for the Ohio and the Souris-Red-Rainy regions, most regions have experienced declining average groundwater levels to varying degrees, with maximal values of the order of −0.2 mm a−1.