In many regions of the world, large systems of shallow water bodies include hundreds and thousands of lakes and wetlands, integrated with climate- and human-controlled groundwater. In some cases, like in China and Mongolia, a dramatic reduction of areas and numbers of such lakes is observed. Disparity between spatial-temporal scales of each surface-water body and the entire system presents grand challenge to their modeling, and practical approaches are needed.
We explore feasibility of modeling for the Nebraska Sand Hills (48,000 km2), a large, grass-stabilized dune region containing thousands of small closed-basin lakes and wetlands in hydraulic connection with the Northern High Plains Aquifer. Instead of tracking lakes individually, we identify lakes as areas where the simulated regional water table exceeds land surface elevation DEM, and wetlands as areas where the water table is within 3 m from the land surface. Baseflow, the total area, and numbers of lakes and wetlands were simulated using a calibrated groundwater flow model.
Major driver of the water table-groundwater recharge (GR), critical to the existence of lakes and wetlands in the semi-arid climate was assessed as decadal averages of the differences between precipitation and evapotranspiration. Both modern GR and the 21st century GR scenarios were considered. Median, wet, and dry GR scenarios were obtained from sixteen downscaled Global Circulation Models and three greenhouse emission scenarios, accounting for uncertainty in forecasts.
Results indicate mild increase of lake and wetland numbers and total areas for median GR scenario by the end of century. More dramatic changes can be expected, if wet or dry GR scenarios will be realized. However, some counterintuitive relationships between different scenarios are possible at intermediate times. Results are consistent with studies of GR in the High Plains and indicate feasibility of proposed approach.
invited by Sven Frei, Hydrology
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