DATA CONSIDERATIONS AND MODELING A MEGA-SYSTEM OF GROUNDWATER-FED LAKES IN THE NEBRASKA SANDHILLS (Invited Presentation)

Studies of large lake-aquifer systems with numerous lakes (e.g. in the USA, Canada, China) encounter challenges of data availability, modeling ability, and their fusion compared to those considering just a few lakes in small watersheds. Coupling of surface and subsurface domains must be done with adequate spatio-temporal resolution of fluxes to reproduce modern and predicted future hydrodynamics of lake-aquifer systems. Although data availability limits the potential of integrated watershed models, data with coarser resolution but larger spatial coverage can be of interest. Data on precipitation and runoff, land-based water-table maps and test hole data, surface DEMs and remote sensing-based data on evapotranspiration are available in many areas of the world. A new approach integrating such data allows the essential system responses to be captured by limiting model complexity and matching the overall water budget. This approach was applied to the Nebraska Sandhills, the largest vegetated sand-dune region in the Western Hemisphere. Large groundwater recharge rates of the High Plains aquifer in these areas contribute to a shallow water table, creating thousands of shallow lakes in inter-dune areas that depend upon the topography-controlled groundwater flow patterns. A MODFLOW-based model was developed using inputs from a regional water-table map, a 30-m digital elevation model, and net groundwater recharge rates inferred from satellite with 1-km resolution. The model has been calibrated to hydraulic heads while honoring total baseflow and satellite-derived recharge estimates as well as reasonably matching total areas of lakes and wetlands from a Landsat survey (10.9% overestimate; 2083 km² vs 1879 km²). Lakes and wetlands are inferred from simulations where groundwater levels exceed or are near the land surface, respectively. This approach avoids using hydrologic data at the lake scale. Results of the simulations are appropriate when conclusions are needed about total area of numerous groundwater-fed lakes or wetlands, including effects of long-term impacts of climate and land use changes.