| Lakes are integral parts of groundwater flow systems in many environments. Yet, few exceptions exist in which large numbers are simulated in regional settings, due to data and computational constraints. The first modeling study of this kind was performed in the Nebraska Sand Hills, the largest (~50,000 km2) grass-stabilized dune region in the Western Hemisphere, containing thousands of small closed-basin lakes (and wetlands) in hydraulic connection with the High Plains aquifer. In the semi-arid climate, groundwater recharge is critical to the high water table and strong discharge that maintains lakes, streams, and wetlands---features providing habitat for many unique species, productivity of a large beef cattle industry, and important to salt dust emissions. Co-evolution of the ecosystems, hydrologic processes, and regional climate over the past several hundred years has created a self-sustaining system, but a system that if disturbed too greatly may not persist. In the context of 21st century climate change, individual lake dynamics at annual time scales are of limited interest. Future groundwater recharge scenarios were developed from decadal changes in the difference between precipitation and evapotranspiration from 16 Global Circulation Models and three emissions scenarios. The central tendency is for minor changes, with a much larger uncertainty range. Vadose zone modeling of steady vertical flow shows large spatial variation of soil moisture lag times in the extant climate, and slight changes under future conditions, with an average around five years, shorter than the decade-centennial time scale of interest. A numerical groundwater flow model was developed, calibrated, and used to simultaneously simulate large-scale aquifer behavior, stream baseflows, and the spatial distribution, and total area and numbers of the many scattered lakes and wetlands. Simulated hydraulic heads were downscaled for comparison with the fine-resolution digital elevation model. Response time of groundwater-controlled surface water features suggests that climate change impacts are greatly delayed and dispersed by the groundwater system, but non-linearly dependent upon groundwater recharge regime. |
