Using cumulative potential recharge for selection of GCM projections to force regional groundwater models: A Nebraska Sand Hills example

Groundwater recharge (GR) controls vegetation, geomorphology, groundwater, wetlands and surface flow, and ultimately, the ecology and economics of semi-arid regions. Therefore, it is critical to assess hydroclimate model scenarios and the uncertainty in future GR to force regional groundwater models. We use basic statistics of downscaled Global Circulation Model (GCM)-projected cumulative potential GR (GR_p) for selecting representative projections. Cumulative GR_p is the net recharge (difference between precipitation (P) and evapotranspiration (ET) rates) over the projection period. The approach is illustrated with an example in the Nebraska Sand Hills (NSH), the largest dune region in the Western Hemisphere, where sandy soils are not conducive to overland flow. Changes in decadal-average GR_p at 1/8° (∼12-km) scale were estimated from spatially downscaled, bias-corrected temperature and P output from 16 commonly used GCMs for years 2010 to 2099. These changes accounted for three greenhouse gas emissions scenarios, and projections were subsequently used as input to the Variable Infiltration Capacity (VIC) land surface hydrology model. For each of the 48 GCM/VIC hydroclimate projections, cumulative GR_p was calculated and averaged over the study area. Three projections (those with cumulative average GR_p nearest the median and ±1 standard deviation) were selected as representing Median, Wet and Dry conditions. These projections allow for rapid screening of the sensitivity of regional groundwater models, using readily available downscaled GCM-projected climate changes, thereby optimizing modeling efforts. Future GR_p was calculated for the NSH using the selected GR projections by adjusting the 2000–2009 baseline GR_p estimates at 1-km scale. The latter was inferred from a previously calibrated groundwater model, with ET based on remote sensing (MODIS) temperature data, and matching regional baseflows. In the NSH by 2099, the Median projection indicates an increase in GR_p of 3 mm/yr (+5%) relative to the 2000–2009 baseline of 52.6 mm/yr. The Wet projection has an average increase of 22 mm/yr (+42%), and the Dry projection shows an average decrease of 15 mm/yr (−29%), relative to the baseline. Effects of projection period duration and time-step averaging on selection of GR projections with this approach are discussed. The new detailed GR_p projections clarify varying trends of past large-scale analyses of the Northern High Plains region and indicate the possibility for substantial future changes in the NSH hydrologic system. This approach can be extended to other arid-to-humid regions with available GCM hydroclimate projections.