Mapping mean annual groundwater recharge in the Nebraska Sand Hills, USA

Mean annual recharge in the Sand Hills of Nebraska (USA) over the 2000–2009 period was estimated at a 1-km spatial resolution as the difference of mean annual precipitation (P) and evapotranspiration (ET). Monthly P values came from the PRISM dataset, while monthly ET values were derived from linear transformations of the MODIS daytime land-surface temperature values into pixel ET rates with the help of ancillary atmospheric data (air temperature, humidity, and global radiation). The study area receives about 73 mm of recharge (with an error bound of ±73 mm) annually, which is about 14 ± 14% of the regional mean annual P value of 533 mm. The largest recharge rates (about 200 ± 85 mm or 30 ± 12% of P) occur in the south-eastern part of the Sand Hills due to smoother terrain and more abundant precipitation (around 700 mm), while recharge is the smallest (about 40 ± 59 mm or 10 ± 14% of P) in the western part, where annual precipitation is only about 420 mm. Typically, lakes, wetlands, wet inter-dunal valleys, rivers, irrigated crops (except in the south-eastern region) and certain parts of afforested areas in the south-central portion of the study area act as discharge areas for groundwater.