From the site: "This tabular data set represents the mean annual natural groundwater recharge, in millimeters, compiled for every MRB_E2RF1catchment of selected Major River Basins (MRBs, Crawford and others, 2006). The source data set is Estimated Mean Annual Natural Ground-Water Recharge in the Conterminous United States (Wolock, 2003).

The MRB_E2RF1 catchments are based on a modified version of the U.S. Environmental Protection Agency's (USEPA) ERF1_2 and include enhancements to support national and regional-scale surface-water quality modeling (Nolan and others, 2002; Brakebill and others, 2011).

This tabular data set was created and used by the U.S. Geological Survey (USGS) specifically to estimate the mean annual natural groundwater recharge, in millimeters, per watershed segment in the application of the national SPAtially Referenced Regression On Watershed attributes (SPARROW) model . The SPARROW model relates in-stream water-quality measurements to spatially referenced characteristics of watersheds, including contaminant sources and factors influencing terrestrial and stream transport. The model empirically estimates the origin and fate of contaminants in streams, and quantifies uncertainties in these estimates based on model coefficient error and unexplained variability in the observed data."