Selected approaches to estimate water-budget components of the High Plains, 1940 through 1949 and 2000 through 2009
The High Plains aquifer, underlying almost 112 million acres in the central United States, is one of the largest aquifers in the Nation. It is the primary water supply for drinking water, irrigation, animal production, and industry in the region. Expansion of irrigated agriculture throughout the past 60 years has helped make the High Plains one of the most productive agricultural regions in the Nation. Extensive withdrawals of groundwater for irrigation have caused water-level declines in many parts of the aquifer and increased concerns about the long-term sustainability of the aquifer. Quantification of water-budget components is a prerequisite for effective water-resources management. Components analyzed as part of this study were precipitation, evapotranspiration, recharge, surface runoff, groundwater discharge to streams, groundwater fluxes to and from adjacent geologic units, irrigation, and groundwater in storage. These components were assessed for 1940 through 1949 (representing conditions prior to substantial groundwater development and referred to as "pregroundwater development" throughout this report) and 2000 through 2009. Because no single method can perfectly quantify the magnitude of any part of a water budget at a regional scale, results from several methods and previously published work were compiled and compared for this study when feasible. Results varied among the several methods applied, as indicated by the range of average annual volumes given for each component listed in the following paragraphs. Precipitation was derived from three sources: the Parameter-Elevation Regressions on Independent Slopes Model, data developed using Next Generation Weather Radar and measured precipitation from weather stations by the Office of Hydrologic Development at the National Weather Service for the Sacramento-Soil Moisture Accounting model, and precipitation measured at weather stations and spatially distributed using an inverse-distance-weighted interpolation method. Precipitation estimates using these sources, as a 10-year average annual total volume for the High Plains, ranged from 192 to 199 million acre-feet (acre-ft) for 1940 through 1949 and from 185 to 199 million acre-ft for 2000 through 2009. Evapotranspiration was obtained from three sources: the National Weather Service Sacramento-Soil Moisture Accounting model, the Simplified-Surface-Energy-Balance model using remotely sensed data, and the Soil-Water-Balance model. Average annual total evapotranspiration estimated using these sources was 148 million acre-ft for 1940 through 1949 and ranged from 154 to 193 million acre-ft for 2000 through 2009. The maximum amount of shallow groundwater lost to evapotranspiration was approximated for areas where the water table was within 5 feet of land surface. The average annual total volume of evapotranspiration from shallow groundwater was 9.0 million acre-ft for 1940 through 1949 and ranged from 9.6 to 12.6 million acre-ft for 2000 through 2009. Recharge was estimated using two soil-water-balance models as well as previously published studies for various locations across the High Plains region. Average annual total recharge ranged from 8.3 to 13.2 million acre-ft for 1940 through 1949 and from 15.9 to 35.0 million acre-ft for 2000 through 2009. Surface runoff and groundwater discharge to streams were determined using discharge records from streamflow-gaging stations near the edges of the High Plains and the Base-Flow Index program. For 1940 through 1949, the average annual net surface runoff leaving the High Plains was 1.9 million acre-ft, and the net loss from the High Plains aquifer by groundwater discharge to streams was 3.1 million acre-ft. For 2000 through 2009, the average annual net surface runoff leaving the High Plains region was 1.3 million acre-ft and the net loss by groundwater discharge to streams was 3.9 million acre-ft. For 2000 through 2009, the average annual total estimated groundwater pumpage volume from two soil-water-balance models ranged from 8.7 to 16.2 million acre-ft. Average annual irrigation application rates for the High Plains ranged from 8.4 to 16.2 inches per year. The USGS Water-Use Program published estimated total annual pumpage from the High Plains aquifer for 2000 and 2005. Those volumes were greater than those estimated from the two soil-water-balance models. Total groundwater in storage in the High Plains aquifer was estimated as 3,173 million acre-ft prior to groundwater development and 2,907 million acre-ft in 2007. The average annual decrease of groundwater in storage between 2000 and 2007 was 10 million acre-ft per year.