Deep unsaturated-zone boreholes (55 to 160 feet to ground water) at nine locations in Colorado, Kansas, Nebraska, and Texas, were instrumented to evaluate recharge fluxes to the High Plains aquifer. Recharge fluxes were estimated using Darcy flow calculations, environmental tracers (chloride, tritium, pesticides), and numerical modeling. These techniques integrate data over different time scales and provide information on the temporal variability in recharge fluxes.

The boreholes are located in native rangeland (three sites) and irrigated agricultural (six sites) land-use settings. Latitudinal variations in climate, geology, crop type, and irrigation practices allow comparison of water and chemical fluxes for a range of environmental settings. Recharge fluxes measured at the rangeland sites were generally less than 1 millimeter per year. Mean annual recharge fluxes in irrigated settings ranged from about 20 to 40 millimeters per year. A component of preferential flow also was present at some of the irrigated sites and appeared to occur at rates of 100 to 200 millimeters per year. The variability in flux rates under the irrigated settings has important implications with respect to vertical chemical transport beneath irrigated fields.