WATER MOVEMENT THROUGH THE UNSATURATED ZONE OF THE HIGH PLAINS AQUIFER IN THE CENTRAL PLATTE NATURAL RESOURCES DISTRICT, NEBRASKA, 2008-2012
Measurements of total water potentials indicated that periodic wetting fronts reached greater maximum depths in the unsaturated zone beneath the irrigated sites than beneath the rangeland sites. The depth of the interface between the pre-bomb/post-bomb 3H and a 3H mass balance within the soil profiles were used to estimate water fluxes that ranged from 10 to 140 millimeters per year (mm/yr). The Cl– mass-balance method yielded a range of water fluxes below the root zone of 4.3 to 21 mm/yr at rangeland sites. Concentrations of NO3– in pore water in the unsaturated zone were larger beneath the irrigated and dryland sites compared to beneath the rangeland sites; NO3– and Cl– concentrations profiles were similar. Movement of Br– through the unsaturated zone indicated greater water fluxes occur beneath irrigated lands than beneath rangeland. Recharge rates determined from age dates of samples from the shallowest wells generally were similar to those determined from the 3H and Cl– mass-balance methods, whereas recharge rates determined from age dates from samples from the deeper wells likely resulted from a regional flow system that receives recharge from distant sources.