ESTIMATING GROUNDWATER RECHARGE THROUGH A HETEROGENEOUS VADOSE ZONE FOR THE CENTRAL HIGH PLAINS AQUIFER USING NUMERICAL MODELING

Despite the importance of groundwater sustainability in the Central High Plains aquifer, relatively few studies have focused on the rate of groundwater recharge for the aquifer under natural and irrigation conditions. As part of a 2000 United States Geological Survey study, two boreholes were drilled through thick unsaturated zones (~150 ft) underlying irrigated agriculture land-use area (USGS designation CAL 121 and CAL 122). Measured soil properties for the heterogeneous (layered) vadose zone, statistically estimated hydraulic retention parameters, and measured atmospheric data were used as input to the numerical model Hydrus 1-D to estimate groundwater recharge rates. Using this method, we obtained average annual recharge rates of 1.54 in/yr at CAL 121 and 0.95 in/yr at CAL 122. These rates are within the range of recharge estimates reported by others previously. Gutentag et al (1984) estimated regional mean annual recharge rates between 0.05 and 1.85 in/yr. Sophocleous et al (2002) used soil-moisture measurements combined with a Darcian flux approach to calculate recharge rates of 0.01 in/yr at CAL 121 and 0.02 to 0.04 in/yr at CAL 122. The USGS High Plains Groundwater Study analyzed tritium and atrazine data and determined that recharge was larger (7 to 8 in/yr) for irrigated fields. Reasons for the differences between the estimates will be discussed. The utility of unsaturated zone numerical modeling is that it provides dynamic assessments of aquifer recharge for varying hydrologic and soil conditions and management strategies. These results are useful for assessing sustainable groundwater levels, as well as for evaluating potential aquifer vulnerability to the many agricultural and industrial contaminants in the High Plains region.