ESTIMATING TRANSIENT GROUNDWATER RECHARGE USING THE SOIL-WATER BALANCE METHOD FOR THE ELKHORN-LOUP MODEL AREA, NORTH-CENTRAL NEBRASKA

Water managers and other decision-makers may consider analyses performed using groundwater-flow models or other approaches when evaluating management alternatives. Such analyses are sensitive to estimated recharge, which varies spatially and temporally and can be difficult to measure. Groundwater recharge is defined here as water that infiltrates past the root zone and that may be available to underlying aquifers. The soil-water balance (SWB) method developed by the U.S. Geological Survey incorporates weather data and landscape properties in geographic information system format to calculate recharge. Daily precipitation and temperature grids are interpolated from weather-station data, and these are used to calculate daily water input and evapotranspiration, respectively. Landscape properties grids include the soil hydrologic group and available water capacity (from Natural Resources Conservation Service soil surveys), land use, and surface flow direction. Thickness of the root zone, curve number, and maximum recharge for each hydrologic soil group are stored for each land use in a look-up table. Daily runoff, interception, infiltration, potential and actual evapotranspiration, rejected recharge that becomes runoff due to saturated soil, and recharge are calculated for each cell in the grid, and each of these values are saved in grid format. Recharge can be calibrated by adjusting the landscape parameters. Recharge estimated with SWB can be used for inputs to surface-water and groundwater models, estimating water budgets, and analyzing impacts of climate change on groundwater resources. Output is in grid format that can be imported directly into a MODFLOW groundwater model or parsed by subarea to provide an important component of the water budget. A case study region is located in north-central Nebraska, where regional groundwater-flow modeling is ongoing for the Elkhorn-Loup Model area. Results will be used in parameter estimation for model calibration. Preliminary results for the transient simulations for the period of 1985-1995 show spatial and temporal variations in recharge, demonstrating the utility of the SWB method at regional scales. Preliminary recharge results agree with expected values, though they have not yet been compared with model time steps.