MODELING GROUND-WATER RECHARGE FROM PRECIPITATION—DEVELOPMENT AND IMPLEMENTATION OF THE GSFLOW MODEL

Flow and storage in the unsaturated zone commonly are neglected when assessing ground-water recharge in humid areas, because the unsaturated zone is assumed to be too thin to store much water. Consequently, most ground-water flow models in humid areas apply recharge directly to the saturated zone. Changing land use and ground-water development have increased concerns regarding ground-water interaction with surface water and have led to more sophisticated techniques to model spatial- and time-varying recharge. GSFLOW is a ground-water/surface-water flow model being developed by the USGS that couples the Precipitation Runoff Modeling System (PRMS) with the Modular Ground-Water Flow Model (MODFLOW). The coupling is via a new soil-moisture zone module in PRMS and a new unsaturated-zone flow package in MODFLOW. GSFLOW simulates spatial- and time-varying recharge on the basis of daily energy and mass balances among precipitation, solar radiation, heat, evapotranspiration, runoff, infiltration, and storage in the snowpack and soil zone, and percolation and storage through the unsaturated zone. Fluxes are computed dynamically among PRMS hydrologic response units and corresponding MODFLOW cells. GSFLOW has a modular design that allows for the use of existing and future PRMS modules and MODFLOW packages.

GSFLOW is being tested on a 27 km² watershed underlain by volcanic rocks in the Sierra Nevada near Truckee, California. Land-surface altitude ranges from 1,925 to 2,800 m. Mean precipitation ranges from 89 cm/yr near the basin outlet to 122 cm/yr at the highest elevations. Mean runoff is 40 cm/yr. Most precipitation occurs from November through March in the form of snow. The snowpack normally begins melting in March at the lower elevations and the melt progresses to higher elevations during April through June. The simulation period is from 1981 through 1997 and includes large variations in precipitation and snowmelt. The simulation of ground-water recharge in lowland areas, where the unsaturated zone is thin or nonexistent, results in a rapid streamflow response. Whereas, the simulation of increased recharge in upland areas following years of above normal precipitation, where the unsaturated zone is more than 20 m thick, results in a slow and gradual increase of ground-water discharge to streams.