A BIOGEOCHEMICAL (BGC) PROCESS MODEL APPROACH FOR ESTIMATING MOUNTAIN RECHARGE TO INTERMONTANE BASIN GROUNDWATER SYSTEMS

Precipitation that infiltrates mountain terrain often comprises the major input of groundwater to intermontane groundwater basins in arid regions. Improved constraints on the patterns and magnitude of recharge from adjacent mountain areas will benefit basin groundwater investigations and modeling efforts, and aid in management of groundwater and surface water resources. This project seeks to assess if commonly available datasets used in conjunction with a climate and landscape driven ecosystem model will provide a realistic tool for basin water resource managers to estimate mountain mass groundwater recharge at the basin scale. This method makes use of a numerical biogeochemical process model, Biome-BGC (Running and Hunt, 1993) in conjunction with the mountain climate simulator MT-CLIM (Running et al., 1987) and a field measured hydrologic and hydrogeologic dataset, for the estimation of the mountain front component of recharge to the Lower Ruby Valley, Montana groundwater basin. Biome-BGC simulates complex interactions between meteorology, vegetation, soil, and hydrology and calculates the water, energy and nutrient cycles of terrestrial ecosystems. Biome-BGC has undergone over a decade of model validation and improvement and has been shown to accurately predict the timing surface hydrologic response (Coughlan and Running, 1997; Running, 1994; White et al., 1998; Kremmer and Running, 1996). Biome-BGC utilizes a simplistic 1-layer bucket model for soil water flux; the BGC-model recharge method calculates the recharge component of the water budget as: mountain groundwater recharge=modeled soil water flux – gaged surface water flux. Preliminary results for a 5.6 km grid of the 24,600 Ha study area show the BGC-model recharge method provides reasonable values of recharge when averaged annually given the gaged surface water flux. Further refinement of model resolution as well as soil and vegetative properties are expected to produce a more refined mountain recharge estimate. Evaluation of the recharge estimate will be further provided by a 3-dimensional groundwater flow model calibrated to field measured water levels, surface water gains and losses, aquifer K, as well as bedrock configuration provided by a basin inversion gravity model.