DESCRIPTION OF REQUEST:

Mr. Stover requested that we provide him with the required numbers for his district’s management plan using the groundwater availability model (GAM) for the northern part of the Gulf Coast aquifer (Kasmarek and Robinson, 2004; Kasmarek and others, 2005). METHODS: We analyzed the GAM results for average recharge from precipitation, average surface-water inflow, average surface-water outflow, average inflow into the district, average outflow from the district, average net interaquifer flow (upper), and average net inter-aquifer flow (lower) using the period 1980 to1999. MODFLOW’s general head boundary (GHB) package was used to simulate all recharge and discharge in the GAM for the northern part of the Gulf Coast aquifer. The GHB package simulates flow into and out of the model using water-level elevations and the conductance of the aquifer materials. The amount of water that recharges the aquifer from rainfall, irrigation return flow, and river leakage and water that naturally discharges from the aquifer in the form of baseflow to streams and evapotranspiration are all accounted for in the model with the GHB package. Evapotranspiration is the water that is lost out of the aquifer due to direct evaporation from the water table (when water table is shallow) and plant transpiration. To calculate surface water interaction, we assumed the uppermost block or GHB cell in the model with a river, stream, spring, lake, or reservoir represented groundwater/surface-water interactions. All other cells using the GHB package were assumed to represent net recharge from precipitation. To address the request, we: ran the GAM for the northern part of the Gulf Coast aquifer and extracted water budgets for each yea