A numerical flow model of the Spokane Valley-Rathdrum Prairie aquifer currently (2006) under development requires values for areally-distributed recharge, often the most uncertain component of water budgets and models. Data from six active weather stations in and near the study area were used in four recharge-calculation techniques: (1) the Langbein method, in which recharge is estimated on the basis of empirical data from other basins; (2) a U.S. Department of Agriculture (USDA) method, in which crop consumptive use, effective precipitation, and actual precipitation yield an estimate of recharge; (3) an approach developed for the Eastern Snake Plain Aquifer Model (ESPAM) Enhancement Project in which recharge is calculated on the basis of precipitation-recharge relations from other basins; and (4) an approach in which recharge is calculated from reference evapotranspiration using the Food and Agriculture Organization (FAO) Penman-Monteith equation and determination of crop consumptive use by a single or dual coefficient approach.

Mean annual recharge calculated by the Langbein method for the six weather stations was 4 percent of mean annual precipitation. Mean monthly recharge calculated by the USDA method ranged from 53 to 73 percent of mean monthly precipitation. Three separate calculations of mean monthly recharge were made with the ESPAM method: for thin-soil parameters, recharge ranged from 10 to 29 percent of mean monthly precipitation; for thick-soil parameters, recharge ranged from 1 to 5 percent of mean monthly precipitation; for lava-rock parameters, recharge ranged from 37 to 57 percent of mean monthly precipitation. Mean monthly recharge calculated using single-coefficient FAO Penman-Monteith parameters ranged from 0 to 81 percent of mean monthly precipitation and yielded a mean monthly recharge of zero during the eight warmest and driest months of the year. Monthly recharge calculated with dual-coefficient parameters and 1990-2005 daily values ranged from 0 to 94 percent of monthly precipitation.