Summary and Conclusions

The Spokane Valley-Rathdrum Prairie (SVRP) aquifer, which straddles the Idaho-Washington state line northeast of the City of Spokane, is the sole source of drinking water for more than 400,000 people. The area is experiencing rapid population growth, and a better understanding of the characteristics of the aquifer and its interaction with the Spokane River is necessary to guide the development and management of the resource. To this end, a multi-year study started in 2003 to gather data and construct a ground-water flow model of the SVRP aquifer that both states can use to evaluate water-management decisions.

Currently (2007), a ground-water-flow model that simulates ground-water/surface-water interaction is under construction by a joint team from the Idaho Department of Water Resources, University of Idaho, Washington State University, and the U.S. Geological Survey. This model requires values for areally-distributed recharge from precipitation, however, such values commonly are the most uncertain components of water budgets and ground-water flow models because it is virtually impossible to measure recharge over large areas. In previous flow models of the SVRP aquifer, various approaches or techniques were used to estimate areal recharge, ranging from assigning a uniform recharge to the entire model domain to calculating evapotranspiration for each model cell.

The National Weather Service has nine weather stations in or within 20 miles of the study area, although only six are active, and only three are within or adjacent to the SVRP study area. The data from these weather stations were used in the calculations of area recharge via several methods or approaches.

Four potential approaches were identified for determining areal recharge for the SVRP ground-water flow model:

A method (Langbein method) developed in which recharge is estimated on the basis of empirical data from other basins;
A method (USDA method) developed in which crop consumptive use and effective precipitation are first calculated and then subtracted from actual precipitation to yield an estimate of recharge;
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
An approach in which reference evapotranspiration is calculated by the Food and Agriculture Organization (FAO) Penman-Monteith equation, crop consumptive use is determined (using a single or dual coefficient approach), and the amount of water which passes through the plant root zone (and thus becomes recharge) is calculated.

Mean annual recharge calculated with the Langbein method for the six active weather stations was 4 percent of mean annual precipitation, yielding the lowest mean annual values of the methods discussed in this report. The primary shortcoming of the Langbein method is that it can only be applied to annual time periods.

Mean monthly recharge calculated with the USDA method ranged from 53 to 73 percent of mean monthly precipitation. Mean annual recharge ranged from 64 to 69 percent of mean annual precipitation. Because the USDA effective rainfall equation was derived empirically using data from throughout the United States, it is difficult to evaluate how applicable the method is to the SVRP study area.

Mean monthly recharge calculations by the ESPAM method were made using thin-soil, thick-soil, and lava-rock parameters. The lava-rock parameters yielded the highest recharge values and the thick-soil parameters the lowest. For thin-soil parameters, calculated monthly recharge ranged from 10 to 29 percent of monthly mean precipitation and annual recharge ranged from 16 to 23 percent of mean annual precipitation. For thick-soil parameters, calculated monthly recharge ranged from 1 to 5 percent of monthly mean precipitation and annual recharge ranged from 2 to 4 percent of mean annual precipitation. For lava-rock parameters, calculated monthly recharge ranged from 37 to 57 percent of monthly mean precipitation and annual recharge ranged from 45 to 52 percent of mean annual precipitation. Because the ESPAM method equations were derived from previous work on the ESRP, for a given precipitation (P), paired slope parameter (K), and coefficient for curvature (N), recharge calculated by the ESPAM method will be equivalent to that calculated for the ESRP.

Single-coefficient FAO Penman-Monteith mean monthly recharge values were calculated for the Spokane WSO Airport station: grass-referenced values of mean monthly recharge ranged from 0 to 81 percent of mean monthly precipitation and mean annual recharge was 21 percent of mean annual precipitation; alfalfa-referenced values of mean monthly recharge ranged from 0 to 85 percent of mean monthly precipitation and mean annual recharge was 24 percent of mean annual precipitation. The single-coefficient FAO Penman-Monteith equations used with mean monthly values yield mean monthly recharge values of zero during the eight warmest and driest months of the year (March-October). Such a result seems unlikely based on ground-water levels.

For all stations the lava-rock parameters yield the highest values of mean monthly recharge and the thick-soil parameters the lowest among the ESPAM techniques. USDA mean monthly recharge values are higher than any ESPAM values for all months. For the Spokane WSO Airport station, the single-coefficient FAO Penman-Monteith mean monthly recharge values are highest in the winter and lowest during the growing season.

The dual-coefficient FAO Penman-Monteith dual-crop evapotranspiration (ET_cd) and deep percolation calculations were applied to daily values from the Spokane WSO Airport for January 1990 through December 2005. The resultant monthly totals show a temporal variability lacking in the single-coefficient mean monthly values and demonstrate that the daily amount and timing of precipitation dramatically affect calculated recharge. For the remaining five weather stations, 1990-2005 daily recharge was calculated using wind speed from the Spokane WSO Airport station and assuming that dewpoint was equal to the daily minimum temperature. For all six weather stations dual-coefficient FAO Penman-Monteith monthly recharge ranged from 0 to 94 percent of monthly precipitation.

Because areal recharge is often the most uncertain component of water budgets and ground-water flow models, it is often calculated as the residual of other components. Without a priori knowledge of probable values of areal recharge, choosing between values of recharge calculated by different methods is a challenging decision. Thus, the larger context provided by water budgets and ground-water flow model calibration is crucial in determining reasonable values.

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