Scientific Investigations Report 2007–5041
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5041
Inflow from highlands and side valleys adjoining the Spokane Valley Rathdrum Prairie (SVRP) aquifer has been the least examined component of the SVRP ground-water budget. Published estimates are few and span a wide range. Drost and Seitz (1978) estimated the total inflow to the aquifer to be about 1,320 ft3/s. Of this total inflow, 323 ft3/s (about 25 percent) was estimated as ground-water inflow from adjoining highlands and side valleys that do not have a lake at the valley mouth. By contrast, Buchanan’s (2000) model does not include ground-water inflow from adjoining highlands and side valleys. A model of the Spokane Valley completed by CH2M HILL (1998) was calibrated using values of 40 ft3/s (autumn 1994 conditions) and 79 ft3/s (spring 1995 conditions) for ground-water inflow from adjoining highlands and side valleys.
Estimates of the mean annual discharge from adjoining highlands and side valleys were obtained using existing regional regression equations developed by the USGS (Hortness and Berenbrock, 2001). These regression equations can be used to estimate the mean annual discharge at ungaged sites on streams in Idaho and in parts of adjacent states that are unaffected by regulations and (or) diversions. The equations were developed using mean-annual-discharge values from long-term gaging stations in the area. The equations relate the mean annual discharge to various physical and climatic characteristics (basin characteristics) of the upstream drainage basin.
Estimates of the mean annual discharge at ungaged locations on streams near the point where they flow onto the SVRP aquifer were obtained by determining the required basin characteristics for the upstream drainage basins and inserting those values into the appropriate regression equation. In the case of the SVRP aquifer, nearly all surface flows from adjoining highlands and side valleys quickly infiltrate into the highly permeable sediments overlying the aquifer. Therefore, estimates of surface flows at locations just prior to where the streams flow onto the SVRP aquifer are assumed to be reasonable estimates of inflow to the aquifer from those streams.
Both ArcGIS-ArcHydro tools (Environmental Systems Research Institute, Inc., 2005) and the USGS StreamStats web application (Ries and others, 2004) were used to obtain the estimates of mean annual discharge. The estimation approaches are identical except for the resolution of the digital elevation model (DEM) used. All highlands and side valleys in Idaho were analyzed using ArcGIS-ArcHydro tools where a 10-meter resolution DEM was available. Because the 10-meter DEM was not available for the Washington area of interest at the time of the analyses, the USGS StreamStats application was used to complete the analyses for the highlands and side valleys in Washington using a 30-meter DEM.
Estimated discharges for 72 basins (fig. A1) adjacent to the SVRP aquifer are shown in table A1. Hortness and Berenbrock (2001) presented a standard error of estimate for each of the regression equations used. Depending on the location of the sites used in this analysis and their corresponding regression-equation region (fig. 2; Hortness and Berenbrock, 2001), the standard errors of estimate for the final results are as follow: Region 1, +57.4 to -36.5 percent; Region 2, +56.5 to -36.1 percent; and Region 3, +18.1 to -15.3 percent. It is important to note that because many of the mean annual discharge estimates are relatively small, even a large percentage of error will not necessarily result in a large-magnitude error range around the value.
Hortness and Berenbrock (2001) stated that the results might not be reliable for sites where the basin-characteristic values are outside of the range of values that were used to develop the equations. Because several of the locations where estimates were obtained had basin-characteristic values, most often drainage area, outside of this range, additional analyses and hydrologic judgment were used to determine whether the final estimates were reasonable. The analyses included a simple comparison with estimates of mean annual discharge per unit area obtained from long-term gaging station data from similar sites in the area.
The results are not reliable for sites where streamflow is affected by upstream diversions and (or) regulations, or by significant spring inflows (Hortness and Berenbrock, 2001). Because most of the highland and side valley areas are relatively undeveloped and produce relatively low amounts of streamflow, it is assumed that diversions and regulations are minimal or nonexistent. Inflow from springs is more likely to occur but is difficult to quantify. Thus, it is important to note that spring inflows in certain highland or side valley areas could result in larger discharge values than those estimated using the regression equations.
Buchanan, J.P., 2000, Unified groundwater flow model of the Rathdrum Prairie-Spokane Valley aquifer system: Prepared for Water Quality Management Program, Spokane County Public Works and Idaho Division of Environmental Quality: Cheney, Eastern Washington University, 23 p.
CH2M HILL, 1998, City of Spokane wellhead protection program phase I-Technical assessment: CH2M Hill report, for the City of Spokane Wellhead Protection Program, 2 vols., variously paginated, 12 appendices.
Drost, B.W., and Seitz, H.R., 1978, Spokane Valley-Rathdrum Prairie aquifer, Washington and Idaho: U.S. Geological Survey Open-File Report 77-829, 79 p., 10 pls.; accessed March 2007 at http://pubs.er.usgs.gov/usgspubs/ofr/ofr77829.
Environmental Systems Research Institute, Inc., 2005, ArcGIS desktop help: accessed June 12, 2005 at http://webhelp.esri.com/arcgisdesktop/9.1.
Hortness, J.E., and Berenbrock, Charles, 2001, Estimating monthly and annual streamflow statistics at ungaged sites in Idaho: U.S. Geological Survey Water-Resources Investigations Report 01-4093, 36 p.; available online at http://pubs.er.usgs.gov/usgspubs/wri/wri014093.
Ries, K.G., Steeves, P.A., Coles, J.D., Rea, A.H., and Stewart, D.W., 2004, StreamStats: A U.S. Geological Survey web application for stream information: U.S. Geological Survey Fact Sheet FS-2004-3115, 4 p.; available online at http://pubs.er.usgs.gov/usgspubs/fs/fs20043115.
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