Scientific Investigations Report 2007–5038
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5038
Nearly all of the Spokane Valley-Rathdrum Prairie aquifer is overlain by two STATSGO soil mapping units: the Kootenai-Bonner-Rathdrum unit mostly north of Rathdrum and near Hayden Lake and the Garrison-Avonville-Marble Variant unit in most of the remainder of the study area (fig. A1; Soil Survey Staff, 2006a, b). Virtually all the soil series comprising these mapping units are classified in hydrologic soil groups A and B (rated on a scale from A to D, ranging from high values of saturated hydraulic conductivity and deep free-water occurrence, (A), to moderately low saturated hydraulic conductivity and shallow free-water occurrence, (D), (Weisel, 1981)). Thus, precipitation may be expected to pass readily into the root zone and beyond. Measured mean vertical permeability ranges from 1.3 to greater than 40 feet per day for both units. Golder Associates, Inc. (2004) reported that during calibration of their integrated ground-water/surface-water model of the Middle Spokane watershed, a uniform change of all soils in the model to type D (thus decreasing vertical hydraulic conductivity by approximately four orders of magnitude) decreased mean weekly recharge by about 50 percent.
By way of comparison, Garabedian’s (1992) ESRP thin soil infiltration rate group contained hydrologic soil groups A–D, but were primarily of groups A and B. His thick soil group contained hydrologic soil groups B-D, but were primarily of groups B and C.
Contor (2004) assumed that the rangeland plants of the ESRP had root systems developed throughout the entire soil profile. Because water in the root zone is available for evapotranspiration, the increased time necessary for water to pass through a thick soil results in less recharge than for a thin soil with the same hydrologic properties. Because most of the series comprising the two STATSGO mapping units in the study area are described as “very deep,” no effort was made in the current report to differentiate soil depths.
Available water capacity values for the soil mapping units range from 0.01 to 0.6 in/in (10-600 mm/m) (Natural Resources Conservation Service, 2006a, 2006b, 2006c).
Figure A1. Major soil mapping units in the study area.
U.S.
Department of the Interior | U.S. Geological
Survey
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