Scientific Investigations Report 2007–5038
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5038
The approximately 370 mi2 Spokane Valley-Rathdrum Prairie (SVRP) aquifer straddles the Idaho-Washington state line northeast of the City of Spokane (fig. 1). Because the aquifer serves as the sole source of drinking water for more than 400,000 people and the area is experiencing rapid population growth, a better understanding of the aquifer characteristics 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 began in 2003 to gather data necessary to construct a ground-water flow model of the SVRP aquifer that both states could use to evaluate potential water-development and management alternatives.
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, but such values are commonly 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 have been used to estimate areal recharge, ranging from assigning a uniform recharge to the entire model domain to calculating evapotranspiration for each model cell. An examination of previously developed recharge-calculation methods and their limitations is therefore needed to allow the SVRP aquifer modeling team to select and apply the most appropriate technique.
This report describes four main methods for estimating areal recharge to the Spokane Valley-Rathdrum Prairie aquifer and provides recharge estimates for each method using data from weather stations in and near the study area. The limitations of each of the methods and the reliability of the recharge estimates are discussed. Because areal recharge is affected by soil hydrologic properties, those properties of the soils overlying the aquifer are briefly described in appendix A. The results of this analysis will be used by the SVRP aquifer modeling team to select an appropriate method for determining areal recharge for use in the flow model of the aquifer.
The Spokane Valley-Rathdrum Prairie aquifer underlies a relatively flat valley bottom with land-surface altitudes ranging from about 1,500 to nearly 2,600 ft. Ten lakes are located along the margins of the aquifer, the largest of which are Coeur d’Alene Lake and Lake Pend Oreille. Because the sediments of the valley floor are highly permeable, few distinct surface-drainage channels have developed other than the Spokane and Little Spokane Rivers.
Bedrock highlands of Precambrian metamorphic, Mesozoic and Cenozoic intrusive, and Tertiary basaltic rocks surround the valley. The valley is filled with Quaternary-age glaciofluvial sediments deposited during several catastrophic flood events from glacial Lake Missoula. The Spokane Valley-Rathdrum Prairie aquifer straddles the boundary between the Northern Rocky Mountain and Columbia Plateau physiographic provinces (Fenneman, 1931).
Ground water is the primary source for public-supply, domestic, irrigation, and industrial water use in the area, which led to the U.S. Environmental Protection Agency (USEPA) giving the aquifer “sole source” designation in 1978 (Kahle and others, 2005). Currently (2007), the aquifer is the source of drinking water for as many as 400,000 people.
A more complete description of the study area may be found in Kahle and others (2005).
Spokane has a mild, arid climate during the summer months but cold, coast-like conditions in the winter (U.S. Department of Commerce, 2005). Approximately 70 percent of the total annual precipitation falls between the first of October and the end of March and about one-half of that falls as snow. The growing season usually extends over nearly six months from mid-April to mid-October (U.S. Department of Commerce, 2005).
The entire study area is classified as Dsb under a modified Köppen system in which D indicates a mean temperature of the warmest month greater than 10°C (40°F) and of the coldest month 0°C (32°F) or less; s indicates that precipitation in the driest month of the summer half of the year is less than 40 mm (1.6 in.) and less than one-third of the precipitation amount in the wettest winter month; and b indicates that the mean temperature of each of the four warmest months is 10°C (40°F) or greater and the mean temperature of the warmest month is less than 22°C (72°F) (Critchfield, 1983; Godfrey, 2000).
The National Weather Service (NWS) has nine weather stations in or within 20 mi of the study area, although only six are active (table 1). Of these, only three are within or adjacent to the SVRP aquifer study area: Bayview Model Basin, Coeur D’Alene 1E, and Spokane Weather Service Office (WSO) Airport. Currently (2007), the closest AgriMet station to the study area is the Seven Bays Marina, Washington, (SBMW), near the confluence of the Spokane and Columbia Rivers, approximately 50 mi west of Spokane (Bureau of Reclamation, 2006).
Mean annual temperatures at the six active NWS stations in and near the study area range from 44.9°F at Bayview Model Basin to 48°F at Coeur D’Alene 1E and at Spokane WSO Airport. The coldest month in the area is January, with mean low temperatures ranging from 18.1°F at Priest River Experiment Station to 21.8°F at Coeur D’Alene 1E. The warmest month is July, with mean high temperatures ranging from 79.6°F at Bayview Model Basin to 85.3°F at Newport (table 2; Western Regional Climate Center, 2006a, 2006b). Mean annual precipitation ranges from 16.1 in. at Spokane WSO Airport to about 32 in. at Sandpoint Experiment Station (Western Regional Climate Center, 2006a, 2006b). July and August are typically the driest months and November, December, and January the wettest.
Of the six active stations, only Spokane WSO Airport collects meteorological parameters beyond temperature, precipitation, and snow depth. This limits the ability to calculate evapotranspiration and recharge at each of the other stations by methods requiring such data as wind speed or relative humidity. Table 3 summarizes the meteorological data necessary to calculate recharge by each of the four main methods discussed in this report.
None of the three currently (2007) active stations in or adjacent to the study area have mean maximum temperatures of less than 32°F for any month (table 2). Therefore, recharge probably occurs throughout the winter and no adjustment is needed to winter recharge timing due to melting snowpack.
Any calculation of areal recharge must account for evapotranspiration as well as precipitation. Because the methods discussed in this report use different definitions of evapotranspiration, short definitions are given here:
Potential evapotranspiration (PET) is the “evapotranspiration rate of short, actively transpiring vegetation (e.g., grass) that: completely covers the ground; is well-supplied with water; and exerts negligible resistance to water movement through the plant” (Tindall and others, 1999). If sufficient water were available, PET indicates the amount of water that could undergo evapotranspiration. PET is a climatic parameter because it denotes the evaporation power of the atmosphere. In order to determine actual evapotranspiration, a crop coefficient must be included.
Consumptive use (or actual evapotranspiration), as defined in U.S. Department of Agriculture (USDA; 1970), is “the amount of water used by the vegetative growth of a given area in transpiration and building of plant tissue and that evaporated from adjacent soil or intercepted precipitation on the plant foliage in any specified time.”
Reference crop evapotranspiration (ET0) is the evapotranspiration rate from a reference surface (a hypothetical grass reference crop with specific characteristics) that is not short of water. It also denotes the evaporation power of the atmosphere and thus is a climatic parameter (Allen and others, 1998). As with PET, in order to determine actual evapotranspiration, a crop coefficient must be included.
Crop evapotranspiration under standard conditions (ETc) is the evapotranspiration rate from “disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions” (Allen and others, 1998).
Crop evapotranspiration under non-standard conditions (ETc adj) is the evapotranspiration rate from fields with less than optimal environmental factors such as “pests and diseases, soil salinity, low soil fertility, water shortage or water logging” (Allen and others, 1998).
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