Scientific Investigations Report 2006–5274

Scientific Investigations Report 2006–5274

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Salmon Creek, a tributary to the Okanogan River (fig. 1), contains two reservoirs that supply irrigation water for out-of-basin use. The irrigation project, called the Okanogan Project, was authorized by the U.S. Department of the Interior in 1905 and has been in operation since the early 1900s. Conconully Reservoir is formed by Conconully Dam (fig. 2), which was completed in 1910, and Salmon Lake Reservoir (also referred to as Conconully Lake in other publications) is formed by Salmon Lake Dam, which was completed in 1921. The Bureau of Reclamation (Reclamation) owns the dams and the project is operated by the Okanogan Irrigation District (OID). As a result of streamflow diversions, the lower 4.3 miles of Salmon Creek typically is a dry creek bed except during the spring snowmelt season during years of high runoff (Dames and Moore, 1999). To continue meeting the water needs of irrigators but also to leave water in lower Salmon Creek for fish passage and to help restore the natural ecosystem, changes in how the Okanogan Project is operated currently (2006) are being considered. In preparation for the potential changes, a draft environmental impact statement that considers different management alternatives was issued in August 2004 (U.S. Department of Energy, 2004).

Reclamation plans to develop a water-operations model to study the water resources of the Salmon Creek Basin and asked the U.S. Geological Survey (USGS) to develop a precipitation-runoff model that will provide input data for the water-operations model. Both models are components of a Decision Support System (DSS) for the Salmon Creek Basin that will be similar to the DSS that Reclamation and the USGS developed previously for the Yakima River Basin in central southern Washington (Mastin and Vaccaro, 2002b; U.S. Geological Survey, 1998). In addition to the two models, the DSS consists of a Hydrologic Database (HDB) that is an interface between the models. The HDB provides daily time-series input to the precipitation-runoff model (described in this report) and receives daily streamflows simulated by the model. The streamflows then are provided as input to the planned RiverWare model (Bureau of Reclamation and CADSWES, 2000), a water-operations planning model that will be used to simulate different water-operations options for the available streamflow. The DSS for the Yakima River Basin was developed as part of the Watershed and River Systems Management Program (WARSMP), a federally-funded program under which Reclamation and the USGS have collaborated since 1995 (U.S. Geological Survey and Bureau of Reclamation, 2006). WARSMP’s purpose is to develop, test, and implement a framework for water-resources management in Reclamation Act states.

The objective of this study was to develop the precipitation-runoff-model component of the DSS. The precipitation-runoff-model component can be used to simulate historical daily unregulated streamflows for different locations in the Salmon Creek Basin and to forecast daily unregulated streamflows for the runoff season (April–July) or longer periods as far as 1 year in the future. Forecasting is based on near-real-time hydrologic conditions in the basin and an assumption that historical climate records will recur with the same probability in the future as in the past. Reclamation plans to use the simulated historical daily unregulated streamflows for long-term-planning studies and may use forecasted daily streamflows to assist with seasonal planning of water operations.

Purpose and Scope

This report documents development of a precipitation-runoff model for the Salmon Creek Basin in Okanogan County, Washington, that can be used to simulate daily unregulated streamflows in the basin. Unregulated streamflows are defined as streamflows unaltered by human activities such as streamflow diversions and impoundments in reservoirs. The precipitation-runoff model that was developed is a modified version of the Precipitation-Runoff Modeling System (PRMS; Leavesley and others, 1983) and was run within the Modular Modeling System (MMS; Leavesley and others, 1996). The modified model version that was used is identical to that used previously to simulate streamflow in the neighboring Methow River Basin (Ely, 2003; Ely and Risley, 2001). Input data used to run the model are based on daily precipitation and daily minimum and maximum air temperatures for three National Weather Service stations (Conconully, Omak 4 N, and Winthrop 1 WSW), a Natural Resources Conservation Service SNOTEL station (Salmon Meadows), a Bureau of Reclamation AgriMet station (Omak OMAW), and a Bureau of Reclamation Hydromet station (Conconully CCR). The model was used to simulate daily streamflows that were aggregated on a monthly basis and calibrated against historical monthly streamflows for Salmon Creek at Conconully Dam. Historical monthly streamflows were estimated by Dames and Moore (1999) and the U.S. Department of Energy (2004) and as part of this study from monthly outflows from Conconully Reservoir and from storage changes in Conconully Reservoir and Salmon Lake Reservoir. Additional calibration data were provided by the snowpack water-equivalent record for the Salmon Meadows SNOTEL station. The precipitation-runoff model was calibrated for water years 1950–89 (a water year starts October 1 and ends September 30) and tested for water years 1990–96.

Description of Study Area

The Salmon Creek Basin is located in the northeastern part of the Cascade Range (fig. 1) and encompasses an area of 152 mi2. Elevations in the basin range from 820 to 8,250 ft and have a mean of 4,050 ft. Upper Salmon Creek Basin, defined as the area upstream of Conconully Dam, encompasses 78 percent of the basin. Elevations in the upper basin range from 2,270 to 8,250 ft and have a mean of 4,450 ft. Much of the upper basin is located in the Okanogan National Forest and is largely vegetated with evergreen forests of Ponderosa pine (Pinus ponderosa) and Interior Douglas-fir (Pseudotsuga menziesii) (Cassidy, 1997). The study area is sparsely populated. Conconully, the only town in the basin, had a population of 185 in 2000 (U.S. Census Bureau, 2006).

The two reservoirs in the basin, Conconully Reservoir and Salmon Lake Reservoir (fig. 2), have a maximum active storage capacity of about 13,000 and 10,500 acre-ft, respectively (U.S. Department of Energy, 2004). (Active storage capacity is defined as the usable reservoir capacity available for seasonal or cyclic water storage.) Most of the water stored in the reservoirs is spring snowmelt runoff that is released later in the season or in subsequent years for irrigation purposes. The Okanogan Project currently supplies irrigation water to about 5,000 acres of agricultural land outside Salmon Creek Basin along the Okanogan River near the town of Okanogan (Bureau of Reclamation, 2006b).

Salmon Lake Dam allows the level of the natural Salmon Lake to be raised. Water in the reservoir behind the dam consists of runoff from the Salmon Lake subbasin and water that is diverted from North Fork Salmon Creek at the North Fork diversion (fig. 2). Releases from Salmon Lake Reservoir flow into Conconully Reservoir, which also receives runoff from West Fork Salmon Creek and any runoff from North Fork Salmon Creek that is not diverted to Salmon Lake Reservoir. South Fork Salmon Creek joins West Fork Salmon Creek about 0.6 mi west of Conconully Reservoir. Seepage, releases, and uncontrolled spills from Conconully Reservoir flow about 12 mi downstream along the main stem of Salmon Creek to the OID diversion (fig. 2) from where water is diverted outside the basin through a series of canals. Any water not diverted flows another 4.3 mi and then discharges to the Okanogan River. Since the early days of the Okanogan Project, the streambed between the OID diversion and the mouth of Salmon Creek has been dry except during those spring-runoff events when uncontrolled spills occurred at Conconully Dam (Dames and Moore, 1999).

Mean annual precipitation in the Salmon Creek Basin based on PRISM data from the Spatial Climate Analysis Service-Oregon State University (2006) during the most recent climate-normal period (1971–2000) was about 21 in. and ranged from about 12 in. near the mouth of the basin to almost 33 in. near the crest (fig. 3). In the upper basin, mean annual precipitation was about 23 in. and ranged from 15.4 in. to almost 33 in. (Spatial Climate Analysis Service-Oregon State University, 2006). About 58 percent of mean annual precipitation occurs from October through March. Minimum precipitation in the basin occurs in September, and maximum precipitation occurs in November (fig. 4A). A secondary precipitation maximum occurs in May. The snowpack that accumulates in the basin during the winter melts off by early summer. The interannual variability in precipitation in the basin is large. For example, based on the National Weather Service climate record for the Conconully station, for which some missing data were estimated in this study, the mean annual precipitation for 1971–2000 was about 15 in. The minimum was 7.8 in. in water year 1979, and the maximum was 26.3 in. in water year 1983 (fig. 4B). The mean minimum air temperature for the Conconully station ranged from about 15 to 53°F, and the mean maximum ranged from about 31 to 82°F (fig. 5). On average, the coldest month of the year is January and the warmest month is August.

The large amount of interannual variability in precipitation in the basin results in a large amount of interannual variability in runoff. For example, based on information provided by Dames and Moore (1999), the U.S. Department of Energy (2004), and T. Sullivan (Okanogan Irrigation District, written commun., 2006), the mean annual runoff from upper Salmon Creek Basin for water years 1949–2004 is estimated to be about 23,300 acre-ft (about 32 ft3/s). The minimum annual mean was 4,400 acre-ft (6 ft3/s) in water year 1966, and the maximum annual mean was 65,800 acre‑ft (91 ft3/s) in water year 1983. The mean annual runoff from upper Salmon Creek Basin is about equal to the total maximum active storage capacity of Conconully and Salmon Lake Reservoirs. During most years, the majority of annual runoff from the upper basin occurs from April through July because of melting of the snowpack that accumulated during the previous autumn and winter.

Most of the Salmon Creek Basin is in steep terrain that consists predominantly of Mesozoic intrusive and metamorphic rocks and Mesozoic-Paleozoic metamorphic rocks (Schuster, 2005). In parts of the lower basin and near the center, including the areas surrounding Conconully and Conconully Reservoir, sedimentary deposits of Pleistocene continental glacial drift comprise the surface. Most of lower Salmon Creek runs along the trace of a fault (Schuster, 2005) that has formed a narrow valley. Based on the topography and surficial geology of the area, at some time in the geologic past, lower Salmon Creek may have drained to the southeast of Conconully Reservoir into what are now Scotch and Johnson Creeks instead of into the present channel (fig. 2).

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