Scientific Investigations Report 2007–5007

Scientific Investigations Report 2007–5007

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Surface water in the Yakima River Basin, in south-central Washington (fig. 1) is under adjudication and the amount of surface water available for appropriation is unknown, but there are increasing demands for water for municipal, fisheries, agricultural, industrial, and recreational uses. These demands must be met by ground-water withdrawals and/or by changes in the way water resources are allocated and used. On-going activities in the basin for enhancement of fisheries and obtaining additional water for agriculture may be affected by ground-water withdrawals and by rules implemented under the Endangered Species Act for salmonids that have been either listed or were proposed for listing in the late 1990s. An integrated understanding of the ground-water flow system and its relation to the surface-water resources is needed in order to implement effective water-resources management strategies in the basin. In order to develop this understanding, a study of the Yakima River Basin aquifer system began in June 2000. The study is a cooperative effort of the U.S. Geological Survey (USGS), Bureau of Reclamation (Reclamation), the Yakama Nation (YN), and the Washington State Department of Ecology (WaDOE).

The overall objectives of the study are to fully describe the ground-water flow system and its interaction with and relation to surface water, and to provide baseline information for a management tool—a numerical model. The conceptual model of the flow system and the results of the study can be used to guide and support actions taken by management agencies with respect to ground-water availability and to provide information to other stakeholders and interested parties. The numerical model will be developed as an integrated tool to assess short-term to long-term management activities, including the testing of potential management strategies.

The study includes three phases. The first phase includes (1) project planning and coordination, (2) compiling, documenting, and assessing available data, and (3) initial data collection. The second phase consists of data collection to support the following Phase 2 work elements: (1) mapping of hydrogeologic units, (2) estimating ground-water pumpage, (3) developing estimates of ground-water recharge, (4) assessing ground water-surface water interchanges, and (5) constructing maps of ground-water levels. Together, these five elements provide the information needed to describe the ground-water flow system, develop the conceptual model, and provide the building blocks for the hydrogeologic framework. In the third phase, six structural basin models and one regional model of the ground-water flow system will be constructed in order to integrate the available information. The numerical models will be used to gain a further understanding of the flow system and its relation to surface water, and to test management strategies.

The results from selected work elements will be described in a series of reports. This report describes the estimates of ground-water recharge to the Yakima River Basin aquifer system, a Phase 2 work element. Recharge is a major control on the flow of ground-water in the aquifer system and on water availability. The estimates of recharge provide information on a little known, but important, water-budget component in the study area. Spatial and temporal estimates of recharge also are needed as input to the ground-water flow models being constructed as part of this study.

Purpose and Scope

This report presents estimates of mean annual ground-water recharge to the Yakima River Basin aquifer system for predevelopment and current land-use and land-cover (LULC) conditions. Predevelopment conditions are an estimate of LULC conditions prior to any human activities in the basin, and current conditions are representative of a recent multi-year (1995–2004) composite.

The report describes the data and methods used to estimate recharge, the factors controlling recharge, and the potential sources of error in the estimates. For this report, recharge is defined as deep percolation past the bottom of the root zone, or in the case of bare soils as deep percolation through the bottom of the mapped soil depth, herein called the soil zone. Recharge is derived from snowmelt, incident rain on the land surface (including throughfall), irrigation of croplands, and septic-system drainfields.

Hydrologic models have proved to be efficient tools for estimating long-term daily average recharge to an aquifer system under predevelopment and current LULC conditions. Recharge to the Yakima River Basin aquifer system was calculated using two different models for the complete system (18 areas or watersheds covering 6,207 mi2) for predevelopment LULC conditions. Predevelopment conditions were estimated by changing the LULC with large human influences to a LULC of sagebrush. Current-condition recharge was modeled for 16 areas with large human influences, such as irrigated agriculture, high-density commercial or residential use, and urban land. Thirteen of these 16 areas were the same areas modeled for predevelopment conditions.

Four previously developed watershed models (Mastin and Vaccaro, 2002a) were used to calculate daily values of recharge for upland areas for water years (WY) 1950–98. A soil water-budget model was used to calculate recharge for WYs 1950–2003 for lowland areas with large human influences. The daily values were then aggregated to calculate monthly, annual, mean monthly, and mean annual values. This report principally describes the mean annual values.

Previous Estimates of Recharge

The most comprehensive previous estimates of recharge to the Yakima River Basin system were made by Bauer and Vaccaro (1990) for the parts of the study area underlain by the Columbia River Basalt Group using the Deep Percolation Model (DPM) (Bauer and Vaccaro, 1987). The DPM was applied to 15 areas or watersheds in the basin, which included most of the irrigated lands in the basin. These areas were generally divided into 1-mi square cells for model simulations. Recharge was estimated for both predevelopment and current LULC conditions for climatic conditions during the period 1956–77. Mean annual recharge for predevelopment conditions was estimated to range from about 0.23 to 10.65 in. for the 15 areas. For current conditions, mean annual recharge was estimated to have increased by more than 15 in. in some of the areas irrigated by surface water.

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