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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2009–5030

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Description of Study Area

The DR2 catchment is a small (5.5 km²) subbasin in the Yakima Valley of central Washington (fig. 1). A detailed description of the catchment is available in Payne and others (2007). Three characteristics of the catchment particularly relevant to the transport and fate of agricultural chemicals are: (1) irrigation practices, (2) crop heterogeneity, and (3) subsurface drainage. These characteristics are discussed here briefly.

Land use in the DR2 catchment is nearly 90 percent agriculture, and, with less than 18 cm of annual precipitation, 95 percent of the agriculture is irrigated. The demand for irrigation water is met by withdrawals from the Yakima River that are delivered to the catchment through the Sunnyside Canal (fig. 1) and dispersed throughout the catchment by secondary delivery canals. At the individual field scale, water is applied to crops by various methods, including rill, drip, and sprinkler systems. Although the use of rill irrigation is slowly diminishing in the catchment as growers convert to more efficient sprinkler and drip systems, it remains the predominant method due to economic, planting, harvesting, and other crop management considerations that make more efficient systems less practical.

A wide variety of crops are grown in the DR2 catchment (Payne and others, 2007). During the study period, crops and other agricultural activities included corn, grapes (for both wine and juice), asparagus, alfalfa, forage grass, pasture, and dairy/feedlot operations. Not only do crops—and therefore irrigation practices and chemical applications—vary considerably across this small catchment, they also vary from year to year in many fields.

Shallow subsurface flow in the area has been modified by an extensive system of buried drains. These drains exert a strong influence on the shallow ground-water system and its connection to surface water. However, most of the drainage system has been in place for many decades, and apart from the large, regional drains, the location and extent of the drains are not well known.

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