Effect of agricultural practices on hydrology and water chemistry in a small irrigated catchment, Yakima River Basin, Washington

Abstract

The role of irrigation and artificial drainage in the hydrologic cycle and the transport of solutes in a small agricultural catchment in central Washington’s Yakima Valley were explored using hydrologic, chemical, isotopic, agedating, and mineralogical data from several environmental compartments, including stream water, ground water, overland flow, and streambed pore water. A conceptual understanding of catchment hydrology and solute transport was developed and an inverse end-member mixing analysis was used to further explore the effects of agriculture in this small catchment. The median concentrations of major solutes and nitrates were similar for the single field site and for the catchment outflow site, indicating that the net effects of transport processes for these constituents were similar at both scales. However, concentrations of nutrients were different at the two sites, suggesting that field-scale variations in agricultural practices as well as nearstream and instream biochemical processes are important components of agricultural chemical transformation and transport in this catchment. This work indicates that irrigation coupled with artificial drainage networks may exacerbate the ecological effects of agricultural runoff by increasing direct connectivity between fields and streams and minimizing potentially mitigating effects (denitrification and dilution, for example) of longer subsurface pathways. 2 Effect of Agricultural Practices on Hydrology and Water Chemistry, Yakima River Basin, Washington In the work presented here, the hydrologic cycle and the transport of solutes in a small, irrigated catchment in the Yakima Valley of central Washington are described on the basis of the results of analyses of various hydrologic, chemical, isotopic, age dating, and mineralogical data from several environmental compartments, including stream water, ground water, overland flow, and streambed pore water. A conceptual model of catchment hydrology and solute transport is developed and an inverse end-member mixing analysis is used to further explore the effects of agriculture in this small irrigated catchment. These data were collected as part of a larger effort by the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) program. To gain insights into how environmental processes and agricultural practices interact to determine the transport and fate of agricultural chemicals in the environment, the USGS NAWQA program conducted in-depth investigations at five agricultural study areas across the country during 2003–04. The design of the overall study is described in detail by Capel and others (2008). Companion studies report on the transport and processing of agricultural chemicals in surface water (Domagalski and others, 2008; Duff and others, 2008); transport in the subsurface (Green and others, 2008; Steele and others, 2008), and ground-water/ surface-water interactions (Duff and others, 2008; Essaid and others, 2008; Puckett and others, 2008). 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.