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

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Introduction

Agriculture is widely recognized as a major source of nutrients and other potential contaminants to water bodies throughout the world. In an assessment of the effects of human activity on water quality, the U.S. Environmental Protection Agency (2002, 2005) listed agricultural practices and hydrological modifications as the two most important causes of water-quality impairment to streams assessed during the 2000 National Water-Quality Inventory. Because irrigation can extensively modify the hydrology of small catchments, these findings indicate that irrigated agriculture may be of particular importance as a source of water-quality impairment.

In the Western United States, between one-quarter and one-third of harvested cropland is irrigated (Schaible, 2004). Due to the nature of the crops and relatively high yields, however, this irrigated cropland accounts for approximately three-quarters of the value of crops sold in the West (Gollehon and Quinby, 2000). It is estimated that the global demand for food will more than double over the coming decades (Green and others, 2005), and it is likely that much of the increased demand will be met by irrigated agriculture.

In the Western United States, irrigation is typically applied to naturally arid or semiarid landscapes, where few natural surface drainage routes exist. To keep such areas agriculturally productive, artificial surface and shallow subsurface drainage networks often are constructed to facilitate runoff. In many of these networks, virtually all water carried by the drains is agricultural runoff, and little or no dilution from naturally occurring ground-water base flow or storm-generated surface runoff occurs. As a result, in addition to the diffuse nonpoint-source contamination typically associated with agriculture, drainage networks in irrigated areas can act as point sources of contaminants to receiving streams and lakes.

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).

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