Classification and Mapping of Agricultural Land for National Water-Quality Assessment

By Robert J. Gilliom and Gail P. Thelin

U.S. Geological Survey Circular 1131

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INTRODUCTION

The U.S. Geological Survey's (USGS) National Water-Quality Assessment (NAWQA) Program is designed to describe the status and trends in the quality of the nation's ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. The program integrates information about water quality at a wide range of spatial scales, from local to national, and focuses on water-quality conditions that affect large areas of the nation or occur frequently within small areas (Hirsch and others, 1988).

The building blocks of the NAWQA Program are Study-Unit investigations, which will be conducted in 60 major hydrologic basins (Study Units) of the nation (fig. 1). The Study-Unit investigations consist of intensive assessment activity for 3 years, followed by 6 years of less intensive monitoring, with the cycle repeated perennially (Leahy and others, 1990). Twenty Study Units will be in an intensive data-collection and analysis phase during each particular year, and the first complete cycle of intensive investigations of all 60 Study Units will be completed in 2002. The 60 NAWQA Study Units cover about one-half of the conterminous United States, encompass 60-70 percent of national water use and of the population served by public water supplies, and include diverse hydrologic systems that differ widely in the natural and human factors that affect water quality.

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The distribution of Study Units ensures that the most important regional and national water-quality issues can be addressed by comparative studies among Study Units. NAWQA National Synthesis projects combine results of Study Unit Investigations with existing information from other programs and studies of the USGS and other agencies and researchers to produce regional and national-scale assessments for priority water-quality issues. The first water-quality issues to be focused on for National Synthesis are nutrients and pesticides.

Comparative studies among study units and national or regional aggregations of data for National Synthesis require consistent data on factors that affect the sources, behavior, and effects of contaminants and other factors that determine water-quality conditions. Natural features, such as geology, and factors related to human activities, such as land-use distribution, provide an environmental framework for assessing influences on water quality in different hydrologic systems. The emphasis of NAWQA on large-scale water-quality issues affected by human activities makes land-use characterization one of the most important aspects in study design and in the analysis of causes of water-quality conditions. Land-use characterization based on a nationally consistent classification system provides a basis for comparing the influences of human activities among Study Units. It also serves as a framework upon which to add many other types of county-based data on human activities, such as waste discharge or chemical use, that are associated with land use.

The purpose of this paper is to describe a system for classifying agricultural land for national water-quality assessment. The system focuses on classification of agricultural land within the 48 contiguous states. Alaska and Hawaii were included in basic data analysis but omitted from classification results because of their unique characteristics. Agricultural land use is one of the most important influences on water quality at national and regional scales, particularly in relation to the first NAWQA National Synthesis topics, nutrients and pesticides. Considered nationwide, agricultural land has large areal extent, a high degree of land disturbance, and high use of agricultural chemicals and water. Individual areas of agricultural land, however, can vary widely in their characteristics. There is a great diversity of agricultural activities in the nation, which follow distinct regional patterns influenced by environmental setting and economics (Sommer and Hines, 1991). Different mixes of agricultural activities characteristic of particular regions can have widely varying influences on water quality because of differences in management practices and natural environmental setting.

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