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Circular 1131

National Water-Quality Assessment Program

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

By Robert J. Gilliom and Gail P. Thelin

Abstract

Agricultural land use is one of the most important influences on water quality at national and regional scales. Although there is great diversity in the character of agricultural land, variations follow regional patterns that are influenced by environmental setting and economics. These regional patterns can be characterized by the distribution of crops. A new approach to classifying and mapping agricultural land use for national water-quality assessment was developed by combining information on general land-use distribution with information on crop patterns from agricultural census data. Separate classification systems were developed for row crops and for orchards, vineyards, and nurseries. These two general categories of agricultural land are distinguished from each other in the land-use classification system used in the U.S. Geological Survey national Land Use and Land Cover database.

Classification of cropland was based on the areal extent of crops harvested. The acreage of each crop in each county was divided by total row-crop area or total orchard, vineyard, and nursery area, as appropriate, thus normalizing the crop data and making the classification independent of total cropland area. The classification system was developed using simple percentage criteria to define combinations of 1 to 3 crops that account for 50 percent or more or harvested acreage in a county. The classification system consists of 21 level I categories and 46 level II subcategories for row crops, and 26 level I categories and 19 level II subcategories for orchards, vineyards, and nurseries. All counties in the United States with reported harvested acreage are classified in these categories. The distribution of agricultural land within each county, however, must be evaluated on the basis of general land-use data. This can be done at the national scale using "Major Land Uses of the United States," at the regional scale using data from the national Land Use and Land Cover database, or at smaller scales using locally available data.

For additional information contact:
Director, California Water Science Center
U.S. Geological Survey
6000 J Street, Placer Hall
Sacramento, California 95819
http://ca.water.usgs.gov/

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Suggested citation:

Gilliom, R.J., and Thelin, G.P., 1997, Classification and Mapping of Agricultural Land for National Water-Quality Assessment: U.S. Geological Survey Circular 1131, 70 p.



Contents

Abstract

Introduction

Classifying Agricultural Land for National Water-Quality Assessment

Existing Classification Systems and Data Sources

Classification of Agricultural Land by Crop Distribution

Application to Land-Use Characterization

Summary and Conclusions

References Cited


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