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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CLASSIFYING AGRICULTURAL LAND FOR NATIONAL WATER-QUALITY ASSESSMENT

Classification and characterization of agricultural land use for national water-quality assessment requires a balance among attributes that often conflict. The primary criteria that governed the development of the classification system described in this circular are summarized below:

1. Relevance to Management Practices - Agricultural land-use categories should be defined with the greatest possible relevance to evaluating water-quality effects in relation to management practices, such as irrigation and chemical use. Categories need to be defined specifically enough that the linkage between regional water-quality effects and management practices can be evaluated.

2. Appropriate Scale - Categories must be either regionally or nationally significant in their extent, and thus have large contiguous areas that can be isolated as distinct land uses that can be focused upon for surface- and ground-water studies. Comparative studies of agricultural areas for national water-quality assessment will involve areas ranging widely in size, but most typically 100-10,000 square miles. Many of the most important comparative assessments, however, will be based on studies of areas of less than 1,000 square miles and categories must be defined specifically enough that important distinctions at this scale are evident.

3. Transferability Among Scales - Definitions and criteria for agricultural land-use categories should be suitable for making consistent and comparable classifications over a broad range of scales, using data of varying spatial resolution.

4. Stability Over Time - Criteria for defining categories should remain consistent over time to facilitate comparison to historical records and to enable updates to reflect new information and support future comparisons.

5. Practicality - The classification system must be applicable to any part of the nation, using available data that are nationally consistent and sufficiently up-to-date to reflect present-day land-use conditions.

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Next - Existing Classification Systems and Data Codes

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