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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APPLICATION TO LAND-USE CHARACTERIZATION

The county-based classifications of row crops and orchards, vineyards, and nurseries are combined with information on the geographic distribution of agricultural land to improve the geographic characterization of agricultural activities. An example of the application of the classification system to the U.S. Geological Survey national LULC categories for cropland and pasture and for orchards, vineyards, and nurseries, is shown in figure 11 for the Lower Susquehanna River Basin NAWQA study unit. As discussed earlier, the LULC land-use data are the highest-resolution, nationally consistent data on land use presently available for the United States. Digital maps, such as the example in figure 11, can be derived for any regional area or river basin in the nation. As improved data on general land use or cropping patterns become available, the same approach can be applied to the new data.

For evaluating national patterns in agricultural land use and cropping patterns, the LULC data is too detailed and requires vast amounts of computer disk space and processing. The more generalized and lower-resolution map of "Major Land Uses of the United States" is used to show broad national and regional patterns (U.S. Geological Survey, 1970). Even though this major land uses map is now three decades out of date, general patterns at this scale have been relatively stable. Figure 12 shows the national analysis of agricultural land use derived from the national extent of cropland and the county-based row-crop classification (fig. 5). Compared to figure 5, in which the distribution of crops is based on counties, the map of classified cropland areas in figure 12 provides an improved geographic view of the nature and distribution of cropping patterns in the United States. The more generalized nature and lower resolution of the major land uses map compared to the LULC data is shown by the comparison of figures 11 and 13.

The classification system developed can potentially be used to evaluate long-term trends in cropping patterns by applying the system to past and future Census of Agriculture data, and to evaluate relations between chemical use (or other management practices) and regional crop patterns by correlating data on these practices with the crop categories. These and other applications will be explored in future investigations of specific water-quality issues.

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