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Correlation of annual variations in water quality and agricultural practices for subbasins of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia

American Geophysical Union
By Douglas A. Harned and Gerard McMahon

Abstract

To define the degree of association between agricultural practices and stream water quality annual variation in water-quality data for 66 small watersheds in the Albemarle-Pamlico drainage area in North Carolina and Virginia were correlated with annual variation in basin agricultural statistics. The data examined were collected from 1970 to 1993 by the U.S. Geological Survey, the North Carolina Department of Environment, Health and Natural Resources, and the Virginia Department of Environmental Quality.

Annual measures of crop acreage and fertilizer sales were compiled to represent agricultural activities; however, these variables are highly intercorrelated. Principal components analysis can be used to combine several intercorrelated variables into a smaller number of synthetic variables. Analysis of annual county reports of crop acreages (1970-90) for corn, soybeans, cotton, sorghum, barley, oats, wheat, peas, tobacco, and potatoes yielded a single agricultural variable (CORNSOY) that accounted for 88 percent of the annual variance in crop acreages. The dominant variables in the CORNSOY principal component are acres of corn (loading=0.60) and acres of soybeans (0.70). A separate analysis of annual county reports (1970-88) of sales for commercial fertilizer, total fertilizer materials, total mixed fertilizer, lime, and land plaster yielded a principal component (FERT) that accounted for 98 percent of the annual variance in fertilizer sales.

Multiple regression analysis was used to test the association of each of the two agricultural principal component variables (CORNSOY and FERT) with instream water quality. Annual streamflow from a centrally located subbasin was included as a second variable in each regression to control for water-quality variations related to discharge, and a third variable accounted for trends over time that were unrelated to agricultural or streamflow characteristics.

Water-quality constituents examined included major ions, nutrients, selected metals, pH, dissolved oxygen, specific conductance, hardness, and solids concentrations. Median annual constituent values were used in the multiple regression analysis. Statistically significant (p=0.05) associations were observed for both agricultural variables for specific conductance, total nitrate, total nitrite, total ammonia, total nitrogen, total hardness, dissolved silica, dissolved solids, and non-filterable volatile solids concentrations. These correlations indicate a distinct association between water quality in the streams of the Albemarle-Pamlico drainage basin and indices measuring the amount of row crops grown and fertilizer used.

Harned, D.A., and McMahon, Gerard, 1995, Correlation of annual variations in water quality and agricultural practices for subbasins of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia: American Geophysical Union 1995 Fall Meeting, Eos, November 7, 1995, p. F278.