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Methods of evaluating water-quality trends in U.S. Geological Survey historical data

Geological Society of America Abstracts with Programs, 93rd Annual meeting, Atlanta, Ga., Nov 17-20, Abstract No. 01039.
By Douglas A. Harned, Charles C. Daniel, III, and J. Kent Crawford

Abstract

Three different methods for evaluating long-term trends in historical water quality data are currently used by the North Carolina Water Science Center office of the U.S. Geological Survey. Identification of long-term changes are important when assessing degradation of water quality by population and industrial growth on the one hand, and improvement in quality resulting from better waste treatment on the other.

These statistical methods have been applied to analysis of preexisting ambient water-quality data that were not collected according to a specific statistical sampling design. Two newly developed methods, discharge normalization and discharge-frequency weighting, take advantage of available long-term discharge records usually associated with Survey sampling stations in estimating trends. Both methods adjust the observed water-quality concentrations as if they were determined at a constant discharge over the period of record.

Discharge normalization shifts daily discharges to a central period of record discharge value, recalculates daily specific conductance from the adjusted discharges, and then calculates constituent concentrations from linear relationships between specific conductance and constituent concentrations. The annual average of normalized values produced by the method can be plotted and regressed against time to illustrate trends.

Discharge-frequency weighting weights each observed concentration by a fraction of the total area underneath the period-of-record discharge frequency distribution, and then sums the weighted concentrations for each year. This technique has the advantage of being simple and easy to use.

A third method, residuals analysis, can also be effectively used to demonstrate trends. Residuals are calculated from the curve produced by a regression of concentration on discharge. When these residuals are plotted against time the resultant graph shows discharge-free concentration trends.

These three trend analysis methods give similar results. Each method has advantages and disadvantages, which must be considered in the frame of reference of data constraints.

Harned, D.A, Daniel C.C. III, and Crawford J.K., 1980, Methods of evaluating water-quality trends in U.S. Geological Survey historical data [abs]: Geological Society of America Abstracts with Programs, 93rd Annual meeting, Atlanta, Ga., Nov 17-20, Abstract No. 01039.