Methods of discharge compensation as an aid to the evaluation of water quality trends

Abstract

Two new methods are described for compensating for discharge when evaluating trends in water quality data. One method, discharge normalization, adjusts daily discharges using a central value calculated for the period of record and recalculates daily specific conductance from the adjusted discharges and discharge versus specific conductance regressions. Normalized concentrations for many constituents can then be calculated from linear relationships between specific conductance and constituent concentrations. The second method, discharge‐frequency weighting, weights each observed concentration by a fraction of the total area underneath the discharge‐frequency distribution of the period of record. This fraction is determined using the stream discharge at the time of sampling and the discharge‐frequency distribution for the period of record. The weighted concentrations are summed for each year. Both normalized values and weighted values can be plotted against time to produce trends essentially independent from discharge effects. Results from the methods are statistically similar to each other and to results from other trend detection techniques.