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.

Trend Analysis of Nutrient Concentrations and Loads in Selected Canals of the Southern Indian River Lagoon, Florida

“…Trend analysis is also often complicated by various exogenous effects (e.g., discharge and seasonality). Nutrient concentrations, for example, may increase with increasing flow and vary in different seasons (Harned et al 1981;Doering 1996). In past decades, many statistical methods have been developed and applied for evaluating trends in water quality over time; these include the Tobit, the Sen's T, the Spearman's Rho, the Mann-Kendall, and the Seasonal Kendall tests (Hess et al 2001;Kahya and Kalayci 2004; Table 1).…”

Use of flow-normalization to evaluate nutrient concentration and flux changes in Lake Champlain tributaries, 1990–2009

“…A number of statistical methods have been developed that provide some kind of flow adjustment to concentration data for the purpose of identifying long-term trends (Alley, 1988;Harned et al, 1981;Hirsch et al, 1991). Conventional methods to evaluate trends in long-term water-quality datasets have provided information on the magnitude and direction or absence of trend, but impose methodological constraints on the analysis, such as requiring the same form of concentration-discharge relationship over the entire period of record and fitting a single linear or quadratic time-trend model over the entire dataset.…”

Cerro Grande Fire Impact to Water Quality and Stream Flow near Los Alamos National Laboratory: Results of Four Years of Monitoring

“…Some analytical technique is required to control for, or to remove, the effects of discharge in order to reveal nonclimatological chronological trends (Harned et al 1981). To estimate changes in TSS concentrations, we used an averaging technique (flow weighting) designed to account for the variation in sediment associated with a changing stream flow regime (Belillas andRoda 1993, Brown andKrygier 1971).…”