Journal Article

INFLUENCE OF DIFFERENT TEMPORAL SAMPLING STRATEGIES ON ESTIMATING LOADS AND MAXIMUM CONCENTRATIONS IN SMALL STREAMS

ABSTRACT

Various temporal sampling strategies are used to monitor small streams to estimate annual loads and maximum concentrations of specific constituents. Extensive water-quality data from eight small streams in Wisconsin were systematically subsampled to simulate and evaluate typically used strategies. These subsets of data were then used with daily average streamflow to estimate annual loads using the regression method (the most commonly used method for streams with limited water-quality data). For each strategy, the accuracy and precision of estimated loads and maximum concentrations of phosphorus were evaluated by comparison with "true" annual loads (computed with the integration method) and maximum concentrations, both estimated using all available data for each site during the period of interest.

Them most cost-effective sampling strategy depends on whether loads or maximum concentrations are examined and the duration of the study. For 1-year studies, fixed-period monthly sampling supplemented by storm chasing was most cost-effective for estimating annual loads, even though loads were overestimated by 25-50%. For 2 to 3-year studies, fixed-period, semi-monthly sampling provided not only the least biased but also the most precise loads. Any strategy that collects samples on the rising limbs of hydrographs was best for detecting maximum concentrations regardless of the duration of the study. The high-flow samples, especially if consistently collected during rising stage, do not represent the average daily concentration and therefore result in imprecise, over-estimated annual loads if used in the regression.

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