Impact of flow variability on error in estimation of tributary mass loads

Annual mass load estimators are evaluated for application to a range of tributary types including highly responsive systems. Evaluations are performed by conducting retrospective studies with comprehensive sets of field data for total phosphorus concentration and discharge. Four estimators from three broad categories are selected for study based on their performance in previous research. Extimators are evaluated using Monte Carlo sampling studies in which load estimates made from random subsamples are compared to true loads calculated from a complete loading record. The performance of the estimators differs strongly among the tributary test cases. Hydrograph characteristics and/or concentration/discharge relationships appear to play a strong role in the precision of all of the estimators and in the bias of averaging and regression estimators. Beale's ratio estimator is the only estimator to provide unbiased estimates for both stable and responsive systems, although stratification is necessary under event sampling. Consistent accuracy of the ratio estimator is valuable because it will allow the design of future sampling programs and accurate estimation of confidence intervals from collected sample data.