ACCURACY OF NUTRIENT RUNOFF LOAD CALCULATIONS USING TIME-COMPOSITE SAMPLING

The accuracy of time-composite sampling as a means to calculate phosph orus loads from agricultural runoff was evaluated analytically and num erically. It was shown that, when hydrographs and concentration graphs were positively correlated, computations using a time-composite sampl ing strategy resulted in load underestimation. Combinations of 11 synt hetically generated and 12 actual hydrographs and concentration graphs of agricultural runoff events were used for numerical analysis. The b ias in load calculation with different sampling frequencies was determ ined The influence of cross-correlation of hydrographs and concentrati on graphs on the direction and magnitude of bias was demonstrated. Alt hough flow-composite sampling is theoretically correct for load comput ation, it is more expensive and, like other approaches, still involves uncertainties in both flow and concentration measurements. The depend ence of the coefficient of variation (CV) of load on CV of flow rate, CV of concentration, and the cross-correlation between flow rate and c oncentration is presented analytically and graphically. To achieve loa d estimates comparable to the accuracy of flow-composite sampling, mul tiple time-composite sampling methods were investigated This analysis indicates that with eight concentration values from eight time-composi te samples of a runoff event, a reliable load estimation can usually b e made.