Peace and Myakka River Water Quality Summary 2002 - Southwest ...

loads for this period. The averaging approach, although simple and flexible, issubjective and prone to bias. The regression approach does not appear applicable tothis data set, because concentrations of most constituents do not show strongrelationships with flow. The two primary assumptions of the ratio estimator — that ascatterplot of load vs. flow approximates a straight line through the origin, whosevariance increases with increasing flow — appear reasonable for most constituentsincluded in the monitoring program. Also, in comparison to the other methods, ratioestimators are considered to be more robust to sources of potential estimation bias(Preston et al. 1989).As a working rule, ratio estimators are best applied in cases where the sample size isgreater than 30 for each time period of interest (Cochran 1977). For typical watershedassessment applications this implies that, for best accuracy and precision in theestimation of constituent loads and yields, a minimum of 30 water quality samplesshould be collected during each time period (e.g., year) for which loading estimates willbe calculated.Using the ratio estimator described by Cochran (1977) and Preston et al. (1989), annualloads of selected water quality constituents (nitrogen forms, phosphorus forms, andTSS) were calculated at each monitoring station for water years 1998 through 2002.The estimator takes the form:)L = ( l / q)Qwhere:L )lqQis the estimated annual loadis the average load on days when both discharge and concentration weremeasuredis the average discharge on days when both flow and concentration weremeasuredis total annual discharge.Estimated annual loads of TN, DIN, NH 4 -N , NO 2+3 -N, TP, PO 4 -P, and TSS calculatedusing this method are shown in Tables 3-9.24