Estimation of nutrient loads from monthly water quality data

Causes of variation between loads estimated using alternative calculation methods and their repeatability were investigated using 20 years of daily flow and monthly concentration samples for 77 rivers in New Zealand. Loads of dissolved and total nitrogen and phosphorus were calculated using the Ratio, L5, and L7 methods. Estimates of loads and their precision associated with short‚Äêterm records of 5, 10, and 15 years were simulated by subsampling. The L7 method generally produced more realistic loads with the highest precision and representativeness. Short‚Äêterm load estimates poorly represented the long‚Äêterm load estimate. The results highlight there is no single preferred load calculation method, the inadvisability of ‚'unsupervised' load estimation and the importance of inspecting concentration‚Äêflow, unit load‚Äêflow plots and regression residuals. Regulatory authorities should be aware that the precision of loads estimated from monthly data are likely to be ‚'optimistic' with respect to the actual repeatability of load estimates.