Methods for water quality sampling and load estimation in monitoring of Norwegian agricultural catchments

Different sampling procedures are applied to monitor water quality in agricultural catchments in the Nordic countries. The need for comparing monitoring results from the Nordic countries was the incentive for establishing a project aimed at comparing estimates of nutrient losses determined using different sampling strategies. Three different sampling methods were compared in three Norwegian catchments: weekly flow-proportional composite sampling (FPCS), weekly composite sampling with temporally equidistant subsampling every second hour (TECS) and temporally equidistant weekly point sampling (PS). Differences in load estimated between the three tested sampling strategies were smaller for nitrogen than for phosphorus or suspended solids. Point sampling tended to miss some of the peaks in concentrations of phosphorus and suspended solids, particularly during flow events, causing significantly lower load estimates for phosphorus and suspended solids by point sampling compared with composite sampling strategies. Flow-proportional composite sampling gave the most reliable data for event-responsible compounds or when the predictability of peaks was low. Based on this investigation, and similar studies in the other Nordic countries, a flow-proportional sampling strategy is recommended for studies of water quality in agricultural streams.