High flows contributed a large part of annual contaminant yields in New Zealand’s rivers

Accurate contaminant yield estimation in rivers is essential to developing water quality policies and monitoring their effectiveness over time. We assessed the contribution of high-flows (≥90th percentile) to total yields of nitrate-nitrogen, total nitrogen, total phosphorus, and E. coli calculated from monthly data over 15 years (310–325 sites) in New Zealand, and at 24 sites with high-frequency (30-min) nitrate-nitrogen and total phosphorus. High flows contributed 51–74% of annual contaminant yields at long-term sites and 48% of nitrate-nitrogen and 63% of total phosphorus in the high-frequency sites. Mean uncertainties in annual yields estimated from monthly monitoring data (compared to the true yield, calculated from high-frequency records) were 29% for nitrate-nitrogen and 52% for total phosphorus. Daily sampling was needed to reduce uncertainty to <10% especially in catchments with a high proportion of agricultural land use. Contaminant yields are strongly influenced by high river flows and the uncertainty in measurements can be reduced by higher frequency in sampling, according to an analysis of monthly and frequent sampling (30 min) data collected in New Zealand.