Estimation of Catchment Nutrient Loads in New Zealand Using Monthly Water Quality Monitoring Data

Causes of variation between nutrient 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. The L7 method generally produced more realistic loads with the highest precision and representativeness. Short-term load estimates poorly represented the long-term load estimate, and deviations frequently exceeded estimated imprecision. 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.