Water quality trends at NRWQN sites in New Zealand for 1989-2007

The New Zealand National Rivers Water Quality Network (NRWQN) has been running for 2 decades. The network consists of 44 sites in the North Island and 33 in the South Island (all located near flow recording stations) at which 14 physical and chemical variables are measured monthly. The most recent water quality trend analysis, undertaken by NIWA for the Ministry for the Environment (MfE) in early 2009, covers the 19- year period 1989-2007. Formal trend analysis was carried out on flow-adjusted data using NIWA’s Time Trends software. The non-parametric Seasonal Kendall Slope Estimator (SKSE) was used to represent the magnitude and direction of trends in flow-adjusted data. Seven water quality variables were examined, namely temperature, dissolved oxygen, visual clarity, and four forms of the nutrient elements nitrogen and phosphorus.

There were strong overall increasing trends in total phosphorus, dissolved reactive phosphorus, oxidized nitrogen and total nitrogen at the national scale. There was a significant increase in visual clarity (an improving trend) for reasons that are obscure given the negative correlation between visual clarity and percent pasture in catchments. The nutrient enrichment trends indicate deteriorating water quality and reflect increasing diffuse pollution, mainly attributable to the expansion and intensification of pastoral agriculture.