Trends in New Zealand's National River Water Quality Network

An analysis of trends was made for the 14 physical/chemical determinands measured monthly over the period 1989-93 at 77 river sites distributed throughout New Zealand's North and South Islands. This has used non-parametric Kendall slope estimators and associated tests, accounting for seasonality, which have been applied to both the raw data and to flow-adjusted data (flows in much of the North Island trended downward over the period). Results indicate a general improvement in water quality, particularly in the South Island. Downward national trends, for both raw and flow-adjusted data, were found for water temperature, BOD 5 , NH 4 -N, NO x -N and TP. Upward national trends, again for both raw and flow-adjusted data, were found for dissolved oxygen, visual clarity, conductivity, and g 340 /g 440 (indexes of dissolved colour). No national trends are reported for pH or DRP. Downward trends for turbidity and TN were seen in the raw data only. A general characterisation of the dependence of these determinands on river flow is presented.