Trends in New Zealand's National River Water Quality Network

Abstract

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, BOD₅, NH₄‐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 g₃₄₀/g₄₄₀ (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.

Keywords:

• temporal trends
• flow dependence
• water quality
• monitoring networks