Effect of climate variability on water quality trends in New Zealand rivers

Abstract

Associations between temporal variability in 10 water quality variables and the El Niño–Southern Oscillation (ENSO) climate pattern, quantified by the Southern Oscillation Index (SOI), were assessed at 56 river sites in New Zealand over a 31-year period (1989–2019). Fluctuations in monthly observations of water quality at each site were correlated to varying degrees with the corresponding SOI value (ro). Trends for rolling windows of 5-, 10- and 15-year duration were evaluated for each site and variable using Kendall’s tau (τw) and for the SOI using linear regression (δSOIw). Aggregate trends for each variable, duration and time window were quantified as proportions of site trends that were increasing (Pw). Fluctuations in τw and Pw between time windows for all durations were explained by the corresponding SOI trend (δSOIw). Between-site and between-variable differences in the trend responses to the SOI trends were explained by the corresponding ro values and the median of the ro values () respectively. These results indicate that climate variability makes a significant contribution to water quality trends, even at timescales longer than ENSO cycles. Our models can be used to quantify this contribution for individual sites (i.e. τw) and for aggregate trends (i.e. Pw).