Learning From Multiyear Meteorological Drought Impacts on Stream Water Quality—Implications for Water Quality Changes Under a Changing Climate

The frequency and intensity of multiyear drought are expected to increase in the future with climate change. This study aims to investigate the impact of multiyear meteorological drought on the behaviour of five water quality constituents (dissolved oxygen, total nitrogen, total phosphorus, electrical conductivity, and turbidity) at 134 sites across Victoria, Australia. A 27-year period (1995–2021) of observed water quality data that spans the Millennium Drought (~1997–2009) was subdivided into meteorological multiyear drought and non-drought conditions. Statistical analysis of these observed data indicates that under multiyear drought, EC increases, while turbidity, TN, TP, and DO decrease in concentration. The magnitude of change in constituent concentrations may be driven by climate characteristics and drought intensity, as inferred from the spatial variability in change in water quality. For salts, TN, and TP, we also identified that constituent loads decrease during multiyear drought periods, regardless of the direction of change of constituent concentrations during drought, highlighting the dominant role of streamflow in driving the trends in constituent loads. Analysis of concentration-streamflow relationships during drought and non-drought periods suggests that parameter values within the concentration-discharge relationship (intercept and slope) shift during multiyear drought, especially in warmer and drier catchments. This indicates that water quality during multiyear drought cannot necessarily be predicted using concentration-discharge relationships calibrated using water quality data from non-drought periods. This investigation can help inform approaches to managing water quality in the future under a changing climate, given persistently drier conditions are expected in this region.