Australia’s water quality trends over two decades

Water quality of rivers and streams can vary over time due to changing hydro-climatic conditions in the interaction with catchment physio-geographic conditions and local human activities. This study is the first exploration of long-term river water quality trends across Australian continent, consolidating 375 catchments with contrasting climate, hydrology, land use and land cover. We focused on five key water quality parameters and estimated their flow-normalized trends over 2000-2019 using the Weighted Regressions on Time, Discharge, and Season method (WRTDS). For each parameter, about half of nation’s catchments have significant trends, which are generally within ±10% per annum relative to the first year (2000). Except for TSS, there is no systematic non-linearity nor abrupt changes over time, while for TSS many catchments had a systematic shift from increasing to decreasing trends since around 2010. A random forest model was developed and found that catchment land characteristics, along with baseline water quality, can explain over a third of the spatial variation of the trends of EC, TN, TP and TSS (32-51% explained), but had limited explanatory power for DO (22% explained). These findings will provide critical information on the waterway health, thereby facilitating natural resources management for Australia.