Fresher Streams After a Prolonged Drought in Victoria, Australia

Abstract

Salinity in waterways changes throughout the year due to interactions between more saline groundwater and fresher surface water as runoff changes seasonally and with rainfall events. Droughts have recently been shown to cause persistent shifts in rainfall runoff partitioning, and here we examine whether stream salinity also exhibits such shifts from the long‐term seasonal cycles in order to advance our understanding of the mechanisms responsible for this hydrological change. We identify shifts in salinity by evaluating the residuals of a model that explains the temporal variability of daily stream salinity. The model was applied to long‐term (1990–2021) daily salinity observations from 23 catchments in Victoria, Australia. Residuals are analyzed in periods of 3–11 years and at a monthly and daily time‐step. We find most streams get fresher in spring during the prolonged Millennium Drought (), and eight (8) persist with lower salinity after the drought. The shifts in salinity closely align with the rate of change of the groundwater table and annual runoff states, not the groundwater head. Often streams were either in a lower runoff state with fresher streamflow as groundwater declined, or a normal runoff state with saltier streamflow as groundwater rose. We further identified long‐term change in the vertical pressure gradient and provide a conceptual diagram that explains this hydrological change. Given our results and existing evidence, we suggest vegetation water use during the drought initiates change in vertical movement of groundwater resulting in less delivery of saline groundwater into the stream and lower streamflows.