DESPITE Gingin receiving above average rainfall this year it is likely there will be lower than average water levels in the Gingin Brook this summer.
According to the Department of Water and Environmental Regulation (DWER) rivers in the south-west of the state had another dry year with the majority recording below average flows compared with the years 1975-2016.
DWER said at the north of the south-west land division, Gingin Brook continued to be one of the state’s most affected areas for climate change, recording another below average year’s flow.
The department’s regulatory services executive director Paul Brown said low flows were a particular concern in summer where no or low rainfall, decreased groundwater seeped into the brook, high temperatures, and increased demand for surface water and groundwater to support irrigated agriculture caused streamflows to be at their lowest.
To manage the risk these low summer streamflows have for water users and water-dependent ecosystems DWER monitors streamflow at its telemetered gauging stations as well as water quality in important refuge pools, to ensure water levels support aquatic health.
“If the department believes there is a risk to the aquatic ecosystem or water availability to surface water users, we consult with landholders along the brook, notifying them of the risk and encouraging them to liaise with their neighbours to stagger their take and use water responsibly,’’ he said.
Mr Brown said streamflow (how much water was flowing through a river or stream) was an indicator of ecological health in Gingin Brook.
But its relationship with rainfall was complex due to the strong interaction between groundwater and surface water throughout the catchment.
“Rainfall pattern, the intensity and time of year of rainfall events, along with cumulative rainfall over a season, all influence runoff, groundwater recharge and groundwater discharge, and therefore the streamflows observed in Gingin Brook.
“The department’s scientists have measured that under climate change in the state’s south west land division, a 10 per cent decrease in rainfall results in around a 30 per cent reduction in run-off, with recent data at some sites suggesting the reduction may be greater than this.’’
He said rainfall should not be attributed as the sole indicator of stream health and that DWER used the south west index of river condition to assess river health.
“The index uses multiple indicators such as aquatic biota, water quality, fringing zone, physical form, hydrological change and catchment disturbance to provide a holistic assessment.
“In addition to published reports, DWER has a number of investigations from 2015-2017 showing that ecological health has been maintained in this area despite the declines in summer streamflow.
“Data from these investigations has been used to revise the low-flow thresholds for Gingin Brook and Lennard Brook and will be published in 2018.’’