It is no secret that the Western Australian grain belt is struggling with reduced rainfall and water supply. Areas with lower rainfall are typically expected to be hardest hit. However, higher rainfall areas, such as Darkan, Arthur River and Wickepin, are also beginning to deplete their water supply before the end of summer.
According to Senior Principal Research Scientist at the Department of Primary Industries and Regional Development (DPIRD), Dr Richard George, less rainfall means up to three times less run-off.
“For every 1mm we lose in rainfall, we lose 3mm in run-off. We’ve also lost a third of our 10mm and 25mm rainfall events, so our natural catchments are becoming less effective,” Dr George said.
These trends are set to continue, particularly in South West WA which is predicted to be Australia’s region most vulnerable to climate change impacts. Global climate models based on a warming of 2.2oC predict that our run-off will drop by 54% by 2060. Worst case scenario modelling predicts a 73% decline, with best case scenarios still expecting a 20% decline.
Dr George spoke at The Sheep’s Back Spring Optimiser event at Arthur River in October (as well as others at Wickepin, Jerramungup, Lake Grace, Kulin) about climate trends and how farmers can improve water storage. His key message was that farmers need to start planning for less reliable surface water. So, where should farmers start?
“An opportunity farmers have this summer is to bulk up what water they have to reduce evaporation,” Dr George said. “Two dams have double the surface area and therefore, double the evaporation compared to a single dam. So, if you put all the water into fewer ‘key’ dams you can reduce the surface area.”
Another option is to maintain or improve the quality of existing water supplies by installing silt traps in front of dams and piping water in to reduce organic matter entering the dam.
In terms of increasing water capture, well-sited low salinity bores are often the most economic option. However, an equally effective but less widely adopted option is the use of roaded catchments, which are water-harvesting structures designed to increase the amount of run-off reaching the dam. This is achieved by compacting clay to make a smooth, impervious surface to reduce infiltration above the dam.
“Roaded catchments in the western Great Southern are few and far between, so farmers in the area have a great opportunity because they have a lot of adoption capacity. Not necessarily on the steep country or deep gravels, but there are soils that would allow it.
“It’s not just about building roaded catchments. You can also make good use of hard surfaces that already exist, such as roads and laneways.”
In some cases, roaded catchment may be ineffective above existing dams, so new dams may need to be built to incorporate a catchment. DPIRD has also investigated using sealants on roaded catchments to capture run-off from lower rainfall events.Another option currently being adopted in the drier areas of the wheatbelt is the construction of bigger dams with bigger roaded catchments. However, Dr George doesn’t think this will be as easy in western areas due to shallow watertables.
“There are only so many sites available for big structures, but it is possible, and something to start thinking about.”
Desalinisation has until recently been considered an unlikely option. However, with 25-30 farmers adopting desalinisation plants in the last two years, that may change as industry learns more about the feasibility of the technology and how to deal with the salt-laden discharge.
The benefit of desalinisation is that it could make use of the increasing availability of groundwater which has developed in line with land salinity.
“Rising water tables is putting 1,000 GL/year into the wheatbelt, so we are building a water resource under our farms. It causes salt, but maybe the flip side to that is a water resource.”
However, for the time being, more feasible options are available to most farmers such as well-sited bores and key dams combined with better roaded catchments.
Dr George’s presentation on both surface and groundwater are available on The Sheep’s Back YouTube channel – https://www.youtube.com/channel/UCMzeMG_9jxj9PpE9Dil2dtA/videos
For assistance with water enquiries, contact DPIRD on 1300 374 731.
For more information on roaded catchments, see the DPIRD publication “Roaded catchments to improve reliability of farm dams” – https://researchlibrary.agric.wa.gov.au/bulletins/109/#:~:text=Improving%20reliability%20of%20dams%20by,the%20catchment%20above%20a%20dam.