5.3.2. Water availability
Annual average rainfall varies widely across the Murrumbidgee Catchment, reaching highs of roughly 1300 mm near the headwaters of the Tumut River and declining to slightly more than 320 mm in Balranald. Average surface water availability under historical conditions has been 4270 gigalitres per year. The current usage level is very high: 53% of available surface water is diverted for urban and agricultural use. Climate change and increasing urban development threaten to increase demand and reduce supply. By 2030, surface water availability is predicted to decline by 9% and end-of-system flows by 17%. Worst-case scenarios suggest a decline of 28% in surface water availability and a 44% decrease in end-of-system flows.
A report by CSIRO predicts that the area under plantation timber will increase by 17,000 hectares by 2030, having significant local impacts on runoff. Over the same period, farm dam storage capacity is expected to increase by 47,000 ML (13%), slightly reducing (<1%) overall runoff for the region. Additionally, groundwater extraction is expected to increase by around 22%, reaching 494 gigalitres per year. It is expected that this will chiefly affect the Lachlan Fold Belt, with a smaller impact in the Mid-Murrumbidgee Alluvium. This is expected to result in a net stream-flow loss of 161 gigalitres per year. The return of surface irrigation water to the river is expected to offset this by 70-80 gigalitres per year. Taken together, these three developments would represent an increase in surface water use of 4% and a reduction in end-of-system flow by 2%. This is likely to have a slight impact (in addition to the direct impacts of climate change) on the Lowbidgee Floodplain and Mid Murrumbidgee Wetlands.
The extent of irrigation in the Catchment depends on the seasonal availability of water. Between 2001 and 2004, the area under irrigated rice decreased from 66,000 hectares to 24,000, owing to drought conditions. While the “best guess” scenario for 2030 is less severe than conditions during the recent drought, ongoing climate change is likely to have a damaging impact on irrigated agriculture in the Catchment. Given that irrigated agriculture amounts to only 3-4% of the catchment but produces roughly $500 million annually, this is likely to have wider consequences within the Catchment, the state, and the country.
During the recent drought, water allocations for the Murrumbidgee Irrigation Area declined from more than 100% of entitlement in 1995 season to only 30% of entitlement in 2003. A 2005 study by Rajinder Pal Singh, John Mullen and Rohan Jayasuriya attempted to predict likely impacts of reduced water availability on the profitability of irrigated farming and the area under certain crops. Using a model farm of 220 hectares, the authors considered the impact of reduced water allocations on three rice-based farming systems: a pasture-based system, a cereal-based system and a split system.
Pasture-based: At 100% allocation, the pasture-based system was expected to yield total farm gross margins of $188,800. Under 80% allocation, this would be reduced to $176,600, under 50% allocations to $113,000, and under 30% allocations to $70,500.
Cereal-based: At 100% allocation, the cereal-based system was expected to yield total farm gross margins of $191,500. Under 80% allocation, this would be reduced to $179,700, under 50% allocations to $115,800, and under 30% allocations to $73,500.
Split: At 100% allocation, the split system was expected to yield total farm gross margins of $203,700. Under 80% allocations, this would be reduced to $185,600, under 50% allocations to $124,400, and under 30% allocations to $82,100.
(Note that these figures are slightly increased when water trading is accounted for).
Short-term climate change predictions are certainly less drastic than the worst-case scenario given here (30% allocations), but reduced river flow and a decline in annual rainfall are likely to have an impact on the profitability of irrigated farming in the region.
In addition to addressing the causes of climate change, improvements in water-use efficiency are needed to avoid or minimise these consequences. Techniques such as the retention of crop stubbles, which helps to prevent moisture loss, have a part to play in this process. Research conducted as part of CSIRO’s “Water for a Healthy Country” project has suggested that 295 gigalitres of water can be saved per year without changing cropping systems.
References and Resources
• CSIRO. 2011. Murrumbidgee water savings.
• CSIRO. 2011. Murrumbidgee region: CSIRO Murray-Darling Basin Sustainable Yields Project.
