Climate change and water resources in the Murray Darling Basin ...

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ABARE CONFERENCE PAPER 02.11 and 6 per cent higher in the SRES A1 scenario than under the SRES B1 scenario in 2050, and between 8 and 18 per cent higher in 2100. The impact of the two climate change scenarios on water quality varies over time and location, reflecting differences in both land use and the underlying hydrology of each land management unit. Salt concentration tends to increase in the tributary rivers above irrigation areas as surface water runoff declines by a greater proportion than salt loads, particularly in the shorter term. There are increases in river salinity in both the Goulburn–Broken and Gwydir Rivers under both climate change scenarios, with the more severe impacts under the SRES A1 scenario. Increasing river salinity is caused by two factors. First, there is an immediate reduction in both surface runoff and ground water recharge. However, there is a substantial delay before a reduction in recharge is fully reflected in a reduction in ground water discharge and a corresponding reduction in salt loads mobilised into the river system. Initially, the decrease in saline discharge from the ground water system is more than offset by the reduced dilution effect of lower river flows leading to increased river salinity. Second, the excess precipitation over evapotranspiration that enters the river system as runoff tends to be large relative to that which enters the ground water system. As a result, the salinity benefits from the eventual reduction in ground water discharge can be more than offset by the reduction in surface water flows available to dilute existing salt loads, even in the longer term. However, the benefits from reduced recharge may outweigh the costs associated with reductions in surface water flows if ground water salinities are quite high. While irrigation areas are generally significant exporters of salt to the river system, they also tend to have saturated soil profiles, meaning that a change in recharge leads to a reduction in discharge relatively quickly. Consequently, the impact of climatically induced changes to precipitation and evaporation below irrigation areas is quite different. Salinity levels tend to increase only marginally or even decline as the volume of irrigation water applied falls and this reduction is quickly translated into a reduction in ground water discharge. As a result, water quality in the Murray River is higher under the SRES A1 scenario even though it tends to generate more adverse impacts on catchment runoff than the SRES B1 scenario. In this instance, the benefits of reduced recharge under the SRES A1 scenario offsets the reduction in surface water flows generating water quality benefits in the Murray River. The fact that increased global warming may generate both positive and negative environmental impacts is further illustrated in figure 3, which shows the areas affected by high water tables in the two scenarios. High water tables reduce agricultural yields, damages transport, communication and other infrastructure though water logging and by depositing salt into the landscape (dryland salinity). The extent of the damage that occurs as a result of high water tables is highly dependent on the salinity of ground water being 24
ABARE CONFERENCE PAPER 02.11 Figure 3: Total area of high water tables in the Murray Darling basin, by SRES scenario 2500 2000 1500 1000 500 ’000 ha 2050 2100 mobilised into the landscape. Production losses are higher with higher ground water salinity. The areas affected by high water tables under the two scenarios do not begin to diverge until after 2050 because of the delayed response of the ground water system. By 2100, however, the area affected by high water tables is lower under the SRES A1 scenario. This is because the reduction in precipitation leads to a larger reduction in recharge. It is important to note that while reductions in ground water recharge tend to have benefits in saline ground water systems, it can also impose costs though reduced ground water availability for irrigation and higher pumping costs in fresh ground water systems. The evaluation of these scenarios did not take into account any economic incentives to adapt to climate change. These economic incentives are potentially quite large as the opportunity cost of water increases with decreasing availability. The opportunity cost of water in the irrigation areas of the Goulburn–Broken and Gwydir catchments is shown in figure 4. 25 SRES A1 scenario SRES B1 scenario Figure 4: The opportunity costs of irrigation water in the Goulburn–Broken and Gwydir catchments, by scenario 60 50 40 30 20 10 % increase Goulburn–Broken SRES A1 Gwydir SRES A1 Goulburn–Broken SRES B1 Gwydir SRES B1 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
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