There’s been a lengthy debate in the comments threads of recent posts about whether the dry weather in much of Australia in recent years can be attributed to climate change, or is just another round in the natural cycle. One point that’s emerged is the crucial role of evaporation in exacerbating drought conditions. This was first observed in relation to the 2002 drought. The steady increase in global temperatures, including average temperatures in Australia, means that even when rainfall is at or near the historical average, conditions are drier than before because evaporation rates are higher. When we get a drought, as at present, conditions that would once have been bad are now extreme.
The combined effects of low rainfall and high evaporation are amplified when it comes to runoff, since the amount (net of evaporation) absorbed by the soil does not change much. And land use changes such as the construction of farm dams have reduced the amount of runoff that makes it into streams (these are now being restricted, but it’s often a case of too little too late). It’s not surprising then, as reported by Mark Neal at the RSMG blog, that, in terms of inflows to the River Murray system, 2006 looks set to be the driest year ever recorded.
So far, the effects on flows and allocations of irrigation water have been offset to some extent by accumulated storage, but with a run of dry years, that can’t be sustained. At the end of September 2006, total River Murray
system storage was 3 550 GL or 37 per cent of capacity, which is only half the long-term average for September of 7 000 GL. With winter and early spring being extremely dry, the chance of significant improvement this year is low, given that 60% of inflow typically occurs during July to October.
But if the situation is bad now, imagine the possibilities if the Cap on extractions hadn’t been imposed back in 1994. We would have started with lower storage levels, and there would have been that much less to draw on.