Our Changing Climate

We all know that Western Australia is a dry State in a dry continent. But although we are now paying more attention to climate change than ever before, average temperatures in WA have actually been slowly rising over the last century and rainfall in the SW has decreased by some 30% in the same period. Over the next 50 years, it is predicted that these trends will continue.

Since the 1970s an accelerated rate of decline has seen a 10-20% drop in rainfall in the southwest of WA and a corresponding reduction of stream flow into water supply dams. Before 1975 droughts still occured, but were balanced by above average inflows roughly every 3 years. Over the last 30 years there have been only one year with significantly above-average rain. At the same time, there have been changes in both the patterns and intensity of rainfall: a later start to winter, fewer rainy days and less rain on each rain day.

Graph of historical and projected water use 1941-2060

Dams replaced by expensive desalinated water

In Perth our public water supply comes from 3 main sources. In increasing order of use, these are:

  1. Dams
  2. Groundwater and, more recently...
  3. Desalination

Between 1998 - 2006 there was 21% less rainfall than the long-term average, which resulted in a drop of 64% in runoff into our dams in the Perth hills. (Roughly speaking, a drop in rainfall of 10% equates to 25% less runoff). Those dams not only supply water to the Perth metro area but to most of the wheatbelt and to Kalgoorlie through the Integrated Water Supply Scheme (IWSS). With increasing demand, surface water catchments now only provide 30% of the demand from the IWSS, as against as much as 80% back in the 1970s. Half now comes from underground or artesian water, and desalination makes up the remaining 20%. This rate of decline is forecast to continue through to 2030.

With a slow reduction in rainfall our groundwater resources can’t continue to meet this level of demand and therefore have to be managed very carefully. At the same time, desalination is a very energy-hungry process that cannot provide the same low-cost water we have previously taken from dams.

Another 30% fall in water use needed!

Over the last two decades there have been significant efforts to reduce water consumption. For example; the introduction of dual flush toilets in 1993, a ban on use of sprinklers during daytime since 1994, the introduction of a two day watering roster in 2001, and incentives to install modern, more efficient appliances. The combined effect of these measures has been to drop average consumption by 20%.

But without further reduction in water use, or new water sources, the gap between supply and demand by 2060 will be 3 times what it is now. As part of the government’s plan to make up the shortfall, average water use per person will have to fall by a further 1/3. We are already seeing the main themes in this programme – education, incentives and technological advances.