Saving water in a drying climate: lessons from south-west Australia

Since 1970, average rainfall in the south-west of Western Australia has decreased by nearly a fifth, andthe sciencesuggests that this is linked to human-caused climate change.

Across Australia,CSIRO predictsrainfall will probably decrease in southern Australia, and thepredictions for the south-westare especially grave where the recent drying has left empty dams and greater reliance on groundwater.

So, what can south west Western Australia tell us about managing water - particularly groundwater - in a drying climate?

Falling rain, rising demandSouth-west Western Australia is home to around 2 million people, some 90% of Western Australia's growing population. This community is fortunate to have substantial groundwater resources, which (on2009 figures) meet around three quarters of its water needs, including for public supply of tap water, and self-supply for horticulture, industry and the irrigation of public and private lawns and gardens. Demand is expected to increase by about one third by 2030. At the same time, rainfall is declining.

In 1987,CSIRO projecteda 20% reduction in average rainfall for Perth between 1970 and 2040 — a projection that proved conservative in light of actual rainfall declines. Throughout the south west the story is similar.

This has had a significant impact on groundwater resources. For example, it hasbeen estimatedthat reduced rainfall between 1979 and 2005 was responsible for falls of up to 4 metres in the important Gnangara aquifer north of Perth, with major impacts on wetlands.

Declining rainfall and increasing demand has prompted the use of desalination for water supplies. Desalination plants now have the capacity to provide about half of Perth's drinking water needs.

But desalination is expensive and energy-intensive, and probably not practical for horticulture and industry away from the coast. If Perth and other parts of the south west really want to adapt to reduced rainfall and groundwater, we need to reform the way we use groundwater.

Three responses

We suggestthat three reforms to Western Australia's water laws to sustainably manage the south west's groundwater in the face of a drying climate and increasing demand.

First, there should be a legal duty to consider and address climate change when setting allocation limits for groundwater resources.

One striking feature of the Western Australian experience is the lag between the climate science and its regular use in groundwater planning. CSIRO made its first predictions in the 1980s, but Perth's water managers didn't include climate change projections in groundwater planning until 2009.

A legal duty would ensure that climate change is properly considered and addressed when were deciding how much groundwater to extract.

Second, water entitlements should be flexible.

Climate science is not a crystal ball - indeed there is a "cascade of uncertainty" from global climate models to regional climate predictions to local water supplies (including groundwater aquifers). Depending on future rainfall, entitlements may need to be adjusted to keep groundwater use within sustainable limits.

In Western Australia, water entitlements are allocated as a fixed volume. For example, on the Gnangara groundwater system the Water Corporation, horticulturalists and others have licences that specify the number of kilolitres they can extract each year. The problem is that in many areas, these volumes add up to an amount that exceeds sustainable levels, especially in dry years. And, under current water laws, it isn't easy to vary these water licences.

TheNational Water Initiativerecommends that water laws define entitlements as a share of a variable pool — this is determined by rainfall and groundwater "recharge". This will ensure collective use can be varied to keep groundwater use within sustainable limits.

For example, a water plan could provide that the pool will be varied each year in line with average rainfall over the previous 10 years. In a declining rainfall trend, this would mean each water user would share the burden of staying within sustainable extraction limits.

Third, there should be better mechanisms to "save" groundwater in wet years. This can be done by keeping extraction below the long term sustainable yield so that more water can be taken in times of extreme drought.

For several years, water managers in Perth used a "variable groundwater extraction rule" to decide how much groundwater could be taken for public water supply. Under this rule, the limit on groundwater taking ("abstraction") for public water supply in any one year was inversely proportional to water storage levels in Perth dams. So, if dam levels were low, more groundwater was allowed to be taken.

The intention was to use groundwater as a "buffer" in dry years. But declining rainfall resulted in sustained high use of groundwater that contributed to reduced groundwater storage, drying of wetlands and damage to vegetation. The average amount of groundwater taken was not adjusted to ensure sustainable groundwater storage in a drying climate.

To fix this, the south west needs laws to ensure groundwater is "saved" in wet years so it can be used, sustainably, during drought. One approach would be to require all water entitlement holders to "bank" a proportion (e.g. 10%) of their allocation, so as to build an individual reserve that is accessible to them only in a severe drought as declared by the Minister for Water. This is a publicly mandated saving of water for severe drought, comparable to compulsory superannuation.

Facing the challengeSome of these reforms have already been implemented in other parts of Australia through the National Water Initiative. However, the "drought reserve" reform we have proposed is, to the best of our knowledge, a new policy measure. It should be considered by other parts of Australia and the world facing a drying climate, and more extreme drought, due to climate change.

The reforms we have proposed are particularly relevant to Western Australia, which is currently rewriting its water laws. The science suggests that a drying south west has suffered from the effects of human-induced climate change over the last three decades, and will experience further reductions in rainfall and groundwater recharge and storage in decades to come.

To what extent will this problem, caused by unsustainable use of fossil fuels, lead to unsustainable use of groundwater resources? The answer to this question will depend, in part, on whether Western Australia's water laws are up to the adaptation task. The three reforms we have suggested would go some way to ensuring that they are.

UPDATE: You can read thefull reportof Michael Bennett's and Alex Gardner's proposed water reforms at the University of Western Australia.

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Taxonomy

• Ecosystem Management
• Drought
• Natural Resource Management