Managing variable water availability
Australia is well known as the driest inhabited continent, with highly variable rainfall and high rates of evaporation. Rainfall is distributed unevenly, both geographically and seasonally, across Australia (Table 1). About 80% of the country has an average annual rainfall of less than 600 mm; about half has an average of less than 300 mm; and over one-third has an annual average of less than 200 mm. The driest part of the continent is around the Lake Eyre Basin, with an annual average of less than 125 mm while in parts of north eastern Queensland and western Tasmania rainfall exceeds 3,000 mm/year.
| City | Average Annual Rainfall (mm) |
|---|---|
| Alice Springs | 270 |
| Adelaide | 500 |
| Hobart | 520 |
| Canberra | 630 |
| Melbourne | 660 |
| Perth | 790 |
| Brisbane | 1180 |
| Sydney | 1220 |
| Darwin | 1690 |
The major influence on rainfall patterns in Australia is the periodic warming of the equatorial Pacific Ocean known as El Nio. All of the major droughts in Australia in the past 100 years, including the one in 2002, have been associated with this phenomenon. Typically, the adjustment in the atmosphere in response to El Nio results in changed rainfall patterns over eastern Australia. Other factors that also influence Australian rainfall include changes in the monsoonal circulation over northern Australia, changes in sea surface temperature that are not directly associated with El Nio, changes in large-scale circulation in the Southern Hemisphere and continental land use changes.
Figure 1: Coefficient of variation for rainfall in various countries
The coefficient of variation calculated as the standard deviation divided by the mean is often used to make comparisons of rainfall variability. This highlights the high rainfall variability for Australia compared with other countries (Figure 1).
Most of this rainfall, even in the wetter catchments, does not run off into rivers. On average, only 12% (less than 3% in the drier areas and up to 24% in the wetter areas) of rainfall enters the rivers; the remaining rainfall is accounted for by evaporation, used by vegetation or stored in lakes, wetlands and aquifers. This results in Australia having only one per cent of the water carried by the world's rivers despite having five per cent of the world's land area. Almost 50% of Australia's average annual run-off enters the Gulf of Carpentaria, a region of relatively limited water resource development, and the Timor Sea (NLWRA 2001).
Highly variable flow is a characteristic of Australian rivers. Flow variability can be described by the coefficient of variation of annual flows (CVR) with Australia and South Africa have CVR values well in excess of the world average (Finlayson and McMahon, 1988). While this relationship is true for all catchment sizes, it is particularly so for Australia's large (greater than 100,000 km2) inland catchments, which have a CVR of 1.12, nearly four times the world average of 0.33. As well as more variable flow, extreme floods occur more often in Australia and South Africa than in the rest of the world (Finlayson and McMahon, 1988).
These factors of high rainfall variability, high evapotranspiration rates and geographic separation of water resources and irrigation development conspire to make the storage and delivery of water an enormous challenge. Historically, water resource development in Australia has taken a surface water focus, with the construction of large dams. The national storage capacity of 79,000 GL in 447 large dams is four times the annual surface water diversions of 19,100 GL (NLWRA, 2001). River regulation by dams reduces the numbers and extent of floods and dry periods. However altered conditions in the natural water flows affect the way the river functions physically and chemically, in turn impacting on aquatic ecosystems.
There are opportunities in using aquifers as "water banks" in conjunction with surface water reservoirs to enable greater flexibility and efficiency in securing water supplies. Subsurface storage complements surface reservoirs and while it may not replace large dams, it could be a robust alternative to expanding storage capacity using reservoirs alone. Enhancing recharge to aquifers during periods of above-average water availability provides a resource for access during droughts. Such a conjunctive approach has benefits when compared with relying solely on large surface water storages. The infrastructure costs are generally cheaper, and there is the potential that the aquifer material can filter and improve water quality. Using aquifers as storages is becoming a viable alternative considering the existing constraints of building new dams due to environmental concerns and general lack of suitable sites. Although the concept is simple, sustainable operations which protect groundwater quality require a sound understanding of the hydrological and biological processes involved, along with careful management.
Relevant Links
Department of the Environment and Heritage SoE 2001 Atmosphere Theme Report
Land and Water Australia Managing Climate Variability Program
Queensland Government Long Paddock
Bureau of Meteorology
References
Finlayson BL, McMahon TA, 1988. Australia vs the World: a comparative analysis of streamflow characteristics. In: Robin F Werner (ed.), Fluvial Geomorphology of Australia, Academic Press, Sydney.
NLWRA, 2001. Australian Water Resources Assessment 2000, Surface water and groundwater - availability and quality, National Land and Water Resources Audit, Canberra.