Australia relies on surface water storages to ensure that water is available when needed and to act as a buffer against drought. However surface storage is inefficient because much water is spilled from dams that are quickly filled during very wet periods and because of ongoing and high evaporative losses during every summer. It is estimated that average annual evaporation from on-farm storages in the Murray-Darling Basin exceeds 1,000 GL – enough to supply Melbourne, Sydney, and Adelaide each year.
The pumping of groundwater for irrigation has created large air spaces in aquifers where irrigation and environmental water can be stored or “banked” for later recovery and use. Researchers at the National Centre for Groundwater Research and Training (NCGRT) believe the opportunities for such “water banking” in aquifers need to be evaluated in farming landscapes.
Storing water in aquifers for later reuse is already practiced with urban waste water in the United States and Australia. The typical approach is for treated waste water to be pumped into depleted aquifers for later reuse on sporting fields or for irrigated agriculture. By comparison, there are few examples of water banking in Australia’s rural landscapes using non-urban sources of water, including river water. The reality is that the reform of water governance in Australia has focused on the goal of achieving increased environmental flows in rivers by reallocating or purchasing irrigation entitlements.
Conservation groups and many scientists are likely to be skeptical of proposals for water banking in farming landscapes, concerned that this unproven technology represents an attempt to ignore the “limits to growth” in the driest continent. Others may confuse water banking with the technologies used for Coal Seam Gas mining. While NCGRT scientists have reservations about suggesting technological solutions to environmental problems, our view is that the magnitude of the challenges being faced by Australia is such that all options need to be considered. Indeed, there may be options that enable us to adapt in ways that are consistent with a desire to respond effectively to our highly variable climate. That is, we expect water banking will present opportunities to “do more with less” water.
The economic and social benefits of water banking for farmers and the communities that depend on irrigated agriculture can be readily understood. There are also opportunities to accomplish environmental objectives. A key point is that water banking can occur within existing allocations of surface and groundwater to the environment and irrigated agriculture. Water for water banking can be sourced from existing entitlements by: reducing evaporation losses from large public and private storages; applying water allocated for the environment; using water markets to buy additional environmental water and store that water underground; and through innovative approaches to conjunctive use of surface and groundwater (i.e., improved delivery of environmental and irrigation water). The potential environmental benefits of water banking include those that flow from:
- removing artificial levees and reconnecting rivers and floodplains in a way that enhances natural recharge of aquifers;
- restoring aquifers so that depleted aquifers are reconnected with surface water systems, thereby maintaining groundwater flows that support groundwater dependent ecosystems;
- returning at least parts of our major rivers to more natural seasonal flow regimes and temperatures (for example, by storing water closer to irrigation districts rather than in upstream dams);
- enabling environmental water holders to store water for longer so they can maximize the benefits of that water, including by accumulating sufficient water to support in-stream ecosystems during drought or by selling water during dry spells when temporary water prices spike;
- storing cool winter flows in aquifers and releasing that water to streams in summer or during droughts in locations where high water temperatures affect aquatic life; and
- recharging aquifers to maintain aquifer integrity or water quality by controlling the groundwater gradient to prevent pollution from nearby saline groundwater or seawater intrusion.
There is the potential for negative environmental impacts as a result of water banking and proposals for water banking will need to be evaluated carefully. Some of these risks were discussed at a national workshop held by the NCGRT, and include those related to the mixing of water from different sources; unaccounted leakage from aquifers; and that recharge or recovery rates will be less than expected. For example, it may not be possible to recharge some depleted aquifers so that groundwater and surface water are reconnected. Workshop participants acknowledged the importance of the social acceptability of water banking.
Key requirements for aquifer recharge include the availability and suitability of aquifer storage and the availability of surplus surface water and the means to convey it. Highly permeable aquifers are relatively easy to recharge, but stored water is mobile and recovery fractions have to be applied to allow for losses. Aquifers with low permeability are harder to recharge but stored water is more stable. Few mechanisms exist to directly inject storm/flood water into aquifers due to the risks of borehole clogging and aquifer pollution. To overcome such issues, temporary surface storages are normally built to capture and stabilize storm water. Aquifer recharge by allowing large floods to spread more readily over floodplains is also possible but adds to the scale and complexity of these challenges.
The development of water banking has also been constrained by the development of large dams, the encouragement of on-farm surface water storage, and the lack of development of entitlements and rules for water banking. Aquifer storage and recovery is not feasible without an entitlement to store water in an aquifer and to recover it. In NSW there is no legal and administrative regime to enable underground storage of water and subsequent recovery. Groundwater enters the “common pool” once it is underground, and can be accessed by anyone with a water use license. Clear ownership and accurate measurement and accounting of stored volumes would be required – taking account of experience with aquifer recharge in South Australia, and with the implementation of guidelines to do that in Western Australia and Victoria. In NSW groundwater users have a water account, and they can carryover water for up to three years. However, this limited carryover does not allow water banking over the full extent of wet dry (El Niño – La Nina) climate cycles. In addition, water releases are not planned to optimize water supply phasing and delivery using intermediate storages, including aquifer storage.
In summary, water banking in aquifers can make a major contribution to efforts to address critical sustainability issues in the Murray-Darling and do so in ways that are consistent with contemporary policy settings and the demands of a highly variable and changing climate.
Professor Allan Curtis is a social researcher in natural resource management at the Institute for Land, Water and Society at CSU. Professor Anthony Jakeman is Director of the Integrated Catchment Assessment and Management (iCAM) Centre at ANU. Associate Professor Bryce Kelly is a member of the Connected Waters Initiative at UNSW.