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When groundwater aquifers are connected to a river, they need to be managed as a single integrated system.  A connected river gains water when an aquifer is higher than the river and loses water when the aquifer is lower than the river. The rate of gain or loss is determined mainly by an aquifer’s gradient towards a river and its capacity to transmit water. Significant time lags can be involved.

How should entitlements be defined and allocations made when ground and river water systems are connected? Should river entitlement holders or the river be given an entitlement to the other resource? Is trade between ground and surface water systems possible? How should any impact of climate change be managed?

Groundwater Entitlement Systems

As is done in NSW, the state of the art when establishing a groundwater entitlement system is to issue unit shares in the system.  In systems where shares have been issued, allocations are made in proportion to the number of shares held.  The system is simple and can cope with changes in supply.

A formal announcement process is necessary.   Each year an assessment of the amount of recharge needs to be made and, once enough water has been put aside for base flow and mandatory inter-system transfers, the remaining recharge can be allocated to share holders.  Shareholders should expect allocations to vary from year to year and, if it gets drier, to receive a smaller allocation.  In the simplest of systems, allocation announcements are varied with changes in the depth to the water table.

To ensure efficient water use, it is necessary to allow entitlement holders to carry forward unused allocations with adjustment for losses and flows out of the system.

Where the aquifer is contiguous and porous, water tends to move quickly from one location to another. In such systems, groundwater trading is possible.  As porosity declines and/or the aquifer becomes fragmented, trading rules become harder to set.

Types of Aquifer

When discussing the effects of aquifers on river flow, Rick Evans has proposed that aquifers be zoned according to the time it takes for the act of extracting water from a bore to reduce river flow.

Aquifers right next to a river

Right next to a river, pumping reduces river flow almost immediately and entitlements are more accurately defined as part of the river system. In much of Australia, however, aquifers right next to a river are not considered to be part of the river system.  This means that those able to pump right next to a river gain access to an entitlement that is more reliable than any river entitlement.

It is interesting to note that Queensland legislation is now written so that a river boundary can be defined to include all groundwater bores within a specified distance of a river.  Reflecting on the merits of such an approach, Evans has suggested that most groundwater licences within 5 kms of the River Murray should be defined as part of this river’s entitlement system and managed accordingly.  In a drought, this would mean that allocations to groundwater and surface water resources would reduced at a similar rate.

Aquifers that can store river water

Further away from a river, there is often a transitional zone where the rate of contribution to or extraction from river flow depends upon river height.  In these zones, allocation and pumping rules need to be based on river height and on the time it takes for changes in the rate of extraction to affect river flow.

These are the aquifers that tempt groundwater hydrologists to suggest they could be managed like a dam. In highly regulated rivers, river height tends to be constant and, hence, opportunity to do this may seem limited.   If river management rules were changed, however, so that river height could be managed strategically with a view to reducing drought risk. Run the river high and the adjoining aquifers could be gradually filled.  Run the river low and the water stored in the aquifer could be gradually returned back to the river.  We think this opportunity is worth evaluating. New accounting would be necessary.

Aquifers further out from the river

The time it takes for groundwater to flow from more distant zones to a river can take many years.  Time lags of 20 to 50 years are not uncommon.  As a result, entitlement trades which involve the movement of the point of extraction closer to the river need to be managed with great care.   The solution is to set trading rules by sub-zone.  Entitlement trading from these sub-zones to a river is possible with adjustment for the time lags involved and to ensure that the trade does not result in long-term or even permanent “borrowing” of water from other zones or from the river.

Dealing with Adverse Climate Change

Having set the scene, we can now explore one of the key questions posed in this droplet.  In a system where a river gains water from an aquifer and it gets drier,

a)  Should river or aquifer users be protected from the impact of this decline in water availability; or
b)  Should the impact be shared?

If the latter sharing approach is taken then one option is to issue a “gaining” river formal shares in the aquifer system and, conversely, a “gaining” aquifer formal shares in the river system.

Under such an arrangement, managers would be forced to manage connected systems as a single interacting system. When recharge increases, those with an interest in a gaining river would get more water and when recharge declines, they would get less water. The result is a regime that would establish a level playing field between ground and surface water users. We think the approach has merit.

Trading among connected river and groundwater systems

Pushing the envelope one step further, it is possible to imagine an entitlement regime where an irrigator or an environmental water manager could purchase a groundwater entitlement and, with appropriate adjustment, arranges either for

a)  The entitlement to be converted into a river entitlement; or
b)  It the entitlement to be tagged so that any allocations made to it are transferred, with an appropriate volumetric adjustment and time delay, to a river account.

The main difference between these two options lies with the way allocation risk is distributed.  When a groundwater entitlement is converted into a surface water entitlement and the exchange rate is wrong, the reliability of all other entitlements in the system changes. Under the second tagged approach, exchange rate errors can be corrected so that there is no long-term impact on the interests of others.  Given the risks involved, in the case of groundwater to river water trading, a tagged approach is likely to result in more efficient decisions.

Where to from here?

Obviously, careful aquifer-specific analysis of the options presented in this Droplet needs to be undertaken. Significant investments in aquifer mapping, connectivity assessment and monitoring would be necessary.

The challenge now is to work out how to get the foundations for such a aquifer-river sharing systems right, cut through the complexity and put systems in place that can be expected to improve with increased understanding and knowledge.

As a bare minimum, we recommend that system managers should start to define the size of each river’s share of the water in each aquifer connected to it and vice versa.  We also recommend that agencies trial the tagged trading of water entitlements among between connected river and groundwater systems.

The good news is that all this is not new.  In places like the Arizona, California and Texas, existing regimes enable people to store water in aquifers.  In these parts of the world, it is also possible to swap surface and groundwater allocations.  In fact, it has even got to the stage that some American States are doing ground-surface water trades with one another.

Examples of the early development of such ideas can also be found in Australia.  In South Australia’s Angas Bremer system, for example, irrigators are given credit for 100% of any surface water they drop into an aquifer on the condition that this water is used within 5 years.  Another example can be found in Queensland’s Burdekin River Delta where up to 250 GL of water per year is pumped into a groundwater recharge system so that cane growers can access groundwater when they need it.   Draft Australian guidelinesfor management of the health risks associated with aquifer recharge have been released.

Authors
Mr. Mike Young
The University of Adelaide
Email: Mike.Young@adelaide.edu.au
Mr. Jim McColl
CSIRO Land and Water
Email: Jim.McColl@csiro.au

Acknowledgements
Comments made on earlier drafts by Rick Evans, Phillip Kalaitzis, John Radcliffe, Greg Raison, Alistair Watson, Nadeem Samnakay, Mark Brindal and Mike Williams are acknowledged with appreciation. We would also like to acknowledge the opportunity to discuss this issue with a significant number of state administrators and the support of our Project Steering Committee

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