Water Allocation
Allocation allows a limited resource to be shared. In the case of water, allocation is currently made on the basis of whether the resource being accessed is currently in surface storage ('surface water') or subsurface storage ('groundwater').Water allocation is based on an estimate of the sustainable yield of a defined resource, derived from an understanding of storage capacity, degree of replenishment and the impacts of extraction. Water allocation frameworks tend not to take into account connectivity. Therefore, connected water resources assessed as not being over-allocated (or over-used) in isolation, may in fact be when considered jointly.
Gaining streams are groundwater fed. Flows in these streams are at risk wherever the sustainable yield for the connected aquifer has been calculated without taking into account the groundwater discharge to the stream. This means that a given volume of water may be allocated twice: once as groundwater and once as (future) stream flow. In many instances groundwater is developed in response to full allocation of surface water. In a gaining stream where flow has been fully appropriated prior to groundwater development, any pumping activity can reduce the water available to existing surface water users. This can be by interception of future stream baseflows or by inducing stream leakage to the aquifer. Equally, for a losing stream which recharges the connected aquifer any new diversions from the stream can reduce the groundwater available to meet the property rights of existing groundwater entitlements. In these catchments it is important that stream management and allocation take into account the requirements of groundwater users.
Potentially, the long-term reduction in streamflow in highly connected systems can equate to the volume of groundwater extracted. However, other mechanisms such as evapotranspiration, leakage between aquifers or discharge to the sea can reduce this volumetric relationship. Also, the time lag between groundwater use and the resulting reduced stream flow can vary from only days where the bores are located close to streams to many years or decades for distant bores. Given the time lags involved with groundwater-surface water interactions, the impact of groundwater development on surface water users may not be recognised for decades. These issues of volumetric impact and time lags need to be considered when managing stream flow depletion due to groundwater extraction.
Depending on the situation, the impacts of over-use across connected river-aquifer systems include:
- A lower flow regime for the river. This can be reflected in greater no-flow days for losing streams or extended periods of flow below a minimum threshold for gaining streams. In extreme cases, seepage fluxes can reverse so that gaining streams become losing. This can have negative impacts for aquatic ecosystems as well as consumptive users;
- Reduction in dilution flows resulting in degraded stream water quality (eg salinity, nutrients) with the extraction of good-quality groundwater;
- Reduction in recharge volumes associated with leakage from highly-developed losing streams, effecting water security for nearby groundwater entitlements;
- Ingress of poor quality water reducing the beneficial uses of the water resource. Intrusion of seawater due to overpumping of coastal aquifers, or further inland, the migration of saline water from surrounding geological formations into the pumped aquifer are examples of this;
- Compaction of the aquifer matrix due to excessive pumping, causing loss of water storage capacity and land subsidence.
Assessment of the sustainable yield for groundwater systems in Australia commonly consider recharge processes , but not necessarily the discharge component. Monitoring of groundwater levels is commonly used to evaluate resource condition, but groundwater flows to streams is not routinely monitored. More focus is required on evaluating and monitoring the discharge of groundwater to surface water resources.