The Murray Darling Basin Groundwater depletion story
The Low flow in the Murray Darling River system is due to low rainfall which is an ACT OF GOD “El Nino” but the depletion of the basin aquifers is an immoral act caused by the mismanagement of a basin divided by state boundaries and controlled by several rival agencies and consultants competing for dwindling dollars essential for their survival.
Groundwater resources of the basin are overexploited:
It is quite clear from the assessment of the data and analysis provided by the numerous MDB reports that the groundwater resources of the basin based on the appropriate definition of “sustainable yield” are overexploited to a large degree in nearly all areas of the basin (See definition of sustainable yield in this site). It is also evident that the entire basin is under stress. Although the degree of stress differs from one area to another, yet most of the basin is extremely stressed and requires immediate adjustment of management strategies based on hydrological and hydrogeological catchments and not on state boundaries.
Although the percentage of groundwater committed in each state which is based on total licence allocations, actual usage and an arbitrary estimate of the sustainable yield, shows that all states except Queensland are using less than 50% of the resource, the continuous and sometimes dramatic drawdown and the fact that the sustainable yield is not a static but a dynamic figure confirm that all states are exceeding the sustainable yield and using groundwater from storage. Due to the decrease in rainfall and runoff in the last ten years and the decrease in recharge which is accompanied by increased discharge from groundwater to compensate for the decrease in surface water, drawdowns are increasing and the groundwater sources in several areas will be irreversibly damaged.
The lack of knowledge regarding surface water – groundwater interaction has led to the double accounting of water, and unmanaged reduction in stream flow due to groundwater extraction. Both these issues have led to the unsustainable use of the water resource. However, in unconfined and alluvial aquifers where extensive irrigation is taking place from groundwater, there are indications that most of the pumped groundwater is induced recharge from surface water systems caused by the differential pressures which increase the inter-connectivity between the surface and groundwater systems.
As surface water is drying up and groundwater is not renewed more groundwater will be abstracted from aquifer storage; this will cause more drawdown which will have to be supplemented by larger quantities of surface runoff and further depletion of the surface water in the next flood.
Groundwater salinity and dryland salinity:
The increasing salinisation of the rivers and landscape of the Murray-Darling Basin is being driven by rising groundwater levels. These are mobilising salt stored in sub soils and bringing it to the soil surface or carrying it sideways into streams. Streams and rivers become saline through salt being washed from the soil surface or where groundwater levels with high salt loads begin to permanently intersect with the base of the stream or river. This process dramatically increases the water salinity levels.
Salinity is also increasing in the landscape due to irrigation:
Irrigation by groundwater: Groundwater salinity in all areas under irrigation by groundwater is increasing, the gradual increase in salinity will lead to decrease in productivity and eventually to total loss of production.
Irrigation by surface water: Similar trends are also noticed in surface water irrigation areas, but due to the leaching of salts by the fresher surface water, the adverse effects are occurring at a slower pace, but salt is accumulating in the interface zone and will require drainage if productivity is to remain the same.
Dryland salinity:
Due to decreasing rainfall and slower rise of water levels in highlands might give a short lapse in the progression of rising water levels and expansion of dryland salinity areas. Nevertheless, saline groundwater discharge will continue to rise due to the long time required by these catchments to adjust to modified recharge/discharge patterns.
Groundwater management:
Due to the fact that most of groundwater management practices in the basin are based on the arbitrary sustainable yield which is derived from recharge, it is evident that all groundwater management arrangements in the basin requires immediate modification and standardisation and to be based on the appropriate classification of sustainable yield and its factual estimate. It is also paramount that all management strategies be based on hydrological and hydrogeological catchments and not on state boundaries.