Impacts of groundwater extraction on surface water flow
Groundwater use across Australia increased 58% from 2,600 GL to 4,200 GL between 1983/84 and 1996/97 (NLWRA 2001). In percentage terms, in some States (such as New South Wales, Victoria and Western Australia) the increase in groundwater use over this time period was more than 200%, refer Figure 1. In volume terms, Western Australia increased groundwater use by almost 800 GL, followed by New South Wales with a 670 GL increase. In Tasmania and the Northern Territory, the corresponding increase in groundwater use was about 100%. However, there was a slight reduction in groundwater usage in Queensland (26%) and South Australia (22%) over these years.
Increase in groundwater use in southeastern Australia has been largely attributed to irrigation in the Murray Darling Basin. From 1 July 1997, the volume of surface water diverted from the Murray Darling river system in New South Wales, Victoria and South Australia was capped by agreement at 1993/94 levels. However, groundwater use in the Basin was not capped and has continued to increase steeply since that time with migration of demand. Low rainfall during the late 1990s has also driven groundwater development. Of concern in Basin water policy is the extent to which projected increases in groundwater extraction will be reflected in decreased base flows to rivers, so impacting on the integrity of the Cap.
Table 1 summarises the water 'account' in the Murray-Darling Basin. Surface water allocations have been capped by intergovernmental agreement at 11,246 GL/yr. Groundwater licences have been issued that could allow the extraction of 3,261 GL/yr, a significant volume at around 34% of the surface water allocation. While there are uncertainties around estimating the proportion of groundwater extraction in connected systems that should be accounted for as surface water, stream-flow depletion is projected to be 330 GL/yr in 20 years (MDBC, 2006). It has also been estimated that 186 GL/yr of streamflow has already been captured due to the growth in groundwater extraction during the period from the introduction of the Cap until 1999/00 (SKM, 2003).
| Issue | Description | Gigalitres/year (GL/yr) |
|---|---|---|
| Average Runoff | Average runoff over the MDB catchment area (NLWRA, 2001) | 23,850 |
| Surface water allocation | Existing allocation of surface water licences in the MDB (SKM,2003) | 11,246 |
| Groundwater sustainable yield | Estimates of the sustainable yields for groundwater management areas in the MDB (MDBC, 2006) | 2,356 |
| Groundwater allocation | Current allocation of groundwater licences in the MDB (SKM, 2003) | 3,261 |
| Double accounting | Range of estimates of streamflow depletion in 20 years due to groundwater pumping (MDBC, 2006) | 275-550 |
| Surface water storage capacity | Water storage capacity of reservoirs in the MDB (MDBC, 2006) Does not include storage associated with the Snowy Mountains Scheme | 25,000 |
Figure 1: Change in groundwater use between 1983/84 and 1996/97 (NLWRA 2001)
References
MDBC, 2006. The shared water resources of the Murray-Darling Basin. MDBC Publication 21/06. Murray Darling Basin Commission, Canberra.
NLWRA 2001, Australian Water Resources Assessment 2000, Surface water and groundwater - availability and quality, National Land and Water Resources Audit, Canberra.
SKM 2003. Projections of groundwater extraction rates and implications for future demand and competition for surface water. Murray Darling Basin Commission Publication 04/03. Sinclair Knight Merz.