Poster Presentation NCGRT/IAH Australasian Groundwater Conference 2019

The cumulative impact of potential groundwater extraction under the Murray-Darling Basin Plan (283)

Glen R. Walker 1 , Quan J. Wang 2 , Avril Horne 2 , Rick S. Evans 3 , Stuart B. Richardson 4
  1. Grounded in Water, Adelaide, SA, Australia
  2. Infrastructure Engineering, Melbourne School of Engineering, University of Melbourne, Melbourne, VIC, Australia
  3. Jacobs, Melbourne, VIC, Australia
  4. CDM Smith, Adelaide, SA, Australia

Under the Basin Plan, extraction limits were set for the first time across the whole of the Murray-Darling Basin, and for groundwater systems at various depths. The aggregate extraction limit is set at 3494 GL/yr, much greater than the average groundwater extraction of 1335 GL/yr. The unused water can be divided into roughly equal buckets, that within extraction limits that existed prior to the Basin Plan (Baseline Diversion Limit) and that above that (unassigned water). There have been concerns that if groundwater use is increased, the impact on streams may be sufficient to partially undermine the recovery of surface water entitlements for the environment under the Basin Plan. This is despite the MDBA and state jurisdictions implementing steps to minimize the volume of unassigned water in highly connected groundwater systems.

There is no evident increase in extraction over the period 2003-2017. It is thought, however, that over the longer term that changes in technology and economic context would lead to increased use of brackish water and desalination, especially as surface water resources were reduced.  The objective of this study was to assess the impact of potential extraction under the Plan on surface water flow. There are two large uncertainties with doing this: 1) the location and timing of future extraction and 2) the connectivity between point of extraction and streams.

A preliminary risk assessment was conducted in which the first uncertainty was addressed by a scenario analysis, and the second by a sensitivity analysis. This showed that:

  1. there is a significant probability that the surface water impact from development that occurred within 40 years would exceed 100GL/yr, but not exceed 400 GL/yr;
  2. very little of this development within 40 years would occur from unassigned water;
  3. most of the impacts were not associated with saline surface water inflows;
  4. spatially, the single largest risk was the Southern Riverine Plain, where impacts would coincide with those from reduced irrigation recharge; and
  5. should the basin aggregate SDL be reached, the impact is likely to be in the range 190 to 630 GL/yr.

The long time scales, associated with growth in extraction and groundwater response times mean that an adaptive strategy could be used to manage risks. Water management plans and associated SDLs could be adjusted to minimize unaccounted impacts, once growth occurred, with triggers and processes being developed beforehand. The results also support an even more precautionary approach to unassigned water.