The ups and downs of groundwater in the Murray-Darling Basin

Posted 15 October 2010

Like a bank account, groundwater levels go up with rainfall recharge and are drawn down by extraction for consumptive usage and discharge to rivers. How much groundwater levels change depends on the hydraulic characteristics of the aquifer, and the volume of water moving in and out of storage.

The Murray-Darling Basin plan has begun to co-ordinate groundwater storage calculations for sustainable diversion limits (SDLs). This is a complex process across 78 distinct groundwater areas in the Basin. Only 10% of current water use in the Basin is from groundwater, although the 1700 GL per year on average that is sourced from groundwater is a strategic store for dry periods. Yet groundwater is an important part of the river and wetland system.

The key water resource management issue in inland Australia is reducing water losses by evaporation. Representative mean annual rainfalls are typically less than a two hundred millimetres per year yet open water evaporation rates may average approximately 2 metres per year. Minimising evaporation will increase available water for both agriculture and the environment. As major rainfall events occur relatively infrequently, replenishing storages of low evaporation loss (usually, those underground) must remain a key strategic approach to future water management. Correspondingly, appropriate management of the large open water bodies, bulk water transfers and irrigation efficiency within inland Australia must also remain a priority.

Why not store more water underground? Evaporation losses of 40% are common from shallow waters, yet ring tank storages have become more common. There would be many benefits from modifying water storage practices and by preparing for the next major flood (say 1 in 20 year return interval). Actively managing flood recharge of connected aquifers along rivers would result in more water to keep the rivers running in dry periods, and for consumptive use.

Actively managing the recharge of poorly connected aquifers could partly address falling groundwater levels. However, additions to storage would need to be limited to evaporative losses that would otherwise occur in the system. Groundwater extractions in regions kilometres away from the river or flood pathways, would also benefit from managed aquifer recharge. In some distal regions it would take hundreds or thousands of years for groundwater levels to return to predevelopment levels by natural processes. The challenges of designing effective recharge projects in fully-allocated systems include accounting for losses and gains in the whole system and water quality issues like turbidity.

How much groundwater levels go up and down can be even more critical than the total volume of groundwater storage. Knowing where and when groundwater levels might change is essential to positioning bore pumps for town water supplies, for secure access to irrigation water for our food and fibre crops and to support our livestock sector. Groundwater levels also impact on the roots of trees that reach down to the fringe of the saturated zone. The level of groundwater can influence the volume of river flow, so both surface water and groundwater needs to be considered using integrated approaches. It is groundwater discharge that keeps rivers running during dry periods.

Why does groundwater go up and down in ways that are so different to surface water? Let's look at just three important features of groundwater systems:

  • The volume of stored groundwater is not a vast bottomless cavern, and is mostly limited to sands and gravels. Groundwater is typically stored in tiny pores in sediment and rock. For example, sandy aquifers typically contain 75% sand particles and 25% pore spaces that can be filled with water. In the Basin, the sand and gravels deposits can be up to 200 m deep, underlain by rock which acts like a bathtub. There's usually no groundwater flow from the underlying rock or with neighbouring catchments in the Basin. There's certainly no groundwater source that might feed the Basin from distant parts of Australia or beyond.
  • Second, groundwater flow movements are slower (100-10,000 times slower than rivers). The time lags for surface water connectivity with groundwater partly explains why 'double accounting' and over allocation has occurred. The Basin plan therefore calculates long term averages for groundwater with scenarios of 50 year duration, which is far longer than the typical 15 year planning horizon for surface water.
  • Third, groundwater can be completely disconnected from local recharge and any interaction with rivers or possibility of rainfall recharge. Disconnected groundwater is fossil water that is not a renewable resource. Some of the water used to irrigate our food and fibre is 10,000 to 20,000 years old. Mining fossil groundwater by pumping causes groundwater levels to fall indefinitely. The Basin plan has allowed for 15% of a fossil groundwater store to be extracted over a 200 year period. This approach enables productivity gains during this period of agricultural development while limiting the rate of groundwater fall.

In parts of the Basin including the Condamine, Gwydir, Lachlan, Namoi and Macquarie areas, groundwater levels have fallen by up to 30 meters since the 1960's. Continuing groundwater drawdown in some areas is evident in recent data, particularly in the summer. Droughts and an absence of floods mean that aquifers have not been replenished over the past decade, exacerbating the downwards trend. Considering the status of water balances, the Basin plan guide has indicated that the groundwater balance is about right in 67 SDL areas, but reductions in current entitlement and use is required in 7 SDL areas. Another 4 SDL areas were identified for a reduction in current entitlement but not use.

Yet there are signs that groundwater levels are stable in some parts of the Basin, or may be starting to stabilise in response to changes in land and water management. Increasing groundwater levels in some spots, highlights the need for monitoring and local management planning. In fact, the Basin plan guide identified 26 SDL areas where groundwater use may be increased subject to water quality and accessibility issues.

Commencing a common approach to groundwater SDLs across so many jurisdictions is a mammoth task that is complex and takes time. The Basin plan acknowledges that there are inherent limitations with data analysis and hydrologic modelling of this scale and complexity. This is particularly the case for groundwater that has not received as much attention as surface water resources, and is relatively expensive to investigate and monitor. Data gaps, simplified accounting and the many assumptions inherent in groundwater studies mean that the precautionary principle is of great importance.

Groundwater studies are accelerating to help ensure that fresh groundwater continues to be available for all licensed users and to keep rivers flowing. Reducing the uncertainty for groundwater users requires knowledge of hydraulic parameters of each groundwater system. There are many unknowns for underground water for example: aquifer pumping test data is rarely available to help measure the system, groundwater level monitoring near rivers is not frequent enough to measure aquifer response to flooding events and there is a large mismatch between estimates of leakage below the root zone and estimated recharge to aquifers.

It is widely recognised that groundwater is neither understood nor managed as well as it needs to be to support the rural communities, agricultural productivity, industries and the environment across Australia. The National Water Commission initiated the comprehensive National Groundwater Action Plan in 2007. This plan is investing in various works including the establishment of the National Centre for Groundwater Research and Training, which is a joint venture with the Australian Research Council to build groundwater knowledge and to tackle the severe skills shortage of groundwater technicians, scientists and engineers. Over 80 groundwater jobs are currently unfilled around the Nation, limiting the know-how available for water development and management.

Agriculture and the environment both benefit from training and research by the UNSW Connected Waters Initiative and the UNSW Water Research Laboratory. For example, in 2008, hydrogeologists from UNSW met with farmers from six Basin catchments from Dalby to Hillston. During these workshops, farmers who use groundwater to grow wheat, sorghum and cotton were able to test their bore water quality, plan for water level monitoring and "experiment" with a groundwater tank model. In 2009, a rare survey of groundwater quality was completed and presented to growers, funded mainly by the cotton industry, and best management practices have been developed for water bores.

Gaps in groundwater knowledge are targeted by new UNSW research programmes in association with the National Centre for Groundwater Research and Training. For the first time, groundwater connectivity and levels are being mapped in three-dimensions, setting a new standard for resource management. Heat tracer technology together with geochemistry and natural isotope tracers are being developed to understand river connectivity. Research on storage and movement of water and salt through thick clay sediments is beginning using state-of-the-art geophysical and geotechnical methods. Current research programmes will provide an integrated picture of surface water and groundwater connectivity by combining field data with improvements in computer modelling. Research outcomes will benefit Basin industries that aim to be the most water efficient in the world and help ensure that fresh water is available when it's needed most for rivers and environmental assets across the Basin.

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