Uncertainty and groundwater sustainable yield

Posted 13 March 2008

Figure 1. Ideal reduced uncertainty in sustainable groundwater yield over time (Brodie 2004)

Wendy Timms, Senior Engineer
UNSW Water Research Laboratory

This article discusses the paradox of scientific certainty as it relates to estimates of groundwater sustainable yield. While sustainable yield is difficult to quantify for many groundwater systems, water users and the community need to know that they can be confident in the science used to determine it.

Estimates of how much groundwater can be extracted without adverse effects should be based on sound science that takes uncertainty into account. Pumping too much groundwater can result in impacts on other bores, increased salinity of bore water, reduction in stream flow, and possibly permanent damage to the aquifer.

In Australia we are not widely using strategies that have already been proven in other countries around the world as effective in reducing uncertainty surrounding sustainable yield calculations. Investing in some of these could reduce uncertainties to do with sustainable yield estimates.

Just as there are no guarantees in life, science offers probabilities rather than guarantees. Uncertainty is occasionally black and while, but usually it comes in shades of gray. The language of probability and common examples are outlined in Table 1.

Probability %TermExamples
1 in 10,0000Virtual improbabilityWinning lotto, contracting virus from reclaimed water
1 in 40,000Virtually no chanceKilled in a plane crash or struck by lightning
1 in 10,000Negligible chanceKilled in car accident
<1Extremely unlikelyContracting hepatitis
1-10Little chance or very unlikelyDeveloping diabetes
10-33Some chance or unlikelyDeveloping asthma
33-66Medium likelihood
66-90Likely or probableLong healthy life
90-99Very likely or very probable
>99Virtual certaintySun rise

Table 1. The language of probability and risk. Modified from Pollard (2003), Aust. Bureau of Statistics (2003). Risks per year.

What contributes to uncertain groundwater yields?

Many factors contribute to uncertainty in how much groundwater can be extracted without adverse effects, including the following:

  1. Community values - uncertainty due to changes over time, with greater value typically placed on the environment.
  2. Conceptual understanding - uncertainty due to the nature and dynamics of the groundwater system including variable hydraulic conductivity, recharge processes, surface interactions and significance of human induced stresses.
  3. Quantifying - uncertainty due to simplifying complex natural systems, interpolating between sparse data points, integrating processes at different scales in space and time. Methods to quantify groundwater sustainable yield are discussed by Kalf and Wolley (2007).

Who is responsible for the risks?

Possible future reductions in water availability mean that it is essential that uncertainties and risk are identified, and the responsibilities of various stakeholders are clear. In Australia, the risk of uncertain groundwater sustainable yields is shared between water users and the government (Council of Australian Governments, 2003). In short, the responsibilities are as follows:

  • Water users - responsible for uncertainty and risks associated with natural events such as drought or climate change.
  • Government - responsible for changes in water access entitlements and policies such as new environmental objectives.

What is an acceptable uncertainty in groundwater yields?

Ideally, the degree of uncertainty in groundwater yield estimates decreases as use of the system increases as shown in Figure 1. This means that over time, monitoring and investigations are able to provide improved estimates of how much groundwater can be extracted without adverse effects (Table 1).

Estimated accuracy*Groundwater quantity assessmentExamples
±10%Based on reliable data and investigations that have required little or no extrapolation or interpolationGnangara aquifer
±10% to 25%Based on approximate analysis and limited investigations. Some measured data and some interpolation/extrapolation to derive the datasetBotany aquifer
±25% to 50%Little measured data, based on reconnaissance informationSome inland alluvial aquifers
>±50%Derived without investigation data. Figures estimated from data in nearby catchments, or extrapolated/interpolated from any available dataCoastal sand aquifers

Table 2. Data reliability categories used for sustainable yield estimates in the Australian Water Resources Assessment 2000 (NLWRA 2001) with aquifer examples proposed by the Water Research Laboratory. *Note that uncertainty could be higher when taking into account surface water interactions and provisions for environmental requirements.

How can uncertainty be reduced?

Investing in groundwater monitoring and hydrogeological investigations can greatly reduce the uncertainty in groundwater yield estimates. Irrigators who have recorded groundwater and water salinity data regularly for several decades have very valuable information on which to base farming decisions.

There are many strategies to reduce yield uncertainty that are not yet practised widely in Australia. Automated monitoring of groundwater levels with real-time reporting on the internet is already in place for some aquifers in the US and New Zealand. In Israel, it is mandatory that all new bores are logged with downhole geophysical sondes, subjected to standard bore pump tests and are tested for a range of water quality parameters.

While "all models are wrong" computer based modelling of groundwater systems is another important tool to explain aquifer behaviour and predict future changes (Figure 2). The usefulness of numerical modelling of aquifer systems is enhanced by realistic understanding of groundwater processes, adequate data inputs and advanced computing techniques (Poeter, 2006). Best modelling practice now provides a range of probable groundwater sustainable yield values, while sensitivity testing indicates what assumptions and data are the most critical.

Managing in face of uncertainty

Decisions and management actions can be made with confidence, even with a degree of uncertainty. Management policies are typically based on principals such as 'erring on the side of caution' and 'a lack of knowledge is no excuse for inaction'. Where uncertainty is unknown or is considered to be too high, further investigation and monitoring can improve confidence in 'how much groundwater can be pumped' and minimise the risk of damage to groundwater systems.

Reproduced from Irrigation Australia, Autumn 2008.

More information:

Brodie, RS (2004). That's Not Right ! - Communicating Uncertainty in our Groundwater Sustainable Yield Estimates. 9th Murray-Darling Basin Groundwater Workshop 2004, Bendigo, Victoria.
Kalf RP and Wolley, DR (2007) Applicability and methodology of determining sustainable yield in groundwater systems. Hydrogeology Journal (2005) 13:295-312.
Poeter, E (2006). All models are wrong - which are useful? Darcy Lecture delivered at UTS, Sydney.
Pollard, HN (2003). Uncertain Science, Uncertain World. Published by Cambridge University Press.
Timms, W and Badenhop, A (2006). Bungendore Regional Groundwater Flow Model - Historic Rainfall Variability and Uncertainty Analysis, UNSW Water Research Laboratory Technical Report 2006/02

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