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Why surface water/groundwater interaction matters Rick Evans

Historically, the major water resource used in much of Australia has been surface water, but much of inland Australia is totally dependent on groundwater. Over the last 30 years or so (especially since the 1982/83 drought), the importance of groundwater has increased gradually. Indeed, the serious drought over the last decade or more has driven total groundwater use to about 10,000GL per annum, with about 600,000 bores in use and 500 cities and towns dependent on groundwater. However, what is generally not understood is that in much of Australia, the groundwater and surface water is interconnected and is an interchangeable resource. Groundwater becomes surface water and surface water becomes groundwater. However, this common physical reality is not reflected in our water management systems, which generally manage groundwater and surface water as separate resources. As a result, the same water may be allocated to both surface water users and to groundwater users. This “double accounting” of the one parcel of water has not generally been recognised because of the common decades-long delay between increased groundwater use in a catchment and the resultant reduction in river base flow. (Base flow is the low flow in a river that occurs during the dry season and is comprised almost totally of groundwater.) As the time lag is often decades or more, the major increase in groundwater use that has occurred over the last few decades is only now beginning to result in reduced stream flows. But because we have often not understood this process, even though we have capped our surface water usage in some parts of Australia, it is still possible to drill a bore near a river and call it groundwater and hence have it approved. Relative to the mean annual flow of most streams in Australia, double accounting is usually very small. However, double accounting may be important during the lowflow times of the year and during drought. Hence double accounting’s importance for low-flow planning. Groundwater use directly affects the security of supply for surface water users and may have a significant environmental impact.

There is little doubt that the drought over the last decade or more has profoundly reduced stream flows. However, the effect of groundwater pumping has also reduced stream flows, a fact that generally not been recognised. There are several important factors that affect the time lag. The most important are the physical properties of the rocks through which groundwater flows and the distance between the extraction bore and the stream. If the bore is close, say within 100 metres, then the time lag is short, in the order of days to weeks. If the bore is far away, say 50km, then the time lag is long, in the order of hundreds of years. For example, for typical groundwater developments in the Murray-Darling basin the time lags have been shown to be commonly from a few years up to 50 years. The magnitude of the ultimate impact also varies greatly, depending on many catchment processes. In some cases the impact may be only a very small percentage of the volume of groundwater pumped. However, where the bores are relatively close to the stream, the impact may be 100 per cent of the volume of groundwater pumped. A rough rule of thumb is that typically about 50 per cent of the volume of groundwater pumped affects the stream. When the significance of groundwater/surface water interaction is recognised, often in a community-driven water-planning framework, a common kneejerk reaction is to stop or reduce groundwater use to “protect” the surface water resources. There is little doubt that if the surface water resources in a catchment are capped (as they are in the Murray-Darling basin) then in most cases (but not all) the groundwater resources should also be capped. But a halt or reduction in groundwater use may not be the best total water management outcome. In many cases groundwater use is a very efficient use of water. Private owners generally meet the capital cost and the distribution and delivery costs are minimal. In addition, application rates for groundwater-based irrigation tend to be lower than those for surface water. In most of our major river systems, the biggest impact on the

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Securing Australia's Water Future  

Securing water supplies has never been easy in Australia. Managing the nation’s rivers and water resources has been a learning experience, r...