Groundwater – surface water interactions and implications

The Level 1 release of AWR 2005 highlighted the relative lack of understanding of surface water – groundwater interactions across most of Australia. Double counting of water resources - where the available resource is counted as a surface water stock and as a groundwater stock - is commonplace and occurred to some extent during the Australian Water Resources Assessment 2000 (NLWRA). Some jurisdictions are now developing integrated water resource management plans to address double counting and to recognise the movement of water between surface water and groundwater systems as part of the hydrologic cycle.

The variation across the country has meant each jurisdiction has developed its own policies and management practices to deal with groundwater and surface water interactions. It has generally been only where the resource is highly developed that impacts of extraction from groundwater on surface water (and vice versa) have been noted, and management plans changed to take this into account. In general, it is groundwater management plans that take into consideration impacts on surface water. Of the 62 surface water management plans (final or draft) across Australia, 33 had considered groundwater, yet 107 of 120 groundwater plans (89 per cent) had considered surface water impacts in their plans.

Many authorities have placed caps on resource use to restrict the growth in extractions. A well-known example of a cap on a water resource is the Murray-Darling Basin Cap on surface water diversions. After an interim Cap in 1995, the upper limit on the amount of water that could be taken from the Murray-Darling Basin river system was defined in 1997 as:

The volume of water that would have been diverted under 1993–94 levels of development.

The Murray-Darling Basin Cap is only on the surface water resource and infrastructure associated with it. There has been an increase in groundwater extractions in the Murray-Darling Basin since the Cap on surface water diversions was introduced. In the absence of new policy signals it could be expected that this trend may pose a serious threat to the long-term availability of surface water in the Murray-Darling Basin, because the Cap does not recognise the importance of surface water – groundwater interactions in resource management.

Groundwater pumping is used to lower the water table as a salinity control measure in West Gippsland
Image by Chloe Wiesenfeld, sourced from SKM

The water balance approaches that are part of AWR 2005 have provided a greater understanding of the importance of surface water and groundwater interactions.

Integrated management of groundwater and surface water requires a detailed understanding of the degree of connectivity between the two resources. As recognised in previous components of AWR 2005, only limited technical information on the degree of connectivity between groundwater and surface water is available across Australia and more detailed on-ground assessments of connectivity need to be completed to improve the level of integrated management of groundwater and surface water resources nationally.

From the water balances, it is clear that data on groundwater – surface water interactions are often limited and unreliable. Even water management areas with available information have generally based their estimates of the volume interchanged between streams and groundwater on the current, or ‘developed’ condition. This means that the current exchange between groundwater and surface waters will already have been affected by the level of development within the basin. Typically, in a ‘natural’ system, where groundwaters are not extracted and the groundwater level has not dropped, the flow to a stream, especially in low flow conditions, would be considerably higher than is currently experienced.

An estimate of the level of interaction from those balances with data on groundwater – surface water interactions has been made (Table 1); but these values will be considerably different to the ‘natural’ conditions for the catchment (before the water resources were developed). Analysis of natural streamflows and groundwater levels prior to development is required to understand this, and information of this type was not available for the water balances compiled for this study. By using the values of surface water flows to groundwater and vice versa, it is possible to assess the importance of groundwater – surface water interactions in relation to the total inflow of water to the water management areas.

Level of groundwater and surface water interactions in the water management area water balances where data were available in 2004–05

Notes:

• Source: AWR 2005
• GW = groundwater; SW = surface water
• n/a = no detected exchange or data not available
• Areas shown as >100% indicate areas with a high level of interaction, based on a cumulative, absolute flow from surface water to groundwater (and vice versa) or a low value of groundwater recharge for the water management area

There are 17 areas where surface water – groundwater interactions are high compared to the total runoff and recharge (more than 10 per cent of total inflows) include the areas listed below. The 10 per cent level of interaction has been used as a criteria in this assessment as it represents a still significant volume of flux between the parameters within the overall water balance.

In making this comparison, it must be recognised that some data unreliability may exist where, for instance, groundwater baseflow volumes to streams have been estimated.

These results are preliminary and based on a desktop assessment, using data provided by the jurisdictions. The commission has recently funded a more comprehensive surface water – groundwater interaction study that will shortly report on current status.

Key observations

• Groundwater – surface water interactions appear to be significant across most of the priority areas. The potential for double counting appears high, given the levels of interaction.
• Seventeen out of the 51 water management areas investigated showed interconnectivity between surface and groundwater of more than 10 per cent of the total of runoff and recharge. This has important implications for measuring extractable water in a way that minimises the double counting of surface and groundwater resources that has occurred in the past.

Surface water and groundwater interaction as a percentage of total runoff and groundwater recharge in water management areas

Surface water and groundwater interaction as a percentage of the total runoff and groundwater recharge in interjurisdictional areas.

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