Groundwater and Surface Water Interactions in the Fractured Rock Areas

An improved understanding of groundwater and surface water interactions in the Wilyabrup and Smiths Brook catchments of Western Australia has resulted from a research project funded by the National Program for Sustainable Irrigation.

The project adds further knowledge about water sources in the state following specific work in irrigation areas including Harvey, various hydrological studies by the Government of Western Australia’s Department of Water, and investigations which were part of CSIRO’s groundwater modeling in south west WA.

A fundamental reason for interest in sustainability of water sources in south west WA is that the region has experienced a 10 to 15 per cent decrease in annual rainfall since 1975 with a big reduction in runoff, raising questions about recharging of groundwater systems.

The contribution made by the Wilyabrup and Smith Brook study, undertaken by the consultancy GHD, has been an intensive data analysis that questions current methods of assessing groundwater and surface water interactions. These methods (which include seepage measurement, hydrographs, hydrometrics, chemical screening and tracers) can each give an insight into surface and groundwater interactions in a given setting. Findings by GHD, however, included a more robust understanding of surface and groundwater processes, and thus their interactions. Their holistic approach and detailed analysis demonstrated, for instance, that the current estimation of annual streamflow based on more limited data from one gauging station was far greater than their calculation.

It was concluded that irrigators and planning authorities like the South West Development Commission and Department of Water would benefit from a standard low-cost method of identifying the nature of interactions and quantifying them.

Principal investigator Fionnuala Hannon said that while the study had provided a valuable body of evidence for authorities and industry it had also raised questions, the most important being about the application of this approach to other catchments, the influence of different land uses over time and the very understanding of what is interflow and baseflow.

For the purposes of this study interflow is defined as a “semi deep” water flow, being above the saturated groundwater zone. It is characterised by water that infiltrates the subsurface and moves both vertically and laterally, typically occurring above discontinuous impermeable layers, before discharging either onto the surface or into other water bodies. For example, groundwater seeps, interpreted as interflow, were observed on hill slopes at one gauging station.

Baseflow is defined as sustained low flow in a river during dry weather conditions, generally all contributed by groundwater and not from surface runoff, particularly in areas without bank storage. Local topography and other factors can influence the importance of baseflow. At Wilyabrup, for example, baseflow is a major contributor in summer months but of no real importance in winter when rainfall runoff is the main contributor to stream flow.

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National Program for Sustainable Irrigation

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id: 3767 / created: 31 August, 2010 / last updated: 31 August, 2010