3D Integration of surface and ground water data for improved catchment knowledge

Posted 6 March 2008

Figure 1: The CWI team installing a multi-port chemical sampling well

B. Kelly, I. Acworth, B. Giambastiani, M. Andersen, A. Greve and A. McCallum

All water recharging a catchment begins as precipitation. Some of this water moves directly to rivers (runoff) and leaves the catchment within a few days or months, unless held back (stored) by dams. A large proportion of the water returns to the atmosphere via evaporation from surface water bodies or transpiration from plants. A much smaller portion migrates downwards to recharge aquifers where it can be stored for periods of between weeks to many thousands of years. These components form a part of the local hydrological cycle.

To understand and manage water movement in the various parts of the hydrological cycle, extensive monitoring of rainfall, evapotranspiration, river flow, dam storage changes (both government and on-farm), groundwater levels, and water usage by all sectors is required. The collection of such an extensive data set requires the collaboration of many government departments, farmers and industries. In order to be able to allocate water on a timely ongoing basis, managers are guided by models that predict how much water will be available at various times throughout the catchment. These models depend on good quality data and accurate conceptualisation of the various hydrological processes within a catchment.

Recent advances in instrumentation and software allow for far more detailed information to be collected and analysed than has been possible in the past. Data loggers are now easy to install in boreholes and this enables data collection on a time scale that can be used to analyse pulse events like storm recharge, or deep drainage recharge from irrigation. Advances in water chemistry techniques, particularly isotope chemistry, allow for better quantification of how waters from various sources are mixing and where they are going. All these data can now be stored and visualised in interactive 3D geological models which provide a better environment to process, analyse and communicate the information.

To demonstrate the integration of these advances, the National Water Commission, Namoi Catchment Management Authority, Cotton Catchment Communities CRC and the Cotton Research and Development Corporation have funded a collaborative project to show how detailed climate, river flow, recharge, irrigation usage, water chemistry, geological and geophysical data sets can be combined to improve our understanding of the processes surrounding the movement of water through a catchment. The project is being led by the Connected Waters Initiative at the University of New South Wales, collaborating with farmers and the NSW Department of Water and Energy.

The large scale movement of water through a catchment is relatively well understood and can be readily modelled. Given enough time (many years), all catchments approach a long-term steady state where the water flows are balanced. What comes into the catchment is balanced by what moves out. Water levels in aquifers stabilize and there is little variation in storage. However, over the shorter term, significant changes occur that we need to understand. For example, when groundwater is abstracted for irrigation or other purposes, the water table elevation frequently falls. If the level is reduced in the vicinity of a river, then space is created in the aquifer for recharge to occur by draining downward through the bed of the river. The CSIRO, in their recent (December 2007 report on the Namoi, have indicated that as much as 99GL/year could be recharged into the aquifers beneath the River Namoi by this process. Clearly, this means that the flow in the river will be reduced by a similar amount and that this water will not be available for downstream diversion or to support environmental uses such as flow to wetlands.

The Maules Creek project being undertaken by the CWI team at UNSW aims to better characterize these important processes and to develop an integrated method for the investigation of surface and groundwater connectivity. A great deal of instrumentation has been installed in the Maules Creek catchment to help address some of these questions. Figure 1 shows installation of a multi-level chemical sampler that will be used to detect the movement of river water into the aquifer. Figures 2 and 3 show the construction of a specialized weir below the outlet of a pumping well so that an accurate record of discharge can be established.

A major component of the project is to develop better visualization tools. The new data collected by the team is combined with historical and current government data and integrated into 3D geological models. Preliminary 3D geological models are shown in Figures 4 to 6. The first model (Fig. 4) shows the simplified 3D geology with two layers, one for the alluvium and the other for the rock. In Figure 5, the alluvium has been removed to display the prehistoric river valleys at depth. In Figure 6, the water quality in the alluvium, indicated by the fluid electrical conductivity, is presented. These 3D geological models will provide the framework for coupled surface and ground water flux models that will be generated as the third stage of the demonstration project. The presentation of the data in 3D greatly facilitates the communication of otherwise complicated geological and hydrogeological concepts.

Reproduced from Australian Landcare Magazine Edition 320, March 2008.

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