Groundwater and surface water are connected resources. Understanding this connectivity is essential for the management of water as a single sustainable resource.

The University of New South Wales Water Research Laboratory (WRL) is currently undertaking a major research project within the Maules Creek Catchment of the Namoi River in New South Wales, funded by the Cotton Catchment Community CRC. Part of this research is to investigate the connection between surface water and groundwater.

One of the challenges presented by this investigation is the fundamental problem of how to measure the movement of water between a stream or river and an aquifer below.

Traditionally, the flow rate (or flux) of water is calculated using the Darcy method. This technique combines measurements of the water gradient and the hydraulic conductivity of the sediments to calculate the flow rate.

However, when the conductivity of the sediments is poorly understood or highly variable, as is the case in most sites including Maules Creek, this method cannot be relied on to produce accurate estimates of flux.

The research work done by Masters student Gabriel Rau and the team at WRL has set out to find alternative methods of measuring flow rates without the detailed knowledge of the system required by traditional hydraulic techniques.

One such method is to study the transfer of heat from the surface into the sediments. This is based on the theory of heat conduction and convection. In a hydraulic system where water travels from the surface into the sediments, there will be a relationship between the temperature at the source (the surface) and the temperature at a point at depth within the sediments, which amongst other factors depends on the rate of water flow. By measuring the temperature difference between these points it is possible to derive the rate of flow of water.

To find out more about the water exchange between Maules Creek and the aquifer below, specially designed arrays of temperature sensors were installed at a number of sites along the creek.

Data obtained from these temperature arrays showed that temperature did vary at different depths in the sediments. These variations were analysed to calculate the flow rate at each site.

Each array consisted of five temperature probes separated by insulators. The arrays were driven down into the stream bed with the uppermost probe at the surface of the sediments. This allowed temperature to be measured at five different depths at 15 minute intervals.

By computer modelling of heat transfer and fluid transport, the temperature variations in the streambed sediments could be predicted. By matching the real data and the results of the computer model the water flux could be determined. A flux of approximately 0.6 m/day downwards at Elfin Creek was computed in this way.

This work demonstrates how natural heat can provide a reliable method for estimating fluxes between surface water and groundwater, and is particularly suitable for situations where knowledge of the sediments is limited and conditions are variable such as in the Maules Creek Catchment.

The research team has recognised the scope for this methodology to be further developed and used in different kinds of sediments and to provide information about water movement in two and three dimensions.