Understanding the fluid exchange between groundwater and surface water is important for effective water resource management. However, groundwater-surface water (gw-sw) interactions also control important ecological variables including water temperature stability, spatial thermal diversity, nutrient availability, and other biogeochemical aspects of water quality. Thus, understanding gw-sw exchange is also important for ecologists who study the health of aquatic habitat in streams, springs, estuaries, lakes and wetlands.
A number of methods exist for quantifying gw-sw exchange rates, but these have predominantly been applied by physical scientists (e.g. hydrogeologists). If methods to quantify gw-sw exchange are to be more widely utilised by other disciplines of aquatic science, it is important that the approaches and associated tools can readily and appropriately applied by non-experts. Here we describe FLUX-LM, a simple Excel spreadsheet tool for quantifying vertical gw-sw flux using measured sediment temperature gradients.
Inputs for FLUX-LM include a temperature-depth profile of the stream or lakebed sediment and some basic thermal properties. FLUX-LM either applies a classic single-layer analytical solution, or a newer multi-layer analytical solution and enables the user to automatically produce an optimal vertical water flux estimate using a clickable ‘solve’ button.
The primary limitation of FLUX-LM is that it assumes steady state temperatures. This condition is violated in shallow portions of the streambed in cases of downwelling, as temperatures vary due the downward propagation of diurnal temperature variations in the surface water. However, upwelling of groundwater to surface water limits diurnal signal propagation to the upper few centimetres of bed sediments, making the FLUX-LM an appropriate tool for the quantification of upward water flux.
We present an example of the tool applied to research regarding native cold water fish spawning habitat in a layered streambed system of contrasting thermal properties. Further potential applications of FLUX-LM will also be discussed.