Streaming Potential and Heat Pulse Methods for High Resolution Water Flux Estimates in Porous Media.

Growing population and resource demands have increased concern about water availability and use.  Understanding the movement of water in soil is critical for estimating water availability as well as refining precision irrigation and fertilizer application.  Currently available instruments are not capable of instantly estimating soil water flux, especially at a resolution necessary to capture naturally occurring processes.  These processes include evaporation, redistribution and deep percolation and occur at rates as low as 1 mm d^-1.  Streaming potential (SP) and heat-pulse (HP) methods have shown promise to capture these low water fluxes.  The SP measurement is highly sensitive to water flow, but influencing factors including electrode design, signal drift, electromagnetic noise, pressure gradients and solution ion concentrations can make interpretation of these measurements difficult.  The HP method is the basis for a Penta-Needle Heat-Pulse Probe (PHPP) which is cable of calculating two-dimensional water flux, as well as soil thermal properties.  Previous work has shown that the PHPP is capable of accurately estimating water fluxes down to 10 cm d^-1, but concurrent research has identifies several modifications to increase PHPP measurement resolution.  Laboratory experiments were performed to understand the behavior and capabilities of these two measurement methods.  Presented results demonstrate water flux resolution thresholds of these two water flux assessment methods. We also provide suggestions for further work toward increasing measurement accuracy.