Soil water flux density measurements near 1 cm d^-1 using an improved heat pulse probe design

The heat pulse probe (HPP) technique has been successfully applied for estimating water flux density (WFD). Estimates of WFD have been limited to values greater than 10 cm d, except for two recent studies with lower detection limits of 2.4 and 5.6 cm d^-1. Although satisfactory for saturated soils, it is recognized that current HPP capabilities are limited for applications in the vadose zone, where WFD values are generally below 1 cm d^-1. Since numerical sensitivity analysis has shown that large heater needle diameters may increase HPP capabilities in the lower flux density range, a HPP with a 4-mm-diameter heater needle was developed and tested. WFD values were obtained by fitting temperature data to the analytical solution for a pulsed cylindrical heat source of infinite length. Effective heater-thermistor distance and soil thermal diffusivity values were determined for specific heat input scenarios with zero WFD, prior to imposing water flow across the HPP needles. We showed excellent results in the range of 1-10 cm d^-1 and satisfactory results in the range of 10-1000 cm d^-1.