Improved Determination of Soil Water Flux and Thermal Properties with a Penta-Needle Heat Pulse Probe.

An accurate method for in-situ determination of soil water flux continues to be the most sought after and yet elusive hydrologic measurement. The penta-needle heat pulse probe (PHPP) employs a central heater needle surrounded by an orthogonal arrangement of four thermistor needles for estimation of both water flux magnitude and direction. An analytical solution and inverse fitting method are presented for simultaneous estimation of thermal properties and soil water flux using PHPP measurements. The approach yields estimates of both components of the flux in a plane normal to the axis of the PHPP needles. The method was evaluated by using PHPP data from a laboratory experiment in which columns of water-saturated sand were subjected to fluxes ranging from 1.2 to 33,000 cm d^-1. Improved flux determination was achieved by performing the inverse analysis with values of apparent heater-thermistor spacing that were determined from PHPP data in the absence of flow. Thermal properties were estimated with coefficients of variation less than 1.4 %, indicating that the inverse problem is well-posed and yields unique parameter estimates when water flux is less than 2,000 cm d^-1. Estimates of the x- and y-components of water flux agreed well (R² > 0.97) with measured water fluxes up to 7,000 cm d^-1. Estimates of flow direction based on the components of the flux were in good agreement with probe installation angles for water fluxes ranging from 10 to 7,000 cm d^-1.