A Penta-Needle Heat Pulse Probe Array for Soil Subsurface Evaporation and Heat Flux Estimates.

Soil transport processes including hydraulic and thermal fluxes are fundamental to hydrology, ecology and agriculture. Estimates of hydraulic fluxes including evaporation are difficult to measure without lysimeters or eddy covariance systems. Heat flux measurements have typically been carried out using heat flux plates, which are limited by their required measurement depth. We propose a novel array of penta-needle heat pulse probes (PHPP) as an alternative method for subsurface evaporation estimates combined with multi-layer heat flux measurements. Each penta-needle heat pulse probe is capable of determining soil temperature and thermal properties (conductivity and diffusivity) as well as providing calculated heat capacity and volumetric water content estimates at each depth increment. These measurements and calculated properties facilitate estimation of soil heat flux and sensible heat storage. Subsurface evaporation rates can be determined based on the sensible heat balance. We evaluated a 3-PHPP array in a sand column where two different brands of soil heat flux plates were installed for comparison using an overhead uniform IR heat source. Subsurface evaporation estimates were compared to a water mass balance using a load cell. Applications for the PHPP array include weather stations, eddy covariance stations and other subsurface soil monitoring needs.