Estimation of Evaporation Rates From a Subsurface Drying Front Using a Penta-Needle Heat Pulse Probe.

Soil water evaporation plays a crucial role for both the soil surface energy balance and the hydrologic cycle. Recently introduced technique allows computing subsurface evaporation rate during stage 2 by in-situ heat pulse probe (HPP) measurements based on sensible heat balance. The sensible heat flux density is calculated by measured soil temperatures at two depths and measured thermal conductivity, and the change in sensible heat storage between these depths is calculated by measured heat capacity. A penta-needle HPP with a 6.5 mm spacing has been developed to determine two dimensional water fluxes. By rotating the penta-needle HPP using a specific angle (27.3 °) thermisters provide temperature every 2.75 mm in a vertical profile, which is finer than tri-needle HPP (6 mm spacing). In the study presented, laboratory experiments of soil water evaporation were conducted to evaluate the capabilities of the penta-needle HPP for estimating subsurface evaporation rates. A soil water evaporation experiment from sandy column with two HPP close to the surface was carried out in a temperature controlled chamber with constant temperature of 30 °C.