Determination of Subsurface Soil Evaporation Using a Heat Pulse Probe Array.

Soil-water evaporation is a crucial component of both the surface energy balance and the hydrologic cycle, coupling heat and water transfer between land and atmosphere. Bare-soil evaporation and plant-soil-atmospheric interactions are important components of the water balance, especially in semiarid and arid regions. The rate of evaporation is affected by atmospheric variables and processes such as temperature, humidity, thermal and hydraulic conductivities, vapor diffusivity, and wind induced convection. Our research initially aims to determine subsurface water evaporation in a soil column exposed to constant atmospheric conditions with a heat balance method. In a second step, the sensible heat balance method will be applied under field conditions to assess the diurnal cycling of energy inputs. In-situ soil water evaporation dynamics are measured using a heat pulse probe designed to determine soil temperature and thermal properties. The probe consists of a heater needle and five or six thermistor needles. A small heat input is applied to a resistance wire in the heater needle, while the remaining thermistor needles measure the temperature response at fixed distances of 6.5mm from the heater. The heat pulse probe is rotated 27.3° from a vertical orientation yielding temperature measurements every 3mm within the soil profile. In order to get finer thermistor spacing near the soil surface, a sixth thermistor is added in between the top two array needles. Measurements of soil temperature and thermal properties (thermal conductivity and thermal diffusivity) obtained with the heat pulse probe array allow calculation of the sensible heat balance below the soil surface. The heat pulse probe array provides an effective means to determine subsurface soil-water evaporation rates and can be coupled with surface evaporation estimates using remotely sensed measurement techniques to allow separation of evaporation from evapotranspiration measurements.