Advances in heat-pulse methods: Measuring soil water evaporation with sensible heat balance

Soil water evaporation is an important process in hydrology, engineering, and agriculture. Few techniques are capable of measuring soil water evaporation in situ. An approach has been developed to measure in situ subsurface soil water evaporation using a soil sensible heat balance (SHB) with measurement data obtained from multi-needle heat-pulse sensors. Terms in the SHB (i.e., sensible heat flux and change in sensible heat storage) are calculated from heat-pulse sensor derived soil temperature and thermal property (i.e., thermal conductivity and heat capacity) measurements for a thin soil layer with thickness corresponding to the sensor geometry. The quantity of latent heat required for soil water vaporization can be determined as the residual to the SHB (i.e., change in heat flux with depth minus change in sensible heat storage with time) for the soil layer. Dividing latent heat (per unit time) by heat of vaporization for water allows data to be converted to evaporation rate. Numerical analysis indicates that the SHB approach is most sensitive to the heat flux component of the SHB. Laboratory and field tests indicate that SHB results compare favorably with mass-balance and micrometeorologic approaches for evaporation measurement, with SHB typically differing by <0.2 mm d(-1) from the reference methods.