A New Algorithm for Determining Soil Evaporation Using Heat Pulse Probe Measurements.

Recently developed heat pulse probe (HPP) methods exhibit great potential for estimating soil subsurface evaporation based on an energy balance approach. Generally, the HPP measures soil temperature and soil thermal properties within a fine grid below the soil surface. Evaporation (i.e., latent heat flux) can be determined from the calculated sensible heat flux and heat storage change within the thin surface layers of the soil profile. A shortcoming of the existing HPP method is that evaporation occurring from the soil surface down to the midpoint of the two top needles (i.e., the undetectable zone) is not sensed. Hence, our objective is to focus on and develop an approach for estimating the evaporation occurring within the undetectable zone. Solving the governing equations within this zone as well as the entire soil profile, we introduce a new method for calculating stage 1 and state 2 soil evaporation using HPP data. The method has been validated using numerically simulated data from a transient evaporation process. Compared with the water balance approach, the results indicate the proposed HPP method significantly improves evaporation estimates. The new method is recommended for improving estimates of both surface and subsurface evaporation and can replace previous methods using HPP data to estimate soil evaporation.