Determination of Subsurface Evaporation and Soil Water Content: Implementing a Heat Pulse Probe Array for High-Resolution Measurements.

A sensible heat balance approach was applied for in-situ determination of subsurface evaporation by means of heat pulse probes (HPP). Hexa-needle and penta-needle heat pulse probes facilitate measurements of vertical soil temperatures and estimates of thermal properties (i.e., thermal conductivity and thermal diffusivity), as required by the heat balance method to determine subsurface water evaporation. In addition, the high density temperature needle array allows direct estimation of soil moisture variations at the receding drying front on a millimeter scale, estimated from soil heat capacity and bulk density. A key to obtaining near-real time estimates is on-sensor fitting of thermal properties from temperature rise data. Obtaining subsurface evaporation and soil water content estimates on a millimeter depth scale and in near real-time is an unprecedented achievement. The presentation will focus on a thorough evaluation of fine-scale measurements of subsurface evaporation rate and water content in soil along with the advantages, and limitations encountered with the applied heat pulse probe design.