Measuring Subsurface Soil‐Water Evaporation with an Improved Heat‐Pulse Probe

Abstract

Recently a three‐needle heat‐pulse probe has been used to monitor subsurface soil‐water evaporation dynamics. Several of these probes have to be installed precisely at different depths to measure subsurface evaporation for multiple soil layers, and approximation of the shallow temperature gradient is limited by needle spacing. In this study, an 11‐needle heat‐pulse probe, including four heating needles among the 11 temperature needles and 1‐mm spacing between the upper‐most needles, is used to determine soil‐water evaporation in the 0‐ to 5‐cm soil layer. A field test showed that 4 d after irrigation, evaporation was observed in the 1.5‐ to 4‐ and 4‐ to 10‐mm soil layers and that the peak evaporation rate in the 1.5‐ to 4‐ and 4‐ to 10‐mm soil layer were 0.26 and 0.18 mm h−1, respectively. After 5 d, the bottom of the evaporation zone propagated to a depth of 17 mm, and the peak evaporation rate in the 10‐ to 17‐mm soil layer reached 0.15 mm h−1. After 6 d, there was significant evaporation occurring in the 17‐ to 23‐ mm soil layer with a peak evaporation rate of 0.15 mm h−1. The new 11‐needle heat‐pulse probe was better able to measure the depth and time dynamics of subsurface soil water evaporation for multiple soil layers by improving on the previous three‐needle design.