Estimation of water transport based on in situ measurements of relative humidity and temperature in a dry Tanzanian soil

[1] In situ measurements of relative humidity ( RH) and temperature have been conducted along the profile of an air-dry soil over the course of 3 months. Soil water content data were calculated from RH measurements and the experimentally determined soil water retention curves at various temperatures. In addition, soil water potential data were directly derived from RH measurements, and the water vapor diffusion flux was calculated from the combined RH/temperature data set. The results indicate that traditional concepts for water movement and evaporation under unsaturated conditions must be revised for soils that have an air-dry surface layer. The hydraulic conductivity coefficients determined under these dry conditions was up to five orders of magnitude higher than predicted with pedotransfer functions found in the literature. Thus liquid water flow contributed more to the overall water transport than would have been expected according to traditional concepts. Water vapor transport only dominated in the top surface layer at very dry conditions. After 3 months of constant evaporation, the evaporation front had not moved deeper than about 3 cm into the soil because water was effectively recharged from deeper layers of the soil.