Effect of vapour transport on soil evaporation under different soil textures and water table depths in an arid area of Northwest China

In arid area, the liquid water and water vapour states in soil profiles and fluxes at the upper and bottom interfaces are extremely complex due to heterogeneity of soil textures and the driving forces of heat and matrix potential. In this study, we used Hydrus-1D to simultaneously simulate liquid water, water vapour, and heat transport based on the observed datasets of atmosphere, soil and groundwater at three soil profiles in an arid area of northwest China. Based on the comparison of the observed and simulated results at the three soil profiles, we show that there are diurnal vapour entry and outlet fluxes at the dry surface layer of 30 cm in the summer season. The vapour entry and re-evaporation account for about 14% of annual precipitation for the heterogeneity soil profile with a mean groundwater depth of 210 cm. Because of limited soil moisture in this arid area, vapour induced re-evaporation occurs shortly in the early daytime. Moreover, the extent of vapour entry, condensation and re-evaporation are strongly dependent on soil properties and water table depth. The deeper water table produces the drier soil surface, allowing more vapour entry, condensation and re-evaporation. Whereas the finer grained soil layers benefit the vapour fixation to produce zero fluxes that substantially inhibit the upward liquid water and vapour fluxes, and thereby reduces soil actual evaporation (AE). The reduced AE correspondingly decreases the capillary effect on phreatic evaporation, given that AE decreases slowly with decline of water table and the large extinct depth of phreatic evaporation for the finer grained soil profiles. The estimated extinct depths are 180 and 200 cm for the soil profiles consisting of silt loam and loamy sand, respectively, much larger than 100 cm of the typical sandy soil profile. Additionally, as water table is comparably higher and lower than the extinct depth, the models neglecting the vapour - heat function could, respectively, overestimate and underestimate soil evaporation.