Effect of vapor transport on soil evaporation under different soil
textures and water table depths in an arid area of northwest China
Abstract
In arid area, the liquid water and water vapor 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 vapor, and heat transports based on the
observed data of atmosphere, soil and groundwater at three soil profiles
in an arid area of northwest China. Comparison and contrast of the
observed and simulated results at the three soil profiles show that
there are diurnal vapor entry and outlet fluxes at the dry surface layer
(DSL) of 30 cm in the summer season. The vapor 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, vapor induced re-evaporation occurs
shortly in the early daytime. Moreover, the extent of vapor entry,
condensation and re-evaporation strong depends on soil properties and
water table depths. The lower water table produces the drier soil
surface, allowing more vapor entry, condensation and re-evaporation.
Whereas the finer grained soil layers benefits the vapor fixation to
produce zero fluxes that substantially inhibit the upward liquid water
and vapor fluxes, and thereby reduces soil evaporation. The reduced soil
evaporation correspondingly decreases the capillary effect on phreatic
evaporation, proven by that soil evaporation decreases slowly with
decline of water table and the large extinct depth of phreatic
evaporation for the finer grained soil profiles. The estimated extinct
depth is 180 cm and 200 cm for the soil profiles consisting of silt loam
and loamy sand, respectively, much larger than 100 cm of the sandy soil
profile. Additionally, as water table is higher and lower than the
extinct depth, the models neglecting the vapor - heat function could
respectively overestimate and underestimate soil evaporation.