Validation of coupled water, vapor and heat flow models with evaporation experiments

Evaporation from bare soil is an important hydrological process and part of the water and energy balances of land surfaces. Comprehensive modelling of this process must include coupled liquid, vapor and heat fluxes. Model concepts of varying complexity have been proposed to predict water loss from soil through evaporation. The objective of our study was to test a coupled water, vapor and heat flow model with data from laboratory evaporation experiments under different boundary conditions. Laboratory evaporation experiments were conducted with a sand and a silt loam under three atmospheric forcings. Pressure heads, soil temperature and the evaporation rates were monitored and the experiments were simulated with a coupled water, vapour and heat flow model which solves the surface energy balance and predicts the evaporation rates. The evaporation dynamics was well captured, in particular the onset of stage-two evaporation and the evaporation rates during stage-two. A valid description of the observed data required the use of a comprehensive model of the soil hydraulic properties which accounts for water adsorption and film flow. However, a slow continuous decrease in the measured evaporation rate during stage-one could not be described with the model under the assumption of a constant aerodynamic resistance. While the addition of established empirical soil resistance parametrizations significantly degraded model performance, the use of a boundary layer resistance improved the evaporation rate predictions for the sandy soil but not for the silt loam. Care should be taken when using resistance parametrizations in coupled modelling of bare soil evaporation.