Lysimeter and In Situ Field Experiments to Study Soil Evaporation Through a Dry Soil Layer Under Semi-Arid Climate

During droughts, soil evaporation is often constrained by water vapor transport through an air-dry soil layer (DSL). Fick's water vapor diffusion is widely regarded as the only process for such transport; however, field studies conducted in arid and semi-arid conditions showed measured evaporation rates higher than those predicted by diffusion. Therefore, transport processes other than diffusion could be relevant. To study the evaporation through a DSL, the same lysimeter column with 70 cm thick DSL as earlier applied in laboratory in Balugani et al. (2021), was installed in the field in Spain applying an original weighing setup to measure evaporation. The correlation between the measured evaporation and possible drivers of the water vapor transport were evaluated. With the DSL thickness of 70 cm in 2012 and 12 cm in 2015, the lysimeter recorded similar groundwater evaporation rates: 1.25 and 1.05 mm days⁻¹, respectively; these rates were much larger than the laboratory recorded rates (0.3 mm days⁻¹) and those estimated in this study using Hydrus1D accounting for non-isothermal liquid water fluxes and water vapor diffusion. The main forcing driver of the field lysimeter evaporation was the soil profile temperature fluctuation, which concealed other less important forcing factors, that is, atmospheric pressure fluctuations and diffusion. A multivariate regression model to estimate evaporation was proposed, based on the profile temperature fluctuation, that, when added to the atmospheric pressure fluctuations, yielded reliable estimates of the cumulative evaporation measured in both 2012 and 2015.