Soils undergoing desiccation exhibit shrinkage and eventually cracking. That may become an environmental problem in practical applications. From a Geotechnical perspective, it is possible to solve the corresponding Thermo-Hydro-Mechanical (THM) equations involved in the problem applied to the soil mass. When this is attempted, the challenge of defining the surface boundary condition arises. The paper presents an analysis of the soil-atmosphere interaction problem in the context of desiccating soils considering experiments and numerical analyses solving the TH equations using an “in house“ Finite Element code. A comparison between measurements and calculations highlights the main variables dominating the evaporation from soil surfaces. The boundary condition applied on the soil surface is discussed in detail and becomes the key aspect in this kind of simulations.