Simulation of an evaporation test in a soil column have been carried out to characterize the properties of the topsoil of the El Cabril Radioactive Waste Disposal Facility and the evaporation process. The conceptual model considers unsaturated liquid flow, vapour flows and heat flows of a sample of 14 cm. of the vegetal soil. Vapour fluxes become the main water flow mechanism under very dry condition. Water leaves the column as vapour only at the top of the column, due to the difference of relative humidity and temperature between the column and the laboratory. Very dry conditions necessitate the modification of the retention curve to represent oven dry condition and it has been calculated considering the experimental values obtained by CIEMAT (Villar et al. 2012). The model has been calibrated with the experimental data obtaining the values of the parameters of the constitutive laws and of the boundary conditions, suggesting that it can be used to assess the behavior of the soil under evaporation conditions. Finally, an abnormal behavior of the sample during the test has been displayed due to a bigger value of β.