Calculation of humidity during evaporation from soil

The relative humidity at the surface of a soil is calculated as a function of time from a realistic, exactly solvable model for unsaturated fluid flow. When the evaporation rate is constant, as in the atmosphere-controlled phase, the surface relative humidity suddenly drops at a particular time, even though the humidity–time function is differentiable. This sudden transition still occurs, but is smoother, when more realistic radiation-type boundary conditions are introduced and when the soil-water diffusivity approaches zero continuously at zero water content. In these cases, solutions are obtained by a numerical method of lines which has been validated against the analytically solvable model. A gradual decrease in surface water content is not inconsistent with a sharp, step-like decrease in relative humidity. This is due to the universal exponential Gibbs–Boltzmann relationship between relative humidity and soil-water potential. q 1999 Elsevier Science Ltd. All rights reserved.