A simple method for estimating water diffusivity of unsaturated soils

Numerical models have been extensively used for predicting water and solute transport in saturated and unsaturated soils. Soil hydraulic properties are required for quantitatively describing water and chemical transport processes in soils by the numerical models. Soil water diffusivity is one of the important hydraulic properties. Several approaches have been developed to estimate the soil water diffusivity, however the intensive calculations and time-consuming measurements required by the methods for determining soil water diffusivity limit the application of the methods. It is necessary to develop a method with easily performed experiments for determining soil water diffusivity. In this paper, the problem of water absorption into a horizontal soil column is solved, and the relationship between cumulative infiltration and infiltration rate with the distance of the wetting front are obtained. Based on the relationships, a new and relatively simple method for estimating soil water diffusivity is presented in this paper. Experiments of water absorption into 60-cm long and 25-cm long horizontal soil columns were conducted to evaluate the method. Estimates of soil water diffusivity by the new method were in good agreement with estimates by the Bruce and Klute method. With the new method short (approximately 8 cm) soil columns can be used, and it is possible to use the new method to estimate soil water diffusivity of undisturbed soils. Therefore, the present method provides simplicity for determining soil water diffusivity