Soil surface sealing and its effects on infiltration runoff and the soil water regime

This study includes: Quantitative modeling of soil surface seal, its hydraulic conductivity, porosity, water retention, thickness, their variation under different rainfall characteristics and their effect on soil water processes. The objective is to develop sound computational tools for prediction of the dynamic infiltration-rainfall-runoff relationships. Conceptual models have been derived to determine the properties of the seal layer formed under different conditions and their effect on the soil water regime. The findings upset some of the conventional conception of the phenomena: It was found that minor changes (invisible by eye observation, could not be well-identified by a regular microscope or scanning electron microscopy, and thus neglected) which are caused by the raindrop impact lead to significant changes in the soil hydraulic properties and therefore on the flow processes in the sealed profiles. Under specific conditions, the conductance of the seal formed on coarse textured soil might be lower than the seal conductance of a fine texture soil. The importance related to the "skin" layer was sometimes extremely exaggerated, and the interpretation of its role during rainfall was often mistaken. It seems to be rather nonexistent as a uniform impermeable layer when the seal is formed under the strikes of large raindrops. Also, the properties of the seal layer differ for each soil and rainfall. Soil sealing under laboratory conditions might differ significantly from sealing process under field conditions not only because of the change in the soil properties but also because of the differences between the natural rainfall characteristics and the simulated rainfall. The importance of this aspect has been analyzed via calculation of its effect on the infiltration curves under different rainfalls. The errors of neglecting the effect of different rainfall was estimated quantitatively.

Anisotropy of unsaturated soils

In order to investigate the flow processes in anisotropic unsaturated media, a conceptual model was developed to determine the variation hydraulic conductivity tensor as a function of the capillary head. It was for the first time that the anisotropy factor was defined at the microscale, and methods of measurement and calibration were suggested as well. The results indicate substantial differences from those known in the past regarding both the characteristics of the anisotropic soil and their engineering implications for the flow rates, the stream lines and the flow paths of miscible contaminants in unsaturated, anisotropic media. Among other interesting results, it was found that under particular ranges of capillary head, the medium became less isotropic than in saturated conditions. At its minimal value, the medium function as an isotropic one, but approaching drying state, the anisotropy factor may increase by several orders of magnitude. Also, the flow direction from the soil surface to the ground water table varies both as a function of the rainfall intensity and elevation. All these findings have practical engineering significance.

Physical characterization of the regional rainfall

A theoretical analysis has been suggested for the formation of the raindrop size distribution. It was found that there is a regional similarity between rainfalls of different intensities. After a transformation, all the drop size distribution converge into a single curve characteristic to rainfall in that region. Furthermore, a similarity was found between rainfall in different parts of the world (in three different continents) and various climatic conditions. On the basis of these surprising findings, it is possible now to characterize regional rainfalls, and predict the kinetic energy at their impact with the soil surface more efficiently and accurately from a limited amount of measured data by far less than before.

Unified theory of the soil hydraulic properties

We extended our domains theory of porous media, which was applied before to describe soil water hysteresis and the hydraulic conductivity, to predict the diffusion and the electrical conductivity in unsaturated conditions. By this approach, it is possible now to derive from a one measured function all the other hydraulic properties.