BIMODAL POROSITY AND UNSATURATED HYDRAULIC CONDUCTIVITY

Soil water characteristic curves determined on undisturbed soil samples in the laboratory were used for the calculation of the unsaturated conductivity function K(h). This computed K(h) was compared to the unsaturated conductivity data measured directly in the field. When the soil water characteristic curve was smoothed according to the van Genuchten procedure and K(h) was evaluated according to Mualem's approach, the coincidence of results of both procedures was poor. When a cubic spline was used for the water retention data, the derived pore size distribution curve showed two peaks. With the assumption on a bimodal porous system, K(h) was computed for each of the subsystem of pores. The function K(h) of the whole porous system is then composed of two parts, one valid close to saturation, the second for medium wetness of the soil. At the intersection of both components of K(h) a singularity exists. The agreement between the K(h) so computed and the directly measured K data has been improved. The hypothesis on the existence of the bimodal porosity within the range of the capillary soil porous system has been therefore confirmed together with the proof on the practical importance of both subsystems when they are detected in the soil.