MODELING THE HYDRAULIC-PROPERTIES OF A MULTIREGION SOIL

Numerical techniques for modeling transport of solutes in multiple porosity systems have recently been developed to better represent the physical attributes of structured soils. Multiregion flow modeling requires characterization of the water content (theta) and hydraulic conductivity (K) as a function of pressure head (h) for each region of the system. A technique is presented for determining the theta(h) and K(h) relationships for a three-region soil system based on a typical water retention data base. The demarcation between Region 1 (macropore) and Region 2 (mesopore) was assumed to occur at h = -10 cm. The effect of this assumption was evaluated against criteria of h = -2 and -25 cm, with h = -10 cm giving the best results. The theta(h) function was described as a Fermi function for Region 1 and combined with the van Genuchten model for describing Regions 2 and 3 (micropore). Continuity in theta(h) and K(theta) between regions was achieved and an excellent agreement between predicted and measured values was obtained for each region. However, the derivative of the theta(h) and K(theta) functions were occasionally not continuous through the junction points. The Fermi function based on measured and assumed parameters gave erroneous predictions of K(theta) when a large (25-cm) range in h was used for Region 1. A modified Fermi function was developed that gave an excellent prediction of K(theta) based on assumed parameters.