Estimating Relative Hydraulic Conductivity from the Water Release Characteristic of a Shrinking Clay Soil

To understand the hydraulic properties of soils, accurate prediction of unsaturated hydraulic conductivity is needed which is commonly derived from the soil water release characteristic. In fine-textured soils the modified Mualem-van Genuchten (MMVG) function has been used extensively to predict the change in relative conductivity with changes in matric potential, but this assumes that the soil is rigid with a constant porosity (CP) which is unrealistic for shrinking soils. We used a triaxial cell apparatus to accurately monitor soil volume changes during water release in a clay soil. From this we derived relative hydraulic conductivity functions based on either MMVG (CP), or based on geometrically-similar shrinkage (GSS) of all pore sizes by a constant factor as calculated from measured shrinking porosity (SP). The MMVG (CP) function predicted a decline in relative conductivity of up to three orders of magnitude from 0 to-400 kPa matric potential. This was similar to published data on the response of conductivity in the same soil to consolidation of larger pores. However, based on SP, the soil remained effectively tension-saturated (>0.97) down to a matric potential of-85 kPa due to normal shrinkage. The corresponding GSS (SP) function predicted only a modest decrease in relative conductivity from 1 to 0.85 over this range. In this case the larger pores, although reduced in size, are assumed to be still water-filled and conducting. The GSS function was probably the most realistic portrayal of hydraulic functioning in a tension-saturated shrinking clay soil. 3 The Mualem-van Genuchten approach (Mualem, 1976; van Genuchten, 1980) has been widely used to predict the change in relative hydraulic conductivity with matric potential, based on the soil water release characteristic (e.g. The central assumption of this approach is that soil is a rigid material and porosity remains constant over the range of matric potentials. This is not realistic for many soils as there is often a loss of porosity upon drying due to In the extreme case, clay soils may exhibit normal shrinkage over a range of matric potentials where the loss of water upon drying is matched by the loss of pore volume by shrinkage such that the soil may remain tension-saturated Over this range of matric potentials, the hydraulic conductivity of the soil may decline due to the reduction in porosity through shrinkage and not due to air entry as is implicit in the theoretical framework of the Mualem-van Genuchten function. However, Childs (1969) …