HYDRAULIC CONDUCTIVITY ESTIMATION FOR SOILS WITH HETEROGENEOUS PORE STRUCTURE

The hydraulic conductivity function, which is required to solve the Ri chards equation, is difficult to measure. Therefore prediction methods are frequently used where the shape of the conductivity function is e stimated from the more easily measured water retention characteristic. Errors in conductivity estimations can arise either from an invalidit y of the prediction model for a given soil, or from an incorrect descr iption of the retention data. This second error source is particularly important for soils with heterogeneous pore systems that cannot be ad equately described by the usually used retention functions. To describ e the retention characteristics of such soils, a flexible theta(psi) f unction was formed by superimposing unimodal retention curves of the v an Genuchten (1980) type. By combining this retention model with the c onductivity prediction model of Mualem (1976), conductivity estimation s for soils with heterogeneous pore systems are obtained. Estimated co nductivities by this model and the classical van Genuchten-Mualem meth od can differ by orders of magnitude. Thus reported disagreements betw een measured and estimated conductivities may in some cases be due to an inadequate description of the retention data rather than due to a f ailure of the prediction model.