The Hydraulic Conductivity Function of Natural Soils.

For the prediction of the unsaturated hydraulic conductivity function, it is customary to use the Van Genuchten model (1980) for the water retention curve (WRC). Following some integration process (Mualem or Burdine), one inevitably ends up in the ballpark of the beta functions. An alternative model for the WRC, the Groenevelt-Grant model (2004), leads one, following the same integration process, inevitably into the ballpark of the gamma functions. While the two models produce virtually identical water retention curves, the strategic advantages of using the GG model over the VG model are considerable. During the integration process, one has to deal with the tortuosity. Using fundamental physical principles in dealing with the tortuosity, it becomes clear that the Burdine integration is preferable over the Mualem integration. The unsaturated hydraulic conductivity function of a natural soil has a strong tendency to be bi-modal. The physical reason for this will be discussed. A generalized Burdine integral produces precise fitting of the hydraulic conductivity functions of all 20 Rijtema soils. The parameter values, used in this generalized Burdine equation, will be discussed.