The characteristic curve of almost all the existing soils can be satisfactorily simulated by an S-shape curve. However, in cases where the medium has a double porosity (macroporous soils, unsaturated fractured rocks and fissured ground water systems), the S-shape curve fails to describe the characteristic curve. Gerke and van Genuchten have described the physical structure of such a medium and Zhang and van Genuchten [33] have proposed a new model with five parameters, which can simulate the double porosity soils. In this article a special algorithm is developed in order to optimize the new function describing the double porosity characteristic curve. The algorithm is based on the theory of the quasi-Newton method (BFGS).
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