Simulation of solute transport and crop growth often requires knowledge of the water pressure in the soil. Depending on the approach chosen to simulate water dynamics, soil water pressure may be obtained from the soil water content or vice versa. In many cases, water conductivity under unsaturated conditions is also required (such as to solve Richards’ equation or, in some approaches, to simulate water uptake by roots).

Soil water retention functions define the relationship between volumetric soil water content and matrix suction. Matrix suction is the tension at which water is retained by the soil matrix. The retention function provides a way of obtaining the water pressure from the soil water content and vice-versa.

The soil water retention functions are analytical and allow efficient comparison of the hydraulic properties of different soils and horizons. They allow data to be managed more efficiently in models that consider water flow in unsaturated and/or layered soils. The most commonly used retention functions are those by Brooks and Corey (1964), Campbell (1974), Hutson and Cass (1987) and van Genuchten (1980). The Campbell function has a discontinuity at the point of entrance of the air into the soil that causes difficulty in obtaining a numerical solution to Richards’ equation when the soil has a high water content. The function demands three parameters (a, b, saturation water content). The function of Huston-Cass is a simple modification of the Campbell’s function, avoiding the discontinuity point. The function of Brooks and Corey introduces an additional parameter in Campbell’s equation: this parameter represents the residual water content (or irreducible water content) of the soil. The curve of van Genuchten is a continuous and flexible function but it demands five parameters (alpha, water content at saturation, n, m, residual water content).

In the present version, the soil water retention functions of Campbell and vanGenuchten have been implemented.

In order to calculate the unsaturated conductivity, the SoilW component can use the Mualem model (Mualem, 1976), the Campbell model (Campbell,1974) or Burdine model (Burdine, 1953). All these models can be derived from a general model (Kutilek and Nielsen, 1994).

The SoilW component offers an option between the soil hydraulic conductivity function of Campbell (1974), and the Mualem (1976) models of distribution of the pore dimensions in order to describe the hydraulic conductivity of unsaturated soils. The Campbell conductivity function is the most appropriate choice when the Campbell or the Huston-Cass retention functions are used, because they are derived from the Campbell model. Mualem conductivity function is derived from van Genuchten retention function and also it is successfully applied for Brooks and Corey function (van Genuchten, 1980).

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