van Genuchten (1980) and Mualem (1976) Estimation 
Several investigators such as Brooks and Corey (1964) and Mualem (1976) have proposed closedform equations for predicting the coefficient of permeability of unsaturated soils based on Burdine’s theory (1953). The Brooks and Corey (1964) equation may not converge rapidly when used in numerical simulations of seepage in saturatedunsaturated soils. The Mualem (1976) equation is in integral form and it is possible to derive a closedform analytical equation provided a suitable equation is available for the soilwater characteristic curve.
The equation proposed for fitting the soilwater characteristic curve by van Genuchten (1980) is flexible, continuous and has a continuous slope. The closedform equation proposed for estimating the coefficient of permeability can be used for saturatedunsaturated seepage modeling.
The use of the van Genuchten (1980) and Mualem (1976) unsaturated hydraulic conductivity estimation requires that the SWCC be represented by the van Genuchten (1980) and Mualem (1976) SWCC equation.
k minimum: The k minimum value entered in this field represents the minimum allowable hydraulic conductivity. The minimum hydraulic conductivity becomes a lower limit permeability value that can be used in modeling. A minimum value of 1e14 m/s is assumed to be the point at which liquid water no longer flows. When selecting the minimum value for input, it is important to note that there will be a significant increase in the number of nodes required for solving the problem when the minimum hydraulic conductivity becomes less than about 1e10 m/s. The user is encouraged to slowly reduce the minimum permeability value to ensure convergence of the solution.
Apply k minimum: The equation originally presented by van Genuchten (1980) and Mualem (1976) did not have a lower limit on the minimum allowable hydraulic conductivity. Applying a reasonable minimum value to the unsaturated portion of the permeability function can improve numerical computations.
Van Genuchten & Mualem Prediction: The Van Genuchten (1980) and Mualem (1976) Predicted field indicates whether the fit algorithm has been successfully executed on the current data.
Van Genuchten & Mualem Error: The Van Genuchten (1980) and Mualem (1976) Error is the difference between the fitted results and the laboratory values in terms of R2.
The Estimate button is provided at the bottom of the dialog in order to allow the user to estimate the hydraulic conductivity function.
The Graph button allows the user to display a graph of suction versus hydraulic conductivity. The interpolated curve is also displayed. It is important to note that laboratory data will be interpolated on an arithmetic scale.
