Modified Campbell (1973) Estimation 
The Modified Campbell (1973) equation is implemented into SVSOILS to provide a hydraulic conductivity equation that decreases and then levels off at some limiting value. The “levellingoff” feature is in keeping with the observation that the hydraulic conductivity of an unsaturated soil becomes essentially constant as the point of residual suction is approached. The residual suction is assumed to be the point where liquid flow in the water phase of an unsaturated soil becomes discontinuous. The Campbell (1974) equation was modified to produce an equation that levels off at approximately the residual suction of a soil.
k minimum: A k minimum value can be entered in this field to represent the minimum allowable hydraulic conductivity. The minimum hydraulic conductivity is reached when the material approaches residual conditions. A minimum value of 1e14 m/s is typical of the point where vapour flow begins to dominate. When selecting the minimum value of k minimum, it is important to note that there will need to be a significance increase in the number of nodes that may be required when the minimum hydraulic conductivity is less than 1e10 m/s. It is therefore recommended that a minimum value of 1e10 m/s be entered for most practical engineering problems. The user is then encouraged to slowly reduce the k minimum value (if necessary) until the model has difficulty in obtaining a solution.
Apply k minimum: The equation presented by Modified Campbell (1973) did not have a limitation on the minimum hydraulic conductivity allowable. Applying a reasonable minimum to the unsaturated portion of the curve dramatically simplifies and improves numerical computations.
MCampbell P: The MCampbell P field needs to be checked when the nonlinear leastsquares fitting algorithm has been used to determine the optimal fitting parameters based on physical data.
MCampbell Error: The MCampbell Error is the difference between the fitted results and 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.
The Graph button allows the user to display a graph of suction versus hydraulic conductivity. The interpolated curve will also be displayed. It is important to note that laboratory data will be interpolated on an arithmetic scale.
