Correspondence between the Campbell and van Genuchten Soil-Water-Retention Models

Two methods are presented that allow converting the van Genuchten (VG) soil-water-retention (SWR) model parameters to equivalent parameters of the Campbell (C) model. The first method builds on an available solution to convert the VG model to an equivalent Brooks-Corey (BC) model, but accounts for the fact that the effective water content of the BC model and the C model are not the same, because the C model lacks the residual water content parameter, ${\theta }_r$ . The second method (II) relies on a curve-fitting exercise (minimizing the differences between the VG and C-SWR curves) to derive simultaneously the optimal capacity-matching point and SWR-matching point. Results indicate that the second method outperforms the first method and yields an improved equivalence. To bypass the time-consuming curve-fitting exercise, based on Method II two empirical regression equations are presented that allow rapid calculation of the two C-parameters, air-entry and Campbell-b, by the corresponding VG-SWR parameters.