The objective of this work was to evaluate eight closed-form unimodal analytical expressions that describe the soil-water retention curve across the complete range of soil water contents. To meet this objective, the eight models were compared in terms of their accuracy (RMSE), linearity (coefficient of determination, m>Rm>2, and adjusted coefficient of determination, m>Rm>2adj), and prediction potential. The latter was evaluated by correlating the model parameters to basic soil properties. Retention data for 137 undisturbed soils from the Unsaturated Soil Hydraulic Database (UNSODA) were used for the model comparison. The samples showed considerable differences in texture, bulk density, and organic matter content. All functions were found to provide relatively realistic fits and anchored the curve at zero soil water content for the coarse-textured soils. The performance criteria were similar when averaged across all data sets. The criteria were found to be statistically different between the eight models only for the sandy clay loam soil textural class. An analysis of the individual data sets separately showed that the performance criteria were statistically different between the models for 17 data sets belonging to six different textural classes. We found that the Khlosi model with four parameters was the most consistent among different soils. Its prediction potential was also relatively good due to significant correlation between its parameters and basic soil properties.
