Hysteresis and Uncertainty in Soil Water-Retention Curve Parameters

Accurate estimates of soil hydraulic parameters representing wetting and drying paths are required for predicting hydraulic and mechanical responses in a large number of applications. A comprehensive suite of laboratory experiments was conducted to measure hysteretic soil-water characteristic curves (SWCCs) representing a wide range of soil types. Results were used to quantitatively assess differences and uncertainty in three simplifications frequently adopted to estimate wetting-path SWCC parameters from more easily measured drying curves. They are the following: (1) alpha w=2 alpha d, (2) nw=nd, and (3) theta sw=theta sd, where alpha, n, and theta s are fitting parameters entering van Genuchten's commonly adopted SWCC model, and the superscripts w and d indicate wetting and drying paths, respectively. The average ratio alpha w/alpha d for the data set was 2.24 +/- 1.25. Nominally cohesive soils had a lower alpha w/alpha d ratio (1.73 +/- 0.94) than nominally cohesionless soils (3.14 +/- 1.27). The average nw/nd ratio was 1.01 +/- 0.11 with no significant dependency on soil type, thus confirming the nw=nd simplification for a wider range of soil types than previously available. Water content at zero suction during wetting (theta sw) was consistently less than during drying (theta sd) owing to air entrapment. The theta sw/theta sd ratio averaged 0.85 +/- 0.10 and was comparable for nominally cohesive (0.87 +/- 0.11) and cohesionless (0.81 +/- 0.08) soils. Regression statistics are provided to quantitatively account for uncertainty in estimating hysteretic retention curves. Practical consequences are demonstrated for two case studies.