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) α^w=2α^d, (2) n^w=n^d, and (3) θ^w_s=θ^d_s, where α, n, and θ_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 αw/αd for the data set was 2.24±1.25. Nominally cohesive soils had a lower α^w/α^d ratio (1.73±0.94) than nominally cohesionless soils (3.14±1.27). The average n^w/n^d ratio was 1.01±0.11 with no significant dependency on soil type, thus confirming the n^w=n^d simplification for a wider range of soil types than previously available. Water content at zero suction during wetting (θ^w_s) was consistently less than during drying (θ^d_s) owing to air entrapment. The θ^w_s/θ^ds 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.