Soil water retention parameters are critical to quantify flow and solute transport in vadose zone, while the presence of rock fragments remarkably increases their variability. Therefore a novel method for determining water retention parameters of soil-gravel mixtures is required. The procedure to generate such a model is based firstly on the determination of the quantitative relationship between the content of rock fragments and the effective saturation of soil-gravel mixtures, and then on the integration of this relationship with former analytical equations of water retention curves (WRCs). In order to find such relationships, laboratory experiments were conducted to determine WRCs of soil-gravel mixtures obtained with a clay loam soil mixed with shale clasts or pebbles in three size groups with various gravel contents. Data showed that the effective saturation of the soil-gravel mixtures with the same kind of gravels within one size group had a linear relation with gravel contents, and had a power relation with the bulk density of samples at any pressure head. Revised formulas for water retention properties of the soil-gravel mixtures are proposed to establish the water retention curved surface models of the power-linear functions and power functions. The analysis of the parameters obtained byregression and validation of the empirical models showed that they wereacceptable by using either the measured data of separate gravel size group orthose of all the three gravel size groups having a large size range.Furthermore, the regression parameters of the curved surfaces for thesoil-gravel mixtures with a large range of gravel content could be determinedfrom the water retention data of the soil-gravel mixtures with tworepresentative gravel contents or bulk densities. Such revised water retentionmodels are potentially applicable in regional or large scale fieldinvestigations of significantly heterogeneous media, where various gravel sizesand different gravel contents are present.
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