A superior soil–water characteristic curve for correcting the Arya-Paris model based on particle size distribution

The soil–water characteristic curve (SWCC) is an essential function for unsaturated soils and describes the relationship between soil suction and water content. The SWCC is a prerequisite for modeling soil water, temperature, and solute transport in hydrology. As a classical and widely used model of the SWCC, the Arya-Paris (AP) model (1981) uses a particle size distribution (PSD) to estimate the SWCC, and the PSD is a parimary property of soils and is easy to obtain by a soil mechanical test. Hence, compared with other models, the AP model is more convenient and accurate for describing the SWCC. In the AP model, a shape coefficient is proposed to describe complex pore structures and correct tortuous pore lengths. However, the shape coefficient changes with the mass of the soil samples, which is a factual and fatal defect for the AP model. To solve this problem, we established a superior Arya-Paris (SAP) model to make the shape coefficient independent of the mass of the soil samples. In the SAP model, the pore length in each divided fraction is the summation of the equivalent diameter of every soil particle in the fraction, and the diameter amplification factor is denoted by the shape coefficient. The total number of soil particles in each fraction is unchangeable in the SAP model but is growing in the AP model. Thus, unlike the AP model, the SAP model has a definite physical meaning, and the SWCCs predicted by the SAP model are closer to the observed data than the AP model. Therefore, the SAP model provides a better alternative for predicting SWCCs and provides new insight into the hydrodynamic characteristics of unsaturated soils. Moreover, a continuous SAP (CSAP) model can be obtained by a continuous function to describe the PSD, which widely expands the application of the SAP.