A modified equation for fitting the shape feature of the entire soil water characteristic curves

Abstract. The present work aims to modify Fredlund and Xeng (FX) model to simulate soil water characteristic curve (SWCC) in order to be applicable in the field of soil physics and agriculture. Laboratory measurements of the SWCC of clay soil, representing the soil type close to river Nile in Egypt, were carried out using a pressure plate extractor. The measured points of the curve include saturation, field capacity (FC), and permanent wilting point (PWP). The estimation of the regression equations and unknown parameter were performed using non-linear estimation in the STATISTICA program through user-specified regression. The flexibility and capability of the proposed equation (PE) for a wide range of soil suction (ψ) values and soil types were also determined. The SWCC of the experimental soil and statistical tools indicated that PE showed a very good fit to the measured data. The three compared models accurately simulated the entire range of SWCCs for different soil textures. The statistical analysis and fitting curves revealed that the PE precisely simulated the SWCCs for the entire range of ψ values for different soil types and performed better than MVG and FX. Furthermore, only PE was able to simulate SWCCs using different forms of soil moisture, distinct from MVG and FX. The study hence recommends the use of the PE in simulating SWCCs.