Modified van Genuchten models for describing the entire soil water retention curve

Abstract This study presents a modified van Genuchten model that replaces the residual water content parameter (θR) with shift soil water content (∆θ) to fully characterize the soil water retention curve (SWRC). Derived from this, we introduce both two-segment and three-segment models. The single-segment model, despite high adjR² (0.984–0.999) and low RMSE (0.005–0.031 cm³ cm⁻³), shows deviations for Dune-sand and Loamy sand soils in specific soil matric suction (h) intervals. The two-segment model, with adjR² ≥ 0.970 and RMSE ≤ 0.020, fits loamy and clayey soils more accurately than sandy soils. The three-segment model excels with adR² over 0.99 and RMSE below 0.010 cm³ cm⁻³, closely matching measured data, outperforming the other models in representing the SWRC across all soil types. The models' junctions at θFC and θMWC, representing field capacity and maximum molecular capacity, hold physical significance and correspond to hFC and hMWC, respectively. While hFC is optimally 102.5 cm, hMWC values of 103.5, 103.75, and 104.0 cm are suitable, with 104.0 cm being the most recommended for extending the dry segment equation to predict SWRD at h > 104.18 cm.