Interpreting Soil Water Retention Curves through Overlapping Pore-Functional Storage Domains

Soil water retention curve (SWRC) models describe a measured curve through a closed-form equation, and pedotransfer functions (PTFs) estimate its parameters where no measurement exists. Neither step passes through an explicit description of the pore-size distribution. This study represents the SWRC of mineral soils as a superposition of six fixed, shared storage domains. Five, identified once from 297 calibration soils, span the capillary range; a sixth, centred by convention at the oven-dryness suction (pF 6.8) rather than fitted, is provisionally interpreted as adsorptive storage and replaces the residual water content θr. This re-parameterizes the residual term rather than adding to it: converting a conventional free-floor fit reproduces every calibration curve to within 2.7 × 10⁻⁸ cm³ cm⁻³ at or below pF 4.2, with representation accuracy unchanged. What it changes is that the vertical scale of a predicted curve reduces to porosity alone. The six domains reconstructed measured curves with R² = 0.973, and 11,964 curves of the independent Global Soil Hydraulic Properties (GSHP) database with R² = 0.993. Leave-one-out prediction of the six weights from sand, clay, organic matter and bulk density gave R² = 0.786; fully external prediction gave 0.716 on GSHP and 0.807 on a second continental dataset (EU-HYDI). The complete specification is 30 regression coefficients with no texture-class lookup table, against 47 fitted quantities for an equivalent conventional specification, without loss of predictive accuracy on independent data. The shared basis generalizes broadly; the pedotransfer equations remain sensitive to texture and regional mineralogy.