Probabilistic and Spatial Variability Characterization of Soil Hydraulic Parameters in Arid Ephemeral Watersheds Based on In Situ Double-Ring Infiltration Tests for Enhanced Hydrological Modeling

Uncertainty in soil hydraulic properties is a major source of hydrological and flood-modeling error, especially in data-scarce, heterogeneous arid soils. In ephemeral stream watersheds, infiltration strongly controls runoff generation and flash-flood response, yet soil parameters are often treated as deterministic, ignoring their inherent variability. This study characterizes the probabilistic and spatial variability of infiltration-related soil hydraulic parameters using in situ double-ring infiltration tests at 25 sites in Jazan Province, Saudi Arabia, during a single field campaign. Data were interpreted using the Horton, Philip, and Green–Ampt models to estimate hydraulic conductivity (K), storage suction factor (ΨΔθ), and soil sorptivity (Sp), with distributions selected via the Kolmogorov–Smirnov (K-S) test. K showed high, log-normal variability, with means of 5.27, 4.59, and 4.92 m/day (CV = 1.21, 1.34, 1.29), respectively; ΨΔθ was even more variable (mean = 0.09 m, CV = 1.63, log-normal), while Sp was less variable (mean = 0.47 m/day0.5, CV = 0.66, Gamma). Directional variogram analysis revealed anisotropic spatial structure, with short along-coast ranges (~4–10 km) and weaker inland structure, supporting nearly independent sites. The novelty lies in the statistically robust, previously unreported probability and spatial-correlation models for infiltration parameters in arid ephemeral streams, enabling direct use in stochastic watershed modeling.