Depth-dependent response functions and rainfall transition zones of soil-profile activation under varying antecedent moisture in the Shandian River Basin, northern China

Abstract

Translating event rainfall into soil-profile wetting requires more than recording whether soil moisture increased; it requires estimating how activation response functions change with depth and antecedent moisture. We analyzed 955 rainfall site-events from 20 co-located rainfall and soil-moisture stations in the Shandian River Basin during the non-frozen periods of 2019–2022. Linear mixed-effect logistic models and nonlinear generalized additive mixed models (GAMMs) were compared for EWD ≥10 cm, EWD ≥20 cm, and a detectable response at the 50-cm monitoring layer. Soil moisture response probability decreased from 0.685 at 3 cm to 0.173 at 50 cm, whereas median synchronous 0–50 cm storage gain increased from 0 mm for small events to 12.90 mm for large events. The nonlinear GAMM improved leave-one-station-out AUC only for the 50-cm monitoring-layer response, from 0.789 to 0.806. At median antecedent profile moisture, R20/R50/R80 rainfall requirements were 6.6/13.0/19.9 mm for EWD ≥10 cm, 10.0/19.7/25.5 mm for EWD ≥20 cm, and 21.2/31.5/41.7 mm for the 50-cm monitoring-layer response. These values show a shift toward larger rainfall totals and a wider activation transition with depth. Antecedent-moisture sensitivity was also strongest for the 50-cm monitoring-layer response: under relatively dry conditions, R50 decreased by about 3.58 mm for each 0.01 m 3 m −3 increase in antecedent moisture. The rainfall × antecedent-moisture tensor smooth was not significant. The results define depth-dependent activation response functions, transition ranges, and antecedent-moisture sensitivities within the monitored soil profile.