Climate change is projected to increase the intensity and frequency of extreme rainfall events, amplifying the risk of surface runoff and associated soil degradation. While relationships between soil organic matter (SOM), bulk density (BD), and soil hydraulic properties are well established, their translation into runoff thresholds under realistic storm structures remains insufficiently quantified.
In this study, we combine national-scale soil observations from the UK Countryside Survey with process-based simulations using HYDRUS-1D to quantify how changes in SOM and BD influence infiltration capacity and the onset of surface ponding and runoff under contrasting storm regimes. Vertical water flow was simulated by solving the Richards equation with van Genuchten–Mualem hydraulic parameterisation across a range of soil textures, BD values (1.15–2.00 g cm⁻³), and storm scenarios representative of UK rainfall structure (25–50 mm over 3–6 h; cyclonic vs frontal).
Results demonstrate non-linear threshold behaviour in runoff generation, with small increases in BD leading to disproportionately large increases in runoff, particularly in medium-textured soils. Across simulated conditions, runoff initiation occurred at lower BD values under short-duration, high-intensity (cyclonic) storms compared to longer-duration events. Analysis of Countryside Survey data indicates a national-scale shift towards lower BD in cropland soils between 2007 and 2020, consistent with modest increases in SOM (~0.003–0.005 g g⁻¹), which correspond to substantial reductions in simulated runoff under extreme rainfall scenarios.
These findings provide a quantitative link between soil structural condition and hydrological response, highlighting critical BD and SOM ranges where management interventions are most effective. The results emphasise that storm structure, in addition to total rainfall depth, is a key control on runoff thresholds, and should be explicitly considered in assessments of soil-based natural flood management strategies.