Mechanistic modeling of the impact of rainfall pumping on soil solute remobilization into runoff

Abstract. The remobilization of solutes from the soil to surface runoff is a critical process for surface water contamination, yet it remains challenging to model mechanistically. This study explores a novel mechanistic modeling approach that explicitly accounts for local advective transport driven by pressure fluctuations induced by raindrop impacts (“rainfall pumping”) on the soil surface. We coupled the HYDRUS-1D model with a time variable, surface pressure head boundary condition, which combines runoff depth and the semi empirical, sinusoidal rainfall pumping wave of Higashino and Stefan (2014) whose amplitude and frequency are estimated from runoff depth and rainfall intensity. This boundary condition was tested for the first time against experimental data on saturated soils using two benchmark studies: one for model development and another for independent verification. Two additional datasets were used to verify the physical consistency of the pressure head values estimated by the pumping formulation. The results were compared with those from a conventional “no-pumping”, fixed runoff depth boundary condition. The rainfall-pumping boundary condition improved estimates of the final soil bromide concentration profile, solute extraction depth, and total remobilized mass, with the pumping model achieving the best predictions across all soils (final concentration profile NSE = 0.997, 0.977, 0.620, 0.881 for the sandy loam, loam and clay soils in the development phase and the loamy soil the verification phase, respectively). The results support the importance of accounting for the physical rainfall pumping process to explain enhanced solute extraction from soils during storms and to avoid unrealistic transport parametrization.