Water Enters the Ground
When rain falls on soil, water enters through the surface in a process called infiltration. The rate at which water can enter depends on the soil's hydraulic conductivity, its current moisture content, and the capillary suction pulling water into dry pores. Initially, dry soil sucks water in rapidly, but as the wetting front advances and the suction gradient diminishes, the infiltration rate declines toward the saturated hydraulic conductivity — the soil's steady-state limit.
The Green-Ampt Model
Developed in 1911, the Green-Ampt model idealizes the wetting process as a sharp front separating fully saturated soil above from uniformly dry soil below. This piston-like approximation yields a tractable equation: f = K_s(1 + ψ_f Δθ / F), where ψ_f is the suction at the wetting front, Δθ is the moisture deficit, and F is cumulative infiltration. Despite its simplicity, the model captures the essential physics — rapid initial infiltration that decays to steady state.
Runoff and Ponding
When rainfall intensity exceeds the soil's infiltration capacity, water ponds on the surface and runs off. This Hortonian overland flow is the primary driver of erosion and flash flooding on compacted or clay-rich soils. The ponding time — the moment runoff begins — depends on rainfall intensity, soil conductivity, and initial moisture. This simulation tracks the wetting front descent and the transition from full infiltration to runoff generation in real time.
From Soil to Aquifer
Water that infiltrates past the root zone continues downward through the vadose zone until it reaches the water table, recharging groundwater. This percolation pathway is also nature's filter: soil particles, organic matter, and microbial communities remove pathogens, heavy metals, and organic contaminants. Understanding percolation rates is essential for designing septic systems, irrigation schedules, stormwater management, and groundwater protection zones.