Saturationexcess

Saturation excess, in hydrology, refers to a form of surface runoff that occurs when the soil profile becomes saturated with water. Once the upper soil layers reach full saturation, the soil’s storage capacity for additional rainfall is effectively exhausted. Any further precipitation cannot be infiltrated and is discharged as runoff at or near the surface. This runoff mechanism is sometimes called saturation-excess runoff or saturation overland flow.

Saturation excess is distinct from infiltration-excess runoff (Hortonian runoff), which occurs when rainfall intensity exceeds the soil’s ability to infiltrate water, typically on dry soils or steep surfaces. Saturation excess dominates when antecedent moisture is high, the groundwater table is shallow, or there is perched or near-surface groundwater that limits the soil’s infiltration capacity. In such settings, rainfall first fills available storage until the saturated zone expands, and excess water flows laterally toward streams.

Mechanisms and settings: The phenomenon is common in humid climates, floodplains, wetlands, peatlands, and agricultural landscapes with shallow water tables or drainage that raises the surface water table. Topography and soil properties influence where saturation excess develops, with low-lying and poorly drained areas more prone to it. In hydrological modeling, saturation excess is often represented by the expansion of a saturated contributing area, as seen in TOPMODEL and related approaches.

Implications: Saturation excess affects flood generation, groundwater–surface water interactions, and land-use planning. Drainage that lowers the water table can reduce saturation-excess runoff, while restoration of high-water-table conditions can increase it.