Saturation Excess Flow

Saturation excess occurs when the soil layers have become saturated and no further water can infiltrate.

It is most common with long-duration, gentle-to-moderate rainfall, or with the latter of successive precipitation and or snowmelt events.

For example, consider a storm that produces a steady rainfall rate of 10 millimeters per hour for four hours. After three hours of infiltration the soil becomes saturated. In the fourth hour, the 10 mm/hr rainfall rate cannot infiltrate the saturated soil and becomes saturation excess overland flow.

Standing water and runoff channels in a grassy field on a rainy day, United Kingdom

Saturation excess overland flow can occur anywhere the soil is wet. It is most common in humid climates with gently sloped or flat basins. Here we see a field where the soil can typically absorb a large amount of water. The standing water in relatively light rainfall suggests that soil saturation is preventing further infiltration of rainwater. Saturation excess overland flow is the result.