Saturation Excess Runoff Generation

Saturation excess overland flow occurs due to surface water input on areas already saturated. Infiltrating water completely saturates the soil profile resulting in the water table rising to the surface as shown in the animation. This is referred to as saturation from below, or the Dunne mechanism for runoff generation. The extent of the area subject to surface saturation varies over time and is referred to as the variable source area. Saturated areas occur most commonly in concave hollows and wide flat valleys. Return flow is water that flows out of the subsurface to add to overland flow in saturated areas.