Paths to Runoff
- There are two types of surface runoff:
- Infiltration excess overland flow (Hortonian flow)
- Saturation excess overland flow
- Infiltration excess overland flow occurs when the rainfall rate exceeds the infiltration capacity and can occur even in dry soil conditions.
- Saturation excess overland flow occurs when the soil is saturated and no more rain or snowmelt can infiltrate.
- Interflow is the subsurface movement of water to the stream and can be a significant component of runoff.
- Transmissivity feedback occurs where macropores are activated by interflow.
- Macropores are natural voids and pipes in the soil that increase interflow and are formed by biological and chemical activity.
- Enhanced interflow may occur along a soil-bedrock interface, especially if the bedrock is sloped.
- A fragipan is an area of relatively low permeable soil and rock that can block the downward percolation of water and increase horizontal water movement.
- Groundwater ridging occurs when surface rain or snowmelt reaches the water table more easily in a given area causing the water table to bulge upward – this can increase interflow.
- Pre-event water is water that exists in the soil profile when a storm begins and gets displaced into the stream by newly infiltrated water. It can be a very important contributor to runoff as interflow.
