Overview of Runoff

  • Runoff is often defined as the portion of rain, snowmelt, and irrigation water that moves toward the stream channel rather than infiltrating the soil.
    • For some purposes, however, runoff also includes the subsurface water known as interflow which also quickly moves toward the stream channel.
  • Runoff is the most important component of flood prediction.
  • The three soil water processes are:
    1. The entry or infiltration of water into the soil
    2. The transmission of water within the soil
    3. The storage of water as soil water
  • Important runoff terms include:
    • Basin – the area that drains to a single outlet point
    • Baseflow – the long-term supply that keeps water flowing in streams
    • Infiltration – the downward movement of water through the soil surface
    • Percolation – the movement of water within the soil profile
    • Surface runoff – the movement of water across the soil surface to the stream channel
    • Interflow – the relatively rapid movement of water beneath the soil surface to the stream channel
    • Runoff – sometimes this is just the surface runoff, but it can refer to a combination of surface runoff and interflow.
    • Infiltration rate – the amount of water able to enter the soil in a specified time
    • Infiltration capacity – the upper limit of the infiltration rate
    • Surface runoff equals the rainfall or snowmelt rate minus the infiltration capacity.