Runoff

When it rains,  runoff is the water that does not soak into the ground; it is the water that runs downhill, into the manmade drainage systems. It is the water that runs to the creeks and rivers to the oceans. It is the water that is not soaked into the earth, and does not recharge the aquifer. Runoff tends to move rapidly carrying with it trash, sediment and debris.

Surface runoff enters the downhill flow almost immediately flowing to the surface waterways. Sub-surface runoff leaches into the soil and moves laterally without joining the water table, to the stream, rivers or ocean is known as sub-surface runoff. Rainfall which infiltrates into the soil and meets the water table and flows to the streams is very slow is referred to as delayed runoff.

Runoff is generated as long as the rainfall intensity exceeds the actual infiltration capacity of the soil. The soil, like a sponge, can only hold so much. Runoff stops when the rate of rainfall drops below the actual rate of infiltration. In other words, when the rain slows down to a rate that is slower than the absorption rate of the soil, runoff drops.

Factors Affecting runoff rate include

Climate factors:

1. Types of Precipitation: It has great effect on the runoff. When it rains runoff starts immediately. When it snows, the snow does not immediately result in surface runoff.
2. Rainfall Intensity: If the rain comes down faster than the soil can absorb it, runoff happens. If the rain comes down slowly, runoff happens when the soil cannot absorb any more water. If it rains a lot then a lot of runoff is generated.
3. Duration of Rainfall: If the rain continues over a longer period of time, runoff happens because the soil cannot absorb all of the rain.
4. Rainfall Distribution: Runoff from a watershed depends on where the rain falls. If rain falls near the outlet of watershed, runoff will be more.
5. Direction of Prevailing Wind: If the direction of prevailing wind is the same direction as the drainage system, it results in peak low. A storm moving in the direction of stream slope produce a higher peak in shorter period of time than a storm moving in opposite direction.
6. Other Climate Factors: Other factors such as temperature wind velocity, relative humidity, annual rainfall etc. affect the water losses from watershed area.

Physiographical Factors:

1. Size of Watershed: A large watershed takes longer to drain than smaller watershed and vice-versa.
2.  Shape of Watershed: Shape of watershed is generally expressed by the term “form factor” and “compactness coefficient”. Form Factor = Ratio of average width to axial length of watershed. Compactness Coefficient: Ratio off perimeter of watershed to circumference of circle whose area is equal to area of watershed. Two types of shape:
   1. Fan shape – tends to produce higher runoff early
   2. Fern shape – tends to produced less runoff
3.  Slope of Watershed: The slope controls the time of overland flow and time of concentration of rainfall. Slopping watersheds result in greater runoff due to greater runoff velocity and vice-versa.
4.  Orientation of Watershed: This affects the evaporation and transpiration losses. The north or south orientation, affects the time of melting of collected snow.
5.  Land Use: Land use and land management practices have great effect on the runoff yield. An area with forest cover or thick layer of mulch of leaves and grasses contribute less runoff because water is absorbed more into soil.
6.  Soil moisture: Magnitude of runoff yield depends upon the initial moisture present in soil at the time of rainfall. If the rain occurs after along dry spell then infiltration rate is greater, creating less runoff.
7.  Soil type: In filtration rates vary with type of soil.
8.  Topographic characteristics: Undulating land has greater runoff than flat land.
9.  Drainage Density: The longer and wider a watershed area the greater the runoff
10.  Storage Characteristics:
    1. Depressions
    2. Ponds, lakes and pools
    3. Stream
    4. Channels
    5. Check dams in gullies
    6. Upstream reservoirs or tanks
    7. Ground water storage in deposits/aquifers

Runoff is part of the water cycle. It cannot be eliminated. We can slow it down so that the water can soak into the soil, reduce erosion and sediment displacement.