Runoff

Runoff can be defined as the portion of the precipitation that makes its way towards rivers or oceans etc., as surface or subsurface flow. Portion which is not absorbed by the deep strata. Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. Precipitation (or rainfall), after satisfying the requirements of evapotranspiration, interception, infiltration into the ground and detention storage, drains off or flows off from a catchment basin as an overland flow (or surface runoff which includes precipitation falling on the stream system too) into a stream channel. Some part of the infiltrating water moves laterally through the upper layers of the soil and returns to the ground surface as interflow or subsurface runoff at some place away from the point of infiltration into the soil. Part of the infiltrating water percolates deep into the ground and joins the ground water storage. When water table intersects the stream channels of the catchment basin, some ground water may reach the surface or join the stream as ground water runoff, also called base flow or dry-weather flow. Thus, the runoff from a catchment includes surface runoff, subsurface runoff and base flow.

The surface runoff starts soon after the precipitation and is the first to join the stream flow. Subsurface runoff is slower and joins the stream later. Depending upon the time taken by the subsurface runoff between the infiltration and joining the stream channel, it may be termed as prompt subsurface runoff or delayed subsurface runoff. The groundwater runoff is the slowest in joining the stream channel but, is responsible in maintaining low flows in the stream during dry season. Based on the time interval between the precipitation and runoff, the runoff is categorized as direct runoff (that enters the stream immediately after precipitation i.e., surface runoff, subsurface runoff) and base flow (i.e., ground water runoff). Runoff is the response of a catchment to the precipitation reflecting the combined effects of the nature of precipitation, other climatic characteristics of the region and the physiographic characteristics of the catchment basin.

Type, intensity, duration and areal distribution of precipitation over the catchment are the chief characteristics of the precipitation that affect the stream flow. Precipitation in the form of rainfall is quicker to appear as stream flow than when it is in the form of snow. For the surface runoff to start, the intensity of rainfall (or precipitation) must exceed the infiltration capacity of the soil which decreases with the increase in the duration of rainfall. It is obvious that a longer duration rainfall may produce higher runoff even if the intensity of rainfall is less but exceeding the infiltration capacity of the soil. Heavy rainfalls in the downstream region of the catchment will cause rapid rise in the stream levels and early peaking of the discharge. A rare occurrence of uniformly distributed rainfall may result in increased infiltration and therefore, increased subsurface runoff and base flow resulting in slow rise in levels and delayed peaking of the discharge. Likewise, antecedent higher soil moisture conditions at the time of precipitation would hasten the rise in the stream levels. Other climatic characteristics influencing the runoff are temperature, wind velocity and relative humidity. These characteristics affect the evapotranspiration and thus influence the availability of the precipitation for runoff.

Types of Runoff

1) Surface Runoff

Portion of rainfall (after all losses such as interception, infiltration, depression storage etc. are met) that enters streams immediately after occurring rainfall. After laps of few time, overland flow joins streams; sometime termed prompt runoff (as very quickly enters streams).

2) Subsurface Runoff

Amount of rainfall first enter into soil and then flows laterally towards stream without joining water table. It also takes little time to reach stream.

3) Base Flow

It is a delayed flow. Water that meets the groundwater table and join the stream or ocean. It is very slow movement and take months or years to reach streams.

Factors affecting runoff

• Climatic factors
• Rain and snow fall
• Duration of rainfall
• Rainfall distribution
• Direction of prevailing wind
• Other climatic factors like Temperature, wind velocity, relative humidity, annual rainfall etc.