In this article we will discuss about:- 1. Meaning of Runoff 2. Process of Runoff 3. Sources or Components.
Meaning of Runoff:
Runoff is that part of precipitation that appears in a drainage channel as surface flow in a perennial or an intermittent from. It is that part of water, which can be used for engineering purposes and hence is also known as yield of catchment.
The yield from a catchment is generally expressed in terms of volume, in a season or a year.
Runoff is expressed as the rate of flow during a specific period of flow, i.e., Q in given time period T. Thus, the unit for runoff is million cubic meters (106 m3 or million m3), or ha m (denoting 1-m depth of water over an area of 1 ha).
Thus 1 million m3 = 102 ha m
The runoff is also sometimes expressed in meters or millimetres as the depth of water spread uniformly over the entire catchment, for instance, 90.0 mm yield from a catchment of 50 km2,
The line that demarcates the drainage area and hence the surface runoff of two adjacent rivers or drainage basins, is called the ridge line. It is also called the topographic divide, topographic water divide, watershed divide or simply divide. The precipitation on either side of the ridge line will flow in the opposite directions. Fig. 2.1 shows a specific case of the topographic divide.
Groundwater Divide Line:
The line that marks the direction of the groundwater movement of the two adjacent river basins is called the groundwater divide. It is also known as the phreatic divide.
It is very difficult to locate groundwater divide in hilly regions as well as in plains. Hence, for practical purpose, the groundwater divide is considered the same as topographic divide.
Process of Runoff:
Whenever there is precipitation, the requirement of interception and depression storage is first met with. The infiltration also starts immediately. If the rate of precipitation is more than these abstractions, overland flow results. Otherwise, all precipitation water is lost in these abstractions. This overland flow is collected in small streams leading to a channel or a river.
Before and after this storm, there may be some discharge in the river. This may be due to interflow, delayed interflow or flow from the groundwater appearing into the channel. The surface flow observed in the river immediately after the precipitation is the combined flow due to the precipitation and the flow in the river before the precipitation.
Sources or Components of Runoff:
The sources and components of runoff from a catchment area are as follows:
a. Direct Precipitation on the Stream Channel:
When the river is in floods, there will be a spread of flood water. The precipitation occurring directly over this water surface contributes to the runoff without any abstractions there from. However, this quantity is very small and normally neglected.
b. Surface Runoff:
This is the major part of the runoff. The water quantity that reaches the stream from the overland flow and is carried by the stream on its surface is the surface runoff. It is also known as quick flow.
All the water, which infiltrates into the ground, may not reach up to the groundwater table if it meets a local impervious layer. This water may move laterally as per the slope of this impervious local layer and may find an outlet into the stream. This component of runoff is known as interflow.
If this inflow is observed in a short period, it is called prompt interflow, otherwise it is called delayed interflow. This inflow moves slowly as compared to the overland flow and its contribution depends upon the orientation of the impervious layer as well as the soil characteristics. This is also known as subsurface runoff, subsurface storm flow, storm seepage, secondary base flow, through flow or quick return flow.
d. Groundwater Flow:
The water that infiltrates may reach the groundwater storage if not obstructed by any impervious layers in between.
If the groundwater table level is higher than the water level in the stream or the bed level of the stream when it is dry, then the water will flow from the groundwater storage into the stream. The flow may lag behind the occurrence of rainfall by days or by weeks, or even by months. But the flow evens out and is regular over a longer period. This represents the long-term component of the total runoff and is important during the long dry spells when there is no precipitation and hence no surface runoff.
When water is supplied to the fields for irrigation, a small portion of its utilized for the growth of the crops. Remaining portion infiltrates. A part of the infiltrated water finds its way into the adjoining stream and is called regeneration. The remaining flows to the groundwater.
For the regenerated water, the tag time is small. Normally, regeneration is considered to be 20% of the water supplied for irrigation.