Introduction to Runoff


Introduction to Runoff

Over the land surface, for the generation of runoff, the primary source of water is Rainfall. A part of rainfall that intercepted by the vegetation, buildings and other objects and prevented to reach them on grand surface is called as interception. Part of rainfall stored in the surface depressions which in due course of time gets infiltrate or evaporated is referred as depression storage [ Initial detention).
When these entire loses are satisfied then excess rainfall moves over land surface is known as overland flow and draining the same into channel or stream is termed as “Runoff”.



Runoff is that portion of the rainfall or irrigation water [or any other flow]. Applied which leaves a field either as surface or as subsurface flow.

When rainfall intensity reaching the soil surface is less than the infiltration capacity, all the water is absorbed in to the soil.  As rain continues soil becomes saturated and infiltration capacity is reduced, shallow depression begins to fill with water, then the over flow starts.

Surface detention/ Detention storage:

The amount of water on the land surface in transit to words stream channels is called detention storage/surface detention.

Surface Runoff:

The runoff which travels over the ground surface to the channels of watershed

Subsurface Runoff:

The portion of unfiltered water which penetrated to shallow depth travels laterally and is intercepted by channels.

Runoff Cycle:

It is that part of hydrological cycles which galls between the phase of precipitation and its subsequent discharge in the stream channels or direct return to the atmosphere through evaporation and evapotranspiration.

Conditions Associated With Runoff Cycle:

  1. This refers to the end of day period and beginning of the intense and isolated storm.

  2. It is the stage after beginning of rainfall causes the overland flow, base flow, and development of channel storage.

  3. It refers to the condition approaching the end of all isolated intense storm.

  4. This is the stage indicating after end of rainfall where rainfall causes the overland low, base plot and development of channel storage.

Types of Runoff:

  1. Surface runoff

  2. Sub-surface runoff

  3. Base flow

a. Surface Runoff:

That portion of rainfall which enters the stream immediately after the rainfall. It occurs when all loses is satisfied and rainfall is still continued and rate of rainfall [intensity] in greater than infiltration rate.

b. Sub-Surface Runoff:

That part of rainfall which first leaches into the soil and moves laterally without joining the water table, to the stream, rivers or ocean is known as sub-surface runoff. It is usually referred is inter-flow.

c. Base flow:

It is delayed flow defined as that part of rainfall, which after falling on the ground the surface, infiltrated into the soil and meets to the water table and flow the streams, ocean etc. The movement of water in this is very slow. Therefore it is also referred a delayed runoff.
Total runoff = Surface runoff + Base flow (including subsurface runoff)

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