Define the runoff. What are the factors that affect the 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

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.

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.

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)

Factors Affecting runoff

Runoff arte and volume from an area mainly influenced by following two factors

A. Climatic factors. B. Physiographical Factors.

A. Climate factors:

It is associated with characteristics of which includes.

  1. Types of Precipitation:

It has great effect on the runoff. E.g. A precipitation which occurs in the form of rainfall starts immediately as surface runoff depending upon rainfall intensity while precipitation in the form of snow does not result in surface runoff.

  1. Rainfall Intensity:

If the rainfall intensity is greater than infiltration rate of soil then runoff starts immediately after rainfall. While in case of low rainfall intensity runoff starts later. Thus high intensities of rainfall yield higher runoff.

  1. Duration of Rainfall:

It is directly related to the volume of runoff because infiltration rate of soil decreases with duration of rainfall. Therefore medium intensity rainfall even results in considerable amount of runoff if duration is longer.

  1. Rainfall Distribution:

Runoff from a watershed depends very much on the distribution of rainfall. It is also expressed as “distribution coefficient” mean ratio of maximum rainfall at a point to the mean rainfall of watershed. Therefore, near outlet of watershed runoff will be more.

  1. Direction of Prevailing Wind:

If the direction of prevailing wind is same as 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

  1. Other Climate Factor:

Other factors such as temperature wind velocity, relative humidity, annual rainfall etc. affect the water losses from watershed area.

B. Physiographic Factors:

It includes both watershed and channel characteristics, which area as follows:

  1. Size of Watershed:

A large watershed takes longer time for draining the runoff to outlet than smaller watershed and vise-versa.

  1. Shape of Watershed:

Runoff is greatly affected by 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 = B/1     or      A/1/1      = A/I2

Compactness Coefficient:

Ratio off perimeter of watershed to circumference of circle whose area is equal to area of watershed.

Two types of shape:

a. Fun shape [tends to produce higher runoff very early]

b. Fern shape [tend to produced less runoff].

3. Slope of Watershed:

It has complex effect. It controls the time of overland flow and time of concentration of rainfall. E.g. sloppy watershed results in greater runoff due to greater runoff velocity and vice-versa.

  1. Orientation of Watershed:

This affects the evaporation and transpiration losses from the area. The north or south orientation, affects the time of melting of collected snow.

  1. Land Use:

Land use and land management practices have great effect on the runoff yield. E.g. an area with forest cover or thick layer of mulch of leaves and grasses contribute less runoff because water is absorbed more into soil.

  1. 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 more, hence it contributes less runoff.

  1. Soil type:

In filtration rate vary with type of soil. So runoff is great affected by soil type.

  1. Topographic characteristics:

It includes those topographic features which affects the runoff. Undulate land has greater runoff than flat land because runoff water gets additional energy [velocity] due to slope and little time to infill rate.

  1. Drainage Density:

It is defined as the ratio of the total channel length [L] in the watershed to total watershed area [A] Greater drainage density gives more runoff

Drainage density = L/A

Factors Affecting Runoff

The various factors which affect the runoff from a drainage basin depend upon the following characteristics.

  1. Rainfall characteristics:

a. Type of storm and season b. Intensity c. Duration d. Arial Distribution e. Frequency f. Antecedent precipitation g. Direction of storm movement

2. Metrological factors:

a. Temperature, b. Humidity c. Wind velocity d. Pressure difference

3. Watershed Factor:

a. Size b. Shape c. Altitude d. Topography e. Geology [Soil type] f. Land use [vegetation], Orientation g. Type of drainage network h. Proximate to ocean and mountain range

4. Storage Characteristics:

a. Depressions b. Ponds, lakes and pools. c. Stream d. Channels. e. Check dams in gullies f. Upstream reservoirs or tanks. g. Ground water storage in deposits/aquifers

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