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.
2. 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.
3. Duration of Rainfall:
It is directly related to the volume of runoff be cause infiltration rate of soil decreases with duration of rainfall. Therefore medium intensity rainfall even results in considerable amount of runoff if duration is longer.
4. 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. There fore, near outlet of watershed runoff will be more.
5. 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
6. 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.
2. 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
Ratio off perimeter of watershed to circumference of circle whose area is equal to area of watershed
Two types of shape:
Fun shape [tends to produce higher runoff very early]
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.
4. 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.
5. 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.
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 more, hence it contributes less runoff.
7. Soil type:
In filtration rate vary with type of soil. So runoff is great affected by soil type.
8. 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.
9. 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
d. Arial Distribution
f. Antecedent precipitation
g. Direction of storm movement
2. Metrological factors:
c. Wind velocity
d. Pressure difference
3. Watershed Factor:
e. Geology [Soil type]
f. Land use [vegetation], Orientation
g. Type of drainage network
h. Proximate to ocean and mountain range
4. Storage Characteristics:
b. Ponds, lakes and pools.
e. Check dams in gullies
f. Upstream reservoirs or tanks.
g. Ground water storage in deposits/aquifers