Factors Affecting Runoff – Physiographic Factors
Physiographic Factors:
Physiographic factors of watershed consist of both, the watershed as with as channel characteristics. The different characteristics of watershed and channel, which affect the runoff, are listed below.
1. Size of watershed
2. Shape of watershed
3. Slope of watershed
4. Orientation of watershed
5. Land use
6. Soil moisture
7. Soil type
8. Topographic characteristics, and
9. Drainage Density.
1. Size of watershed: Regarding the size of watershed, if all other factor including depth and intensity of rainfall are being same them two watershed irrespective of their size, will produce about the same amount of runoff .However a large watershed takes longer time for raining the runoff to the outlet as result the peak flow expressed are depth is being smaller and vise versa.
2. Shape of watershed:The shape of watershed has a great effect of runoff. The watershed shape is generally expressed by the terms "from factor and "compactness coefficient".
3. Shope of watershed: The shope of the watershed has an important roel over runoff but its effect is complex. It controls the time of overland flow and time of concentration of rainfall in the drainage channel which provide accumulative effect on resulting peak runoff. For example in case of a sloppy watershed. The time to reach the flow at outlet is less, because of greater runoff velocity which results into formation of peak runoff very soon and vice -versa.
4. Orientation of watershed: This factor affects the evaporation and transpiration losses from the area by making influence on the amount of heat to the received from the sun. The north or south orientation of watershed, affects the time of melting of collected snow. In a mountainous watershed the part of wind ward side of the mountain receives high intensity of rainfall resulting into more runoff yield while the part of watershed typing towards leeward side has reverse find of the same.
5. Land use: The land use pattern and land management practices used have great effect on the runoff yield. For example an area which is under forest cover, where a thick layer of much of leaves and grasses etc. has peen accumulated there formed a little surface runoff due to the fact that more rain water is absorbed by the soil. While in a barren field where not any type of cover is available a reverse trend is obtained.
6. Soil Moisture: The magnitude of runoff yield depends on the amount of moisture present in the soil at the time of rainfall. If rain occurs over the soil which has more moisture the infiltration rate becomes very less which results in more runoff yield. Similarly if the rain occurs after a long dry spell of time when the soil is dry, causing to absorb huge amount of rain water. In on the other hand, if the rain occurs in a close succession as in the rainy season; runoff yield has reverse effect.
7. Soil Type: In the watershed surface runoff is greatly influenced by the soil type as loose of water from the soil is very much dependent on inflientation rate which varies with the types of soil.
8. Topographic Characteristics: Topographic characteristics include mores topographical features of watershed which create their effect on runoff it is mainly undulating nature of the reason that runoff water gets additional power to flow due to slope of the surface and altitude time to infiltrate the water into solid.
Regarding channel characteristics to describe their effect on runoff the channel cross-section, roughness storage and channel density are mainly considered. These also have significant effect on runoff.
9. Drainage Density: The rain age density is defined as the nation of the trial channel length in the watershed to two total watershed areas it is expressed at.
(Tranned length (Total))
Drainage density = ——————————
Watered area
I
D.D. = —
A
A watershed having greater D.D. and incites formation of peak rain off very shortly to that of lesser D.D. watershed.
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