It is defined as, “Rainfall that does not soak (absorb) into the soil but flows into surface waters.” OR; “After infiltration and percolation when soil becomes saturated than water flows on the surface called as runoff.”
TYPES OF RUNOFF:
It is the amount of water which is drained over the surface of the soil of watershed after saturation of micro- and macropores of soil, and under gravitational pull from a higher altitude to lower altitude having a much-defined rate of flow.
The amount of runoff depends upon amount and intensity of rainfall, soil structure, texture, and porosity of soil and extent and degree of slope.
When surface storage of soil in the w-shed is completed (ie upper surface layer) and it becomes saturated water begins to flow in the surface of soil up to the depth of one meter from a higher altitude to lower altitude at the rate much less than surface runoff. It takes hours and even days to reach the stream. It is clear water and maintains the stream flowing after precipitation.
FACTORS AFFECTING RUNOFF:
- Rainfall: Higher rainfall; higher runoff
- Area of w-shed: Greater area; greater runoff
- The shape of w-shed: More circular/ square (as compared to other shapes); more runoff
- The aspect of w-shed: On Northern Aspect more runoff.
- Vegetation: More vegetation; less runoff
- Land moisture: More moisture; more runoff
- Slope: More slopes; more runoff.
CHARACTERISTIC WHICH INCREASE/ DECREASE RUNOFF:
With respect to Soil:
- Texture: (Size of soil particle)
- Soil particles are divided into Sand, Silt and Clay particles with relative diameters 2mm – 0.05mm, 0.05mm – 0.002 mm, and < 0.002mm respectively.
- In sand particles, more runoff
- In clay particles, less runoff
- Structure: (Arrangement of soil particles)
- When soil particles are arranged alternatively and irregular, there is less runoff, on the other hand if particles are arranged in lines regularly, there is more runoff.
- Organic Matter:
- More organic matter, less runoff
- Darker the soil less will be the runoff.
Features of Watershed:
- More slope, less absorption, infiltration, and percolation hence more runoff.
- Vegetation cover:
- More vegetation, less runoff.
COMPONENTS OF RUNOFF:
The water reaches the stream flow in the following four ways.
Direct rainfall over the stream:-
- It is a very small portion of the precipitation because open water channels do not occupy much of catchment area.
Surface runoff or overland flow: –
When the intensity of rainfall is higher than the infiltration rate, then after infiltration the excess water will flow over the land surface to the channel and is called surface runoff. The quantity of water moving towards the stream is called overland flow. The surface runoff becomes streamflow for the nearest channels. It is almost accompanied by erosion of the soil.
Subsurface flow or interflow:–
- Some of the water which infiltrates the soil surface may move laterally through the upper soil layers until it enters a stream and is called subsurface or interflow. It reaches the stream later and its basin (watershed).
Base flow, groundwater flow or Dry weather flow:-
- Some of the rainwater percolates downward under gravity through the soil and merges into the water table and if the water table intersects the stream channels and water appears in streams, it is called base flow, groundwater flow or dry water flow. It does not contribute to streamflow surge but rather sustains the base flow of streams at times of no precipitation.
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