Correct Answer - Option 3 : Direct runoff
Explanation:
Hydrograph:
(i) It is the graphical representation of runoff with time at the given point on the stream.
Runoff:
(i) It is defined as the flowing away of precipitation from a catchment.
(ii) The flow which takes place all the time over the surface as overland flow and through the channels as open channel flow is surface runoff.
Runoff = Direct runoff + Base flow
Direct runoff : This is the part of runoff which immediately reaches the stream after precipitation. This includes surface runoff, prompt interflow and precipitation on the stream surface.
Base flow = Total runoff - Direct runoff
Direct Runoff hydrograph:
(i) The hydrograph obtained after deducting the base flow from the flood hydrograph is known as DRH.
Unit Hydrograph:
(i) It is a DRH resulting from one unit depth of rainfall excess (Direct runoff) occurring uniformly over the basin at a uniform rate for a specified duration of ‘D’ hours.
The theory of unit hydrograph was developed by Sherman in the year 1932 and assumptions are given below-
1. Time invariance-
- According to this assumption the DRH for a given effective rainfall is always the same in the catchment irrespective of the time, when the rainfall or storm takes place.
2. Linear response-
- This is the single most important assumption in the theory of unit hydrograph. According to this assumption any change in the input value is proportionately reflected in the output value.
