A hydrograph plots the variation of river discharge over time. It has components like the rising limb, peak flow, and recession limb. There are different types of hydrographs including storm hydrographs, which show discharge from precipitation over a short period, and flood hydrographs, which measure river characteristics. A unit hydrograph represents the runoff from 1 unit of effective rainfall uniformly over a watershed. Hydrographs are constructed to analyze a basin's discharge patterns, develop flood forecasts, and extend flood records based on rainfall data.

1. HYDROGRAPH
Components,Types, Unit Hydrograph, Uses and
Limitations

2. What is a Hydrograph?
■ A Hydrograph by definition is a plot
of the variation of discharge with
respect to time.
■ The discharge of a river: the volume
of water that flows past a point in the
river’s course per second.
■ Unit: m3s-1.
■ 1m3s-1 = cumec. Cusecs = 1 ft3/sec.
■ Discharge is normally measured at
gauging stations that are situated at
different points along the river.
■ Discharge is effected by weather.

3. Components of a Hydrograph
■ Rising Limb

4. ■ Peak Flow

5. ■ Recession Limb or
GroundWater
Depletion Curve

6. ■ Basin LagTime

7. ■ Base Flow

8. ■ Storm Flow

9. Types of Hydrograph
Storm Hydrographs:
■ Drainage pattern response to
rainfall
■ Show discharge that originate
as precipitation
■ Variation in discharge over a
short period of time.

10. Flood Hydrographs:
■ A visual representation of the
variation in the stream or river
depth over time.
■ This depth is usually indicated
on a hydrograph with a
horizontal line called the
base flow.
■ Measures the ‘characteristics’
of a river: the gradient, the
velocity etc.
■ The flood hydrograph is basically
a dual plot of river discharge and
rainfall.

11. Annual Hydrographs or River Regimes
■ Shows the average daily flow of a river
over the course of one year rather
than a period of hours.
■ This hydrograph shows the long term
balance of precipitation, evaporation
and stream flow in the watershed.

12. Unit Hydrograph
■ Defined as the direct runoff hydrograph
(DRH) resulting from one unit (e.g., one cm
or one inch) of effective rainfall occurring
uniformly over that watershed at a uniform
rate over a unit period of time.
■ It is the response of the watershed to a unit
input of rainfall.
■ A UH is specific to a particular watershed.
■ The UH is specified as being the 1-hour,
6-hour, or 24-hour UH, or any other length of time.

13. ■ UH quantifies the effect of a unit of rainfall on the corresponding runoff from a
particular drainage basin.
■ UH theory assumes that a watershed's runoff response is linear and time-invariant,
and that the effective rainfall occurs uniformly over the watershed.

14. Limitations of UH
■ Unit hydrographs assume uniform distribution of rainfall over the catchment and
uniform intensity during the duration of rainfall excess. In practice, these two
conditions are never satisfied.
■ Under conditions of non-uniform areal distribution and variation of intensity, the unit
hydrograph theory can still be used if the areal distribution is consistent between
different storms.
■ The upper limit for use of the unit hydrograph method is 5000km2.

15. Why do we construct hydrographs?
■ i.To find out the discharge patterns of a particular drainage basin.
■ ii. For the development of flood hydrographs for extreme rainfall magnitudes
■ iii.To obtain extension of flood flow records based on rainfall records
■ iv. For the development of flood forecasting and warning systems based on rainfall.