HYETOGRAPH AND HYDROGRAPH
3. Classification of runoff
5. Unit Hydrograph Analysis
6. Hydrograph Vocabulary
A hyetograph is a graphical representation of
the relationship between the rainfall intensity
It is the plot of the rainfall intensity drawn on the
ordinate axis against time on the abscissa axis.
The hyetograph is a bar diagram.
The area under the hyetograph gives the total
rainfall occurred in that period.
This chart is very useful in representing the
characteristics of storm, and is particularly
important in developing the design storm to
predict extreme floods.
Runoff is that portion of rainfall that is not
The runoff is defined as the portion of the rainfall
that makes its way towards river or ocean as
surface or subsurface flow.
The discharge flowing in a river is the runoff
from the basin drained by that river.
Runoff is usually expressed as volume per unit
time, the common unit being m3/s or cumec.
When rainfall starts, some gets caught in retention
storage depressions where it will be evaporated
again, but some enters the ground (infiltration,
including some from detention storage) where it will
contribute to the channel flow at some later time.
Classification of Runoff
Water flows over the land and is first to reach the
streams and rivers which ultimately discharge the
water to the sea.
Inter flow or subsurface flow:
A portion of rainfall infiltrates into surface soil and
depending upon the geology of basins, runs as
subsurface runoff and reaches the streams and rivers.
Ground water flow or base flow:
It is that portion of rainfall which after infiltration,
percolates down and joins the ground water reservoir
which is ultimately connected to the ocean.
A hydrograph is a graphical plot of discharge of
a natural stream or river versus time.
It shows variations of discharge with time, at a
particular point of a stream.
It also shows the time distribution of total runoff
at the point of measurement.
Discharge is plotted on Y-axis and the
corresponding time is plotted on X-axis.
Flood analysis and derivation of unit hydrograph
a single peaked hydrograph is required.
Hydrograph analysis is the most widely used
method of analyzing surface runoff.
Hydrograph determines the peak flood
magnitude of flood for the design of hydraulic
structures i.e. a dam, spilway, bridge, culvert, etc.
Hydrographs are also described in terms of the
following time characteristics :
1) Time to Peak, tp:
Time from the beginning of the rising limb to the
occurrence of the peak discharge.
The time to peak is largely determined by
drainage characteristics such as drainage
density, slope, channel roughness, and soil
infiltration characteristics. Rainfall distribution in
space also affects the time to peak.
2.) Time of Concentration, tc:
Time required for water to travel from the
most hydraulically remote point in the basin to
the basin outlet. For rainfall events of very
long duration, the time of concentration is
associated with the time required for the
system to achieve the maximum or
The drainage characteristics of length and slope,
together with the hydraulic characteristics of the
flow paths, determine the time of concentration.
3.) Lag Time, tl:
Time between the center of mass of the effective
rainfall hyetograph and the center of mass of the direct
The basin lag is an important concept in linear
modeling of basin response. The lag time is a
parameter that appears often in theoretical and
conceptual models of basin behavior. However, it is
sometimes difficult to measure in real world situations.
Many empirical equations have been proposed in the
literature. The simplest of these equations computes
the basin lag as a power function of the basin area.
4.) Time Base, tb:
Duration of the direct runoff hydrograph.
UNIT HYDROGRAPH ANALYSIS
Sherman (1932) first proposed the unit
hydrograph concept. The Unit Hydrograph (UH) of a
watershed is defined as the direct runoff
hydrograph resulting from a unit volume of excess
rainfall of constant intensity and uniformly
distributed over the drainage area. The duration of
the unit volume of excess or effective rainfall,
sometimes referred to as the effective duration,
defines and labels the particular unit
hydrograph. The unit volume is usually considered
to be associated with 1 cm (1 inch) of effective
rainfall distributed uniformly over the basin area.
Rising limb, a falling limb, and a recession. The
rising limb and falling limb are separated by the
hydrograph crest, and the limbs are separated
from the recession by inflection points. The
rising limb is typically steeper than the falling
The rising limb :-
The rising limb is the ascending portion of the
hydrograph corresponding to the increase of discharge
due to gradual accumulation of storage in the channels
existing in the area and also over the watershed
surface. The rising limb is also known as the
The peak or crest :-
The peak or crest segment includes the part of the
hydrograph from the inflection point on the rising limb
to an inflection point on the recession limb. The peak
segment is the most important part of the hydrograph
because it indicates the peak flow rate. The peak
represents the arrival of flow at the outlet from all parts
of the basin.
The recession limb :-
After the inflection point, there is no inflow to
the stream due to surface runoff. The recession
limb extends from the inflection point, to the point
of recommencement of the natural base flow or
ground water flow. The recession limb
represents the withdrawl of water from the
storage already built up in the catchment during
the earlier phase of the hydrograph when
surface runoff was occuring.