Geoscience Reference
In-Depth Information
graphs (without the baseflow component) for a
corresponding rainfall event.
The unit hydrograph
The idea of a
unit hydrograph
was first put forward
by Sherman, an American engineer working in the
1920s and 1930s. The idea behind the unit hydro-
graph is simple enough, although it is a somewhat
tedious exercise to derive one for a catchment. The
fundamental concept of the unit hydrograph is that
the shape of a storm hydrograph is determined
by the physical characteristics of the catchment. The
majority of those physical characteristics are static
in time, therefore if you can find an average hydro-
graph for a particular storm size then you can use
that to predict other storm events. In short: two
identical rainfall events that fall on a catchment
with exactly the same antecedent conditions should
produce identical hydrographs.
With the unit hydrograph a hydrologist is trying
to predict a future storm hydrograph that will result
from a particular storm. This is particularly useful
as it gives more than just the peak runoff volume
and includes the temporal variation in discharge.
Sherman (1932) defines a unit hydrograph as
'the hydrograph of surface runoff resulting from
effective rainfall
falling in a unit of time such as
1 hour or 1 day'. The term effective rainfall is taken
to be that rainfall that contributes to the storm
hydrograph. This is often assumed to be the rainfall
that does not infiltrate the soil and moves into the
stream as overland flow. This is infiltration excess
or Hortonian overland flow. Sherman's ideas fitted
well with those of Horton. Sherman assumed that
the 'surface runoff is produced uniformly in space
and time over the total catchment area'.
Deriving the unit hydrograph: step 2
Take a single storm hydrograph and find out the
total volume of water that contributed to the storm.
This can be done either by measuring the area under
the stormflow hydrograph or as an integral of the
curve. If you then divide the total volume in the
storm by the catchment area, you have the runoff
as a water equivalent depth. If this is assumed to
have occurred uniformly over space and time within
the catchment then you can assume it is equal to the
effective rainfall. This is an important assumption
of the method: that the effective rainfall is equal
to the water equivalent depth of storm runoff. It is
also assumed that the effective rainfall all occurred
during the height of the storm (i.e. during the
period of highest rainfall intensity). That period of
high rainfall intensity becomes the time for the unit
hydrograph.
Deriving the unit hydrograph: step 3
The unit hydrograph is the stormflow that results
from one unit of effective rainfall. To derive this
you need to divide the values of stormflow (i.e. each
value on the storm hydrograph) by the amount
of effective rainfall (from step 2) to give the unit
hydrograph. This is the discharge per millimetre of
effective rainfall during the time unit.
Deriving the unit hydrograph: step 4
Deriving the unit hydrograph: step 1
Repeat steps 2 and 3 for all of the typical hydro-
graphs. Then create an average unit hydrograph
by merging the curves together. This is achieved by
averaging the value of stormflow for each unit of
time for every derived unit hydrograph. It is also
possible to derive different unit hydrographs for
different rain durations and intensities, but this is
not covered here (see Maidment, 1992, or Shaw,
1994, for more details).
Take historical rainfall and streamflow records for a
catchment and separate out a selection of typical
single-peaked storm hydrographs. It is important
that they are separate storms as the method assumes
that one runoff event does not affect another. For
each of these storm events separate the baseflow from
the stormflow; that is, hydrograph separation (see
p. 102). This will give you a series of storm hydro-
