Geoscience Reference
In-Depth Information
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That the surface runoff occurs uniformly over the
catchment because the rainfall is uniform over
the catchment. Another assumption that is
difficult to justify.
Using the unit hydrograph
The unit hydrograph obtained from the steps
described here theoretically gives you the runoff
that can be expected per mm of effective rainfall in
one hour. In order to use the unit hydrograph for
predicting a storm it is necessary to estimate the
'effective rainfall' that will result from the storm
rainfall. This is not an easy task and is one of the
main hurdles in using the method. In deriving the
unit hydrograph the assumption has been made
that 'effective rainfall' is the rainfall which does not
infiltrate but is routed to the stream as overland flow
(Hortonian). The same assumption has to be made
when utilising the unit hydrograph. To do this it is
necessary to have some indication of the infiltra-
tion characteristics for the catchment concerned, and
also of the antecedent soil moisture conditions. The
former can be achieved through field experimen-
tation and the latter through the use of an ante-
cedent precipitation index (API). Engineering
textbooks give examples of how to use the API to
derive effective rainfall. The idea is that antecedent
soil moisture is controlled by how long ago rain has
fallen and how large that event was. The wetter a
catchment is prior to a storm, the more effective
rainfall will be produced.
Once the effective rainfall has been established it
is a relatively simple task to add the resultant unit
hydrographs together to form the resultant storm
hydrograph. The worked example shows how this
procedure is carried out.
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The relationship between effective rainfall and
surface runoff does not vary with time (i.e. the
hydrograph shape remains the same between the
data period of derivation and prediction). This
would assume no land-use change within the
catchment, as this could well affect the storm
hydrograph shape.
Given the assumptions listed above it would seem
extremely foolhardy to use the unit hydrograph as a
predictive tool. However, the unit hydrograph has
been used successfully for many years in numerous
different hydrological situations. It is a very simple
method of deriving a storm hydrograph from a
relatively small amount of data. The fact that it does
work (i.e. produces meaningful predictions of storm
hydrographs), despite being theoretically flawed,
would seem to raise questions about our under-
standing of hydrological processes. The answer to
why it works may well lie in the way that it is
applied, especially the use of effective rainfall. This
is a nebulous concept that is difficult to describe
from field measurements. It is possible that in
moving from actual to effective rainfall there is a
blurring of processes that discounts some of the
assumptions listed above. The unit hydrograph is a
black box model of stormflow (see end of this
chapter) and as such hides many different processes
within. The simple concept that the hydrograph
shape is a reflection of the static characteristics and
all the dynamic processes going on in a catchment
makes it highly applicable but less able to be
explained in terms of hydrological theory.
Limitations of the unit hydrograph
The unit hydrograph has several assumptions that
at first appearance would seem to make it
inapplicable in many situations. The assumptions
can be summarised as:
The synthetic unit hydrograph
The
synthetic unit hydrograph
is an attempt to
derive the unit hydrograph from measurable
catchment characteristics rather than from flow
data. This is highly desirable as it would give the
opportunity to predict stormflows when having no
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The runoff that makes up stormflow is derived
from infiltration excess (Hortonian) overland
flow. As described in Chapter 5, this is not a
reasonable assumption to make in many areas of
the world.
