Geoscience Reference
In-Depth Information
conditions, ranging from the underlying geology
and catchment shape to the antecedent wetness and
storm duration. The temporal and spatial variations
in these underlying conditions make it highly
unlikely that two hydrographs will ever be the same.
Although there is great variation in the shape of a
hydrograph there are common characteristics of
a storm hydrograph that can be recognised. These
have been described at the start of Chapter 5 where
terms such as
rising limb
,
falling limb
,
recession limb
and
baseflow
are explained.
To overcome the problem of a level baseflow
separation a point has to be chosen on the receding
limb where it is decided that the discharge has
returned to baseflow. Exactly where this point will
be is not easy to determine. By convention the point
is taken where the recession limb fits an exponential
curve. This can be detected by plotting the natural
log (ln) of discharge (
Q
) and noting where this line
becomes straight. The line drawn between the start
and 'end' of a storm may be straight (dotted line, see
Figure 6.1) or curved (thin solid line, see Figure
6.1) depending on the preference of hydrologist -
Arnold
et al
. (1995) provides a summary of different
automated techniques.
In very large catchments equation 6.1 can be
applied to derive the time where stormflow ends.
This is the fixed time method which gives the time
from peak flow to the end of stormflow (
):
Hydrograph separation
The separation of a hydrograph into baseflow and
stormflow is a common task, although never easy.
The idea of
hydrograph separation
is to dis-
tinguish between stormflow and baseflow so that the
amount of water resulting from a storm can be
calculated. Sometimes further assumptions are made
concerning where the water in each component
has come from (i.e. groundwater and overland flow)
but, as explained in the previous chapter, this is
controversial.
The simplest form of hydrograph separation is to
draw a straight, level line from the point where the
hydrograph starts rising until the stream discharge
reaches the same level again (dashed line in Figure
6.1). However, this is frequently problematic as the
stream may not return to its pre-storm level before
another storm arrives. Equally the storm may
recharge the baseflow enough so that the level is
raised after the storm (as shown in Figure 6.1).
=
D
n
(6.1)
where
D
is the drainage area and
n
is a recession
constant. When
D
is in square miles and
in days,
the value of
n
has been found to be approximately
0.2.
The problem with hydrograph separation is
that the technique is highly subjective. There is no
physical reasoning why the 'end' of a storm should
be when the recession limb fits an exponential curve;
it is pure convention. Equally the shape of the curve
between start and 'end' has no physical reasoning.
It does not address the debate covered in Chapter
5: where does the stormflow water come from?
Furey and Gupta (2001) have recently provided
a hydrograph separation technique that ties into
physical characteristics of a catchment and therefore
is not as subjective as other techniques, although
it still requires considerable interpretation by the
user. What hydrograph separation does offer is a
means of separating stormflow from baseflow,
something that is needed for the use of the unit
hydrograph (see pp. 103-106), and may be useful
for hydrological interpretation and description.
0
1
Q
Time
Figure 6.1
Hydrograph separation techniques. See text
for explanation.
