Geoscience Reference
In-Depth Information
(a)
storm
runoff
baseflow
(b)
Time, t
Figure 2.5
Hydrograph separation into “storm runoff” and “baseflow” components: (a) straight line separa-
tion (after Hewlett, 1974); (b) separation by recession curve extension (after Reed et al., 1975).
any easier to interpret the effective rainfalls derived in this way so that it is easier to predict the effective
rainfalls for other storms.
There is one further problem in applying the unit hydrograph technique. In any storm hydrograph
that occurs in a perennial stream, some of the stream discharge would have occurred even without the
rainfall event that causes the hydrograph response. This is usually called the
baseflow component
of the
stream and, if there has been a dry period since the previous storm event, is usually assumed to be derived
from subsurface flow. Early on it was found that the effective rainfall is more linearly related to stream
discharge if the total hydrograph is separated into a baseflow component and a
storm runoff
component
(Figure 2.5a). Hydrograph separation then became an important component of the application of the
unit hydrograph model: the problem is that there are no satisfactory techniques for doing hydrograph
separation. Indeed, some very strange methods of hydrograph separation are reported in the literature
(see review in Beven, 1991). About the only physically justifiable technique for hydrograph separation
is to try to estimate the flow that might have occurred if the storm had not happened. However, such a
procedure tends to lead to storm runoff hydrographs with very long tails and can get quite complicated
in the case of several storms in quick succession (Figure 2.5b) so is not commonly used (but see Reed
et al.
, 1975). In fact, the best method of dealing with hydrograph separation is to avoid it all together, as
discussed in Section 2.3.
In many texts, the storm runoff component is called the “surface runoff” component. This is correct
in that the total stream hydrograph is measured as surface runoff in the channel, but the name also subtly
