Geography Reference
In-Depth Information
7 Prediction of flow duration
curves in ungauged basins
Contributors:
A. Castellarin,* G. Botter, D. A. Hughes,
S. Liu, T. B. M. J. Ouarda, J. Parajka, D. A. Post,
M. Sivapalan, C. Spence, A. Viglione and R. M. Vogel
7.1 For how long do we have water?
A wide variety of approaches have been adopted to quan-
tify runoff variability, with differing degrees of emphasis
on high flows (floods), low flows, seasonal variation of
flows, and annual flows, and many of these are discussed in
other chapters of this topic. An additional feature of runoff
variability that has considerable practical relevance is the
period of time runoff remains higher than a specified mag-
nitude, otherwise known as
The FDC can be constructed for the entire runoff record
giving a long-term representation of the runoff regime, or
as an ensemble of annual FDCs (AFDCs) estimated for
each year of record (Vogel and Fennessey,
1994
,
1995
).
Together, these offer a perspective of the between-year, or
inter-annual, variability of the FDCs, which can be consid-
erable in some locations, and enable the estimation of the
mean or median of the AFDCs as well as the variances or
confidence intervals of the runoff quantiles. The mean or
median AFDC is a hypothetical AFDC, which describes
the annual runoff regime for a typical hydrological year.
Normally the median AFDC is preferred to the mean
AFDC because it is less sensitive to the presence of abnor-
mally wet or dry years in the observed runoff time series
(Vogel and Fennessey,
1994
).
Figure 7.1
presents an
example of the construction of the long-term FDC, and
the ensemble of AFDCs based on the daily runoff record of
the Kamp River taken at Zwettl in Austria.
Because of its ability to condense a wealth of informa-
tion about runoff variability into a single graphic image,
and because of the relevance of runoff variability to both
human water use and the maintenance of environmental
health, the FDC is used in a wide range of applications
(Vogel and Fennessey,
1995
). The FDC can help quantify
the ability and reliability of a river to meet demand for
water by humans (for municipal and industrial uses, irri-
gated agriculture), and has been the basis for the design of
small reservoirs or schemes for water uptake from rivers
(Dingman,
1981
; McMahon,
1993
). FDCs are heavily used
in the design and operation of hydropower schemes, for
maximising hydropower production. Increasingly, as
humans interfere with the runoff of the river for hydro-
power production or through extractions for domestic and
industrial consumption, the downstream environment
tends to suffer. FDCs are increasingly used to determine
and set environmental flow standards to protect the aquatic
habitat and maintain and restore ecosystem health (Poff
Fish and Wildlife Service (Milhous et al.,
1990
) use FDCs
for determining the suitability of
'
'
. The flow dur-
ation curve (FDC), which is the subject of this chapter, is a
graphical representation of the frequency, or the fraction of
time (hence the word duration) during which a specified
magnitude of runoff is equalled or exceeded. Representa-
tion of the entire runoff hydrograph time series (typically
daily runoff, but it can also be hourly, or even monthly) in
the form of the FDC makes the latter a compact signature of
runoff variability, and a valuable tool to diagnose rainfall
flow duration
-
runoff responses in gauged catchments at a holistic level,
and to regionalise them to ungauged catchments. However,
by representing runoff variability in the frequency domain
as the FDC, information on the timing of the runoff
response is lost. The latter is better reflected in the basin
s
runoff seasonal flow regime (
Chapter 6
) and, of course, in
the complete runoff hydrograph (see
Chapter 10
).
The FDCs (for daily runoff) can be constructed empiric-
ally for gauged sites by (i) ranking observed runoff in
ascending order and (ii) plotting each ordered observation
versus its corresponding duration (e.g., in days), or its
fractional duration (which is dimensionless). Comparisons
of FDCs between catchments of different sizes or in differ-
ent climatic regions can be assisted by expressing the FDC
in terms of normalised runoff (normalised by drainage area
or by mean annual runoff). If a stronger emphasis is needed
to be given to either the low flow or the flood portion of the
FDC, then it can be plotted semi-logarithmically, express-
ing the logarithm of runoff as a function of (fractional)
duration.
'
river corridors as
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