Prediction of low flows in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 163

8 Prediction of low flows in
ungauged basins

Contributors: G. Laaha,* S. Demuth, H. Hisdal,
C. N. Kroll, H. A. J. van Lanen, T. Nester, M. Rogger,
E. Sauquet, L. M. Tallaksen, R. A. Woods and A. Young

8.1 How dry will it be?
Rivers often undergo periods of low flows that sometimes
last for a long time. In some places, and river reaches, the
river may even cease to flow entirely. While both ecosys-
tems and humans have adapted to low flow conditions in
various ways, the actual amount of water in the stream is
critically important for a number of life-sustaining func-
tions. River water is often used for drinking and household
use, irrigation and energy production as well as industrial
uses. These demands for water often remain constant
throughout low flow periods. Rivers and streams also have
important ecosystem functions. The health of a range of
aquatic and riverine ecosystems is closely linked to the
runoff variability in the stream, of which low flow periods
are an integral part (Gustard and Demuth, 2009 ; Smakhtin,
2001 ). With increasing human population and improving
lifestyles, the demand for water has increased dramatically,
putting more pressure on available water resources. This is
especially true in places where water abstractions, water
transfers, damming of rivers and pumping of groundwater
have exacerbated the low flow problem. Managing water
resources efficiently and effectively during low flow
periods has therefore become an important part of inte-
grated water resources management.
To assist in all these management tasks, accurate predic-
tions of low flows are needed. Additionally, understanding
low flows in the context of the natural hydrological regime
and the underlying climate, landscape and human controls
is an interesting science question in itself.
Depending on the application and the science question,
different low flow characteristics may be of interest (see
Smakhtin, 2001 ; Hisdal et al., 2004 ; Gustard and Demuth,
2009 ): (i) characteristics that represent low flow runoff
with a certain probability; (ii) characteristics that represent
durations or deficit volumes of low flows; and (iii) charac-
teristics that represent how quickly the low flow runoff

decreases with time. Indices that relate to the first group
(i.e., representing runoff) include the following:

Flow quantiles (Q x ): runoff values exceeded x% of the
time corresponding to points on the flow duration curve
( Chapter 7 ). For perennial streams, 90th or 95th quan-
tiles (i.e., Q 90 and Q 95 ) are often used, while for inter-
mittent or ephemeral streams, quantiles based on runoff
within a typical runoff season, or quantiles with a lower
exceedance probability (e.g., 60%) may be more appro-
priate (Smakhtin, 2001 ). Flow quantiles are relatively
robust to measurement errors and anthropogenic effects
(Laaha, 2000 ), which has made them widely used
around the world (Smakhtin, 2001 ).

Mean annual minimum flows over d consecutive days
(MAM d ): long-term average of runoff during the driest
period of each year. A moving average time window of
7 or 10 days is often used to remove fluctuations of the
hydrograph due to measurement errors or anthropogenic
effects (Laaha, 2000 ). The choice of the moving window
size may also depend on the problem at hand. The
magnitudes of mean annual minimum flows over 7 or
10 consecutive days are often similar to Q 95 (e.g.,
Smakhtin, 2001 ; Laaha et al., 2005 ).

Instead of the mean, annual low flows Q d,T of a given
return period T can be used, where Q d,T (d-day, T-year
runoff) relates to the annual minimum d-day low flow that
is expected to occur, on average, once every T years. They
are estimated by extreme value statistics from a d-day
moving average of the daily hydrograph. The 7-day runoff
with a return period of 10 years (Q 7,10 ) is often used in the
USA and Canada (e.g., Kroll et al., 2004 ).
The second group, runoff deficit indices, measure the
runoff volume below a runoff demand function, which
may be related to irrigation water requirements, cooling
water for industrial plants, drinking water supply, min-
imum water depth for navigation, or environmental flows
to support stream ecology (Yevjevich, 1967 ; Nathan and
McMahon, 1990 ; Hisdal et al., 2004 ). Duration indices
measure the maximum or average duration of dry spells