Geography Reference
In-Depth Information
Figure 8.10. Uncertainty of Q 95 low
flow estimates in ungauged basins
in Austria expressed as confidence
bounds (expected low flow plus or
minus the error standard deviation).
From Laaha and Blöschl ( 2007 ).
a) Lower confidence limit
q95 (Ls -1 km -2 )
0 - 1
1 - 2
2 - 4
4 - 6
6 - 8
8 - 10
10 - 12
> 12
b) Upper confidence limit
q95 (Ls -1 km -2 )
0 - 1
1 - 2
2 - 4
4 - 6
6 - 8
8 - 10
10 - 12
> 12
50 km
hydrologically. The uncertainties of the predictors may
significantly vary in space. Laaha and Blöschl ( 2007 )
proposed an error model that accounts for both low flow
measurement errors and the prediction errors of multiple
regressions. An example of the uncertainty is given in
Figure 8.10 in terms of the confidence bounds, defined as
the expected low flow plus and minus the error standard
deviation. They note that these uncertainties provide
regional information. In an application case, this should
be complemented by local information from site visits (in
particular about any anthropogenic effects) and hydro-
logical reasoning should be exercised based on this add-
itional local information.
observations or improve at-site estimates with few obser-
vations. The most common regional frequency method is
the index method, first introduced for floods (Dalrymple,
1960 ), and later extended to low flow studies (Clausen and
Pearson, 1995 ; Madsen and Rosbjerg, 1998 ). The method
assumes that the low flow distribution, scaled by an index
low flow, is uniform within a region. If the index low flow
is known at an ungauged site, the low flow associated with
a given return period can be estimated as the product of the
index low flow and the scaled distribution. The index low
flow is usually taken as the mean or median of the annual
minimum runoff. Clausen and Pearson ( 1995 ) tested the
method to estimate both the maximum duration and max-
imum volumetric deficit of runoff in New Zealand. Catch-
ments from three regions with different climate and
physical properties were used in their study. For both low
flow indices the index low flow (i.e., the mean of the
annual minima) was found to vary with catchment and
climatic characteristics except for one region subject to
very high annual precipitation, where the index low flow
8.3.2 Index low flow methods
Low flow runoff of a given return period, such as T-year
annual d-day minima (Q d,T ), is estimated by statistical
frequency analysis. Regional frequency methods have
been developed to obtain estimates at sites with no
Search WWH ::




Custom Search