Geography Reference
In-Depth Information
(
2011
), the first-order control on the normalised FDC is the
duration curve of precipitation (i.e., PDC, see
Figures 7.4
and
7.9
). This is especially true in the case of the FDC
estimated for the fast flows (FFDC, see
Figure 7.9
). Cheng
et al.(
2012
) derived a climatic index, P
max
α
p
, based on the
precipitation time series, and found that it had considerable
explanatory power for FFDC, where P
max
is the maximum
daily precipitation and
index), Holmes et al.(
2002
; HOST soil classes, see
Chap-
ter 4
), Claps and Fiorentino (
1997
; baseflow index) and
Rianna et al.(
2011
; percentage of volcanic/carbonatic
substrata). Li et al.(
2010
) found the leaf area index
(LAI) to be related, together with the elevation difference,
to the standard deviation of runoff, thus being inversely
proportional to the slope of the FDC: this may be due to
their differential impacts on evaporation between summer
and winter.
Despite this valuable work, the literature on process-
based approaches is still sparse. From a process perspec-
tive, the shape of the FDC (especially the middle part of
the FDC, quantified by the slope of the FDC) can be
influenced by the catchment
α
p
is the probability of zero precipi-
tation (i.e., fraction of non-rainy days within the year). As
in the case of annual runoff (Sivapalan et al.,
2011b
) the
results of Cheng et al.(
2012
) also showed that there is a
certain level of space-time symmetry, the variability of the
FDCs between catchments being matched by their variabil-
ity between years.
Other climatic controls on the shape of the FDC could
be seasonality of precipitation and regional potential evap-
oration, including their relative magnitudes and phase dif-
ference. For example, Cheng et al.(
2012
) considered the
value
of the seasonality index, which represents a measure
of within-year variability of precipitation, as a potential
climatic index for the regional patterns of the FDC for
slow flows (i.e., SFDC), although the relationship was
not very strong.
s storage capacity (both sur-
face and groundwater stores) and associated residence
times (Lane and Lei,
1950
), and how they interact with
the seasonality of precipitation and potential evaporation.
Without substantial storage from which to derive subse-
quent baseflow, catchments will be characterised by steep
FDCs and probably also experience a high frequency of
zero runoff. Catchments with adequate storage to support
baseflow, on the other hand, will have FDCs with flatter
slopes. This is usually also reflected in the magnitude of
the baseflow index, the ratio of total volume of subsurface
flow to precipitation on an annual time scale. This is
confirmed by the work of Cheng et al.(
2012
), who found
a significant relationship between the shape parameter of
the FDC,
'
Catchment similarity
The majority of the studies reported in the literature on the
regionalisation of FDCs have followed statistical
approaches, in which case they attempt to relate quantita-
tive measures of the FDCs (slope of the FDC, parameters
of statistical distributions) to appropriate catchment char-
acteristics. Catchment characteristics usually considered as
potential indicators of the magnitude and shape of the
FDCs are catchment size, vegetation cover (Ouarda et al.,
2000
) and surficial geology (e.g., Holmes et al.,
2002
;
that the parameter representing the shape of the FDC
depended on the overall catchment soil permeability. Sau-
quet and Catalogne (
2011
) found that the catchment yield
and the percentage of impermeable substratum, both repre-
senting the effect of geology, controlled the slope of the
FDC curve. They also found that the slope of the FDC
decreased with increasing catchment size, and suggested
that this may be due to increasing storage capacities, and
the combinations of different river runoff patterns originat-
ing from upstream tributaries. Catchment area, percentage
of permeable area, and areal average of the Soil Conser-
vation Service Curve Number (SCS-CN), along with mean
annual precipitation, were found to be correlated with the
shape of the FDC by Viola et al.
(2011)
. Soil and geo-
logical factors were statistically related to the shape of the
FDC by several authors: Croker et al
. (2003
; soil classes
and baseflow index), Mohamoud
(2008
; available water
capacity, soil depth, soil
baseflow index (BI), as
shown in
Figure 7.11.
Whereas
Figure 7.11a
focuses on the
variability between catchments, the results in
Figure 7.11b
show the nature of the variability between years for a
subset of eight selected catchments, demonstrating consid-
erable space-time symmetry in these relationships.
The existence of significant relationships between quan-
titative indices characterising the shape of the FDC (e.g.,
slope of the FDC, parameters of statistical distributions)
and climatic and catchment characteristics, such as the
aridity index, baseflow index and the precipitation index,
can enable hydrologically sound regionalisations, includ-
ing grouping of similar catchments with the use of more
readily available climatic characteristics and catchment
characteristics, instead of only using distance, as shown
in
Figure 7.8b
.
κ
, and the catchments
'
7.2.3 Catchment grouping
The grouping of catchments helps towards the estimation
of FDCs in two ways: (i) the classification of catchments
underpinning the grouping contributes towards increased
understanding of catchment behaviour, and (ii) the pooling
of similar catchments increases the sample size, and thus
improves the accuracy and robustness of the estimation of
FDCs in ungauged basins. Nevertheless,
texture classes and baseflow
the scientific
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