Prediction of flow duration curves in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 138

similarity indices that can be used to group similar catch-
ments, and to develop relationships to extrapolate FDCs
from ungauged to gauged catchments in hydrologically
similar (i.e., homogeneous) regions.

(see Section 5.2.1 ); and (iii) the lower part is governed
by baseflow recession behaviour over dry periods for
which the dominant control is the competition between
geologically controlled deep drainage and riparian zone
evaporation.
The changing process controls can be recognised in the
FDCs presented in Figure 7.3 . For example, the head-
waters of the Alpine catchment in northern Italy (top) are
characterised by the presence of glaciers. Glacial melting
leads to maximum flows in summer, which due to the type
of geology contributes to significant groundwater recharge
in summer and releases substantial amounts of water to the
river in winter as well. The presence of glaciers, and of
snow processes in general, is the reason for the low
between-year variability in the annual FDCs because the
seasonal energy input is very stable on an annual time
scale. The high baseflow results because of the summer
groundwater recharge, reflected in the flatness of the curve.
In contrast, the catchment in New Mexico (bottom) has a
semi-arid climate (with mean annual precipitation below
500 mm). The FDC mainly reflects precipitation variability
(both within-year and between-year), the dominance of
mostly surface and near-surface fast flow processes, and
the absence of substantial subsurface storage, which
would, if present, contribute delayed flows. These factors
result in an ephemeral stream, with a FDC that is, on
average, characterised by a steeper slope than the moun-
tainous Italian catchment.

7.2.1 Processes
The FDC represents a distillation of the within-year, or
intra-annual, variability of runoff, presented in the fre-
quency domain. The FDC arises from the interplay of
climate regime, catchment size and morphology, vegeta-
tion cover, and the properties of the subsurface domain,
which together control the various runoff components. The
shape of the FDC is therefore governed by both precipita-
tion variability and how water moves through the catch-
ment. Deciphering the controls of both climate processes
(e.g., precipitation, temperature, radiation or potential
evaporation) and catchment characteristics (i.e., soil, top-
ography, vegetation type and functioning, catchment size,
human impacts) on the shape of FDCs is the key to their
prediction in ungauged basins.
As the major climate control on the FDC of a river basin
is precipitation, one can expect signatures of precipitation
variability to be reflected, to varying degrees, in the runoff
variability of a catchment. For example, the FDC at the
high flow end may closely resemble the statistics of pre-
cipitation due to the dominance of fast flow processes,
whereas for intermediate flows the dominant control may
be represented by soil water storage and its partitioning
into evaporation and slow flows. On the other hand, low
flows could be governed, in the absence of precipitation,
by the competition between deep groundwater flows and
riparian evaporation (see Chapter 4 ).
Figure 7.4 presents two illustrative examples, both based
on numerical simulations, which demonstrate that different
parts of the FDC can be governed by different process
controls. Figure 7.4 (top) presents model-predicted parti-
tioning of total hillslope runoff according to whether it
relates to slow matrix flow (slow and uniform water move-
ment through the subsurface) or both rapid and slow pref-
erential flow (water movement through wormholes, cracks
etc.) (Beckers and Alila, 2004 ), and their representation in
the corresponding FDCs. Yokoo and Sivapalan ( 2011 )
carried out independent work on the basis of which they
postulated that the FDC can be partitioned into three dis-
tinct parts, each of which is governed by different mechan-
isms or process controls ( Figure 7.4 bottom): (i) the upper
part, which represents high flows, is governed by flood
processes for which the dominant control is the interaction
of extreme rainfall and fast runoff processes; (ii) the middle
part relates to the mean runoff and its seasonality, for
which the dominant control is the competition and seasonal
interaction between available water, energy and storage

Climate forcing
Climate impacts the FDC in several ways. The annual
mean of the FDC (or the annual runoff) is governed by
the aridity of the climate, as measured by the ratio of
annual potential evaporation to precipitation, E p /P, which
reflects the competition between water and energy avail-
ability (as shown in Chapter 5 : Annual runoff). The slope
of the FDC in the middle part of the curve, which is related
to the variance of daily runoff, can be affected by the
competition between the seasonality of precipitation and
that of potential evaporation (including whether they are
in-phase or out-of-phase), as mediated by subsurface drain-
age. Additional aspects of climate that can impact the FDC
include the amount and timing of precipitation as snowfall,
the eventual melting of accumulated snow, and also sea-
sonal and spatial variations of vegetation cover and func-
tioning (i.e., phenology), which affect the amount and
timing of evaporation, and therefore the amount and timing
of runoff (see Section 6.2 ). The seasonality of both snow
processes and phenology can be attributed to the seasonal
variations of air temperature (which is a climatic feature).
The net effect of these within-year interactions between
climate seasonality and storage processes is clearly
reflected in the seasonal flow regime (seasonal variability