Geography Reference
In-Depth Information
Figure 7.5. Smoothed seasonal flow
regimes (using a 30-day moving
window) (top), and long-term FDCs
(bottom), based on 50-year record
of observed daily runoff, normalised
by mean annual daily runoff,
from USA catchments located in
Pennsylvania (PA), Montana (MT),
Northern California (North CA),
Kansas (KS) and Virginia (VA).
From Yaeger et al.(
2012
).
4.0
3.0
2.0
1.0
0.0
A
A
S
O
D
J
F
M
M
J
J
N
10
2
10
1
10
0
10
-1
PA, Snow, Sub Q
f
, Vegetation
MT, Snow
North CA, Subsurface-influenced Q
f
KS, Vegetation
VA, Snow
10
-2
10
-3
0
10
20
30
40
50
60
70
80
90
100
Percent of time exceeded
two key messages: (i) the seasonal flow regime (discussed
in detail in
Chapter 6
) serves as the connective tissue
between the high and low flow ends of the FDC, and
(ii) due to the elimination of the timing of processes,
catchments with different regime curves may yet give rise
to similar shapes of the FDCs.
and soil permeability, vegetation cover and dynamics (with
attendant impacts on root zone depth), and human-induced
land use and land cover changes.
Work over the past few decades has contributed to the
accumulation of considerable empirical knowledge on the
effects of these catchment characteristics upon the shape of
FDCs. Musiake et al.
(1975)
investigated the effects of
geology and climate type on the shape of FDCs in moun-
tainous catchments in Japan. Ward and Robinson (
1990
)
provided a summary of the effects of dominant soil types
on FDCs in UK catchments. Fennessey and Vogel (
1990
)
documented the important influence of catchment relief on
the shape of FDCs. Burt and Swank (
1992
) investigated
the effects of vegetation type on the FDCs.
Figure 7.6
presents an illustration of the effect of geology on the
shape of the FDCs for two catchments in the UK. The
Eden at Penshurst is a rural catchment with scattered
settlements developed on sands and clays, while the Test
at Broadlands is underlain by permeable formations, 90%
of which consist of chalk but with some tertiary deposits.
The FDCs of the two catchments are very different. That of
the Test is much flatter, which is related to storage in the
chalk aquifer and the close stream
Catchment characteristics
The within-year variability of runoff that is reflected in the
FDC arises through the interaction of within-year variabil-
ity of climate forcing (precipitation, radiation and tempera-
ture, including seasonal vegetation dynamics) with the
landscape, including the subsurface, and the resulting
filtering of the variability. The nature and extent of such
filtering determines the shape of the FDCs. For example,
catchments that are dominated by rapidly responding near-
surface runoff processes will have steeper FDCs (with the
possibility of high frequency of zero runoff), whereas
catchments where slow processes dominate runoff gener-
ation may have less steep FDCs (see
Figure 7.4
). Key
catchment characteristics that impact on the shape of the
FDC include surface soil and vegetation characteristics that
determine the partitioning of incoming precipitation into
interception, infiltration and overland (fast) runoff. Water
that infiltrates into the soil is then partitioned into subsur-
face storage, subsurface (slow) drainage and evaporation
through plant uptake and transpiration. These are all
governed by geology through its impact on soil depth
-
aquifer interactions.
Environmental change
When extrapolating FDCs to ungauged basins it is
important to recognise that the FDCs may be modified
by environmental change, i.e., land use changes, water
Search WWH ::
Custom Search