Geography Reference
In-Depth Information
15
Wights
10
Salmon
5
0
1974
1975
1976
1977
1978
1979
1980
1981
Figure 10.6. Runoff hydrographs observed in Salmon and Wights catchments in the south-west of Western Australia demonstrating change in
the runoff dynamics due to land use change. Joint monitoring started in 1974, forest clearing on Wights occurred in 1976, whereas Salmon
remained forested. Adapted from Sivapalan et al.(
1996
).
forest in 1976 (2 years after monitoring started, which
continued until recent times), and replaced by shallow-
rooted pasture. The removal of the deep-rooted Eucalyptus
forest meant sharp reduction of evaporation, increased
recharge, gradual water table rise (over a 6-year period),
and overall increase in the wetness, and the net result is
increased runoff. This can be clearly seen in
Figure 10.6
,
most notably after 1977, resulting even in summer flows in
a previously ephemeral catchment.
Urbanisation may shortcut the flow paths and therefore
may result in flashier runoff hydrographs, which will cause
lower baseflow, and higher flood peaks (e.g., Konrad and
Booth,
2005
). Urban runoff is controlled by the distribu-
tion and connectivity of sealed areas, by the topography as
well as the characteristics of any sewer systems.
Runoff similarity
What makes two catchments similar in terms of the com-
plete runoff hydrographs? There are many facets to a run-
off hydrograph, so this will depend on which facets we are
interested. Clearly, one or a combination of the signatures
of runoff variability discussed in
Chapters 5
to 9 (i.e.,
annual, seasonal, flow duration curve etc.) are obvious
candidates to be used in similarity measures, especially if
used in a hierarchical manner, considering that they cap-
ture different parts of the full spectrum of variability
embedded in runoff hydrographs.
Figure 10.7
shows how the individual runoff signatures
are related to the complete hydrograph for two catchments
in Australia: Harvey River catchment (148 km
2
), 120 km
south of Perth, Western Australia, and Seventeen Mile
Creek catchment (619 km
2
) located in the Katherine
region, near Darwin in northern Australia. According to
the Köppen climate classification, Perth is located in a
temperate climate region, with a distinctly dry summer
period, with high precipitation during the cold winter
period (May
10.2.2 Similarity measures
In order to transfer information on the runoff hydrograph
across the landscape, e.g., from gauged to ungauged
catchments, one needs to identify what hydrological
similarity is. A very simple measure of hydrological
similarity is spatial proximity, based on the rationale that
the controls on the runoff hydrograph may vary
smoothly in space, so the runoff hydrographs in catch-
ments that are close to each other will be similar. How-
ever, hydrological processes are often more complex
than this as the runoff generation processes may vary
tremendously over short distances. An alternative there-
fore is to define hydrological similarity in terms of run-
off signatures (runoff similarity) and in terms of climate
and catchment characteristics (climate and catchment
similarities, see
Figure 2.8
)thatarerelatedtothedom-
inant processes of
Section 10.2.1
.
October) and almost no precipitation during
the November
-
April summer period (annual precipitation
is 928 mm). Darwin is located in the tropical region of
Australia, with the majority of precipitation during the wet
season (November
-
April), and very little precipitation in
the dry season (May
-
October) (annual precipitation is 979
mm, only slightly higher than Perth). The annual potential
evaporation in Perth is 1757 mm, and in Darwin it is 2220
(30% higher).
Both catchments exhibit strong seasonality of runoff
(Perth due to its Mediterranean climate and Darwin due
to its monsoon climate, although the timing is different),
yet with considerable inter-annual variability in the annual,
monthly and daily flows (i.e.,
-
flow duration curves),
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