Geoscience Reference
In-Depth Information
in the Euphrates 90% of the total runoff comes
from the mountains of north-eastern Turkey,
with virtually zero runoff from areas such as
Iraq (Beaumont 1989). Similarly the Nile flows
from highlands in Ethiopia, which supply most
of its water, but most of its course is through
the arid lands of Egypt. Before the construction
of the Aswan dam the Nile annual discharge
ranged from a low of 1000 cumecs (cubic metres
of water per second) in spring to 11,000 cumecs
in late summer/early autumn (Beaumont 1989).
Perennial rivers in general form the focus of
Chapter 3, so will not be discussed further in this
section. Ephemeral channels dominate in arid
zones and thus form the focus of the rest of this
section. These channels are only occupied by
water following a rainstorm as they are mainly
supplied by storm runoff. They tend to show less
regular discharge patterns than perennial rivers,
and suffer large transmission losses downstream.
(a)
10
4
Perennial
Ephemeral
10
3
10
2
10
1
10
0
10
−
1
10
0
10
1
10
2
10
3
10
4
10
5
(b)
10
3
10
2
5.2.3.1 Flow characteristics of ephemeral channels
10
1
Arid river systems cover a diversity of forms
on a variety of scales. Cooper Creek, Australia,
for example, is the largest internally draining
catchment in the world (1.3
10
0
10
6
km
2
; Knighton
& Nanson 1997) and lies entirely within an
arid zone. Owing to the hydrological character-
istics of the slopes feeding ephemeral channels
(section 5.4.3), runoff is a typically more signific-
ant component in streamflow than in more humid
areas, where throughflow and groundwater may
dominate. For example, in New South Wales,
Australia, only 16 mm of rainfall is required to
produce runoff in the arid west, whereas 35 mm
is required in the more humid east (Cordery et al.
1983). In Australia large, tropical monsoonal
storms can penetrate large distances inland of the
arid zones. In most arid systems, however, rain-
fall is derived from small, often convectional
storm cells of less than 10 -14 km width (Renard
& Keppel 1966; Diskin & Lane 1972). Even
though these may activate only a small propor-
tion of a larger drainage basin, large floods can
be generated rapidly. In the USA the top 12
largest floods recorded have all occurred within
arid/semi-arid areas (Costa 1987). So, although
×
10
−
2
10
−
1
10
0
10
1
10
2
Drainage area (km
2
)
Fig. 5.5
A comparison of perennial (Maryland) and ephemeral
(California) channels. (a) Differences in mean annual runoff,
with largest values associated with perennial rivers. (b) A
demonstration of the similarity in the size of peak flood
discharges generated from perennial and ephemeral drainage
areas. (Adapted from Wolman & Gerson 1978.)
mean annual runoff is lower for most ephemeral
streams, compared with perennial streams, peak-
flood discharges are similar (Fig. 5.5).
Owing to the transport-limited nature of the
system, arid-zone rivers tend to have high peak
values of suspended sediment concentrations
and they are much more efficient transporters of
bedload material than humid systems. In part
this is reflected in the wide channels that typify
ephemeral river systems (Fig. 5.6). Thus a unique
aspect of ephemeral rivers is their multiple
terrace and channel cross-section. Large events
cut the widest channel and upper 'terraces' of
