Geoscience Reference
In-Depth Information
South Africa. This is not normal behaviour for this
type of storm and in so doing it created large
floods in the headwaters of rivers draining into
Mozambique. Flood warnings were issued by
Zimbabwe and South Africa but the poor state of
communications in Mozambique (exacerbated
by the previous floods cutting communication
lines) meant that they were not available to warn
people on the ground. In all 700 people died as a
result of the floods and 45,000 people were
displaced. It is estimated that it will cost US$450
million to repair damage to the infrastructure
in Mozambique (Christie and Hanlon, 2001). This
is not the total cost of the flood, which is far higher
when loss of income and loss of private property
are included. These costs will never be fully
known as in many lesser-developed countries the
costs are borne by individuals without any form of
insurance cover.
In many ways there are no new lessons to learn
from the Mozambique floods of 2000. It is well
known that adequate warning systems are needed
(but expensive) and that people should be
restricted from living in flood-prone areas; but this
1,200
1,000
800
600
400
200
0
1950-51
1960-61
1970-71
1980-81
1990-91
Figure 5.15
Rainfall totals during the rainy season
(smoothed with a two-year average) at Maputo
airport, with vertical bars indicating the strength of
La Niña events (on a scale of three: strong, medium,
weak).
Sources
: Rainfall data from Christie and Hanlon
(2001); La Niña strength from NOAA
is difficult to achieve in a poor country such as
Mozambique. The cause of the flood was a huge
amount of rainfall and the severity was influenced
by the antecedent wetness of the ground due to a
very wet monsoon.
in climate. Many studies have suggested that future
climate change will involve greater extremes of
weather (IPCC, 2007), including more high-
intensity rainfall events. This is likely to lead to an
increase in flooding, particularly while a channel
adjusts to the differing flow regime (if it is allowed
to).
have a lower nutrient level than water that moves
slowly through the soil as throughflow and/or
groundwater. However, water that has travelled as
overland flow may have a higher level of suspended
solids picked up from the surface, so it may appear
less pure.
In considering issues of land-use change and
water quality, an important consideration is the
time taken for water to reach the stream. It is
important to realise that groundwater is frequently
operating as a pressure wave response to rainfall
recharge. Where groundwater responds to a rainfall
event by emitting water into a stream it is a pressure
wave response, i.e. the water entering the stream is
not the same water that infiltrates and causes the
response. This means that water entering the stream
may be several years (or more) older and unaffected
by the current land use change.
RUNOFF IN THE CONTEXT
OF WATER QUALITY
The route that water takes between falling as
precipitation and reaching a stream has a large
influence on water quality. The nutrient level of
water is heavily influenced by the length of time
water spends in contact with soil. Water that moves
quickly into a river (e.g. overland flow) is likely to
