Water Management for Disease Vector Control - Biological and Environmental Control of Disease Vectors - page 168

reservations, it has, however, been possible to
implement AWDI on a large scale. In China, for
example, savings in water provided an important
incentive for farmers to adopt this method, as
water charges were based on volumetric use
(van der Hoek et al ., 2001; Qunhua et al ., 2004).
In India and Sri Lanka, AWDI is adopted by more
and more farmers, though here for more
opportunistic reasons as water is becoming
increasingly scarce (van der Hoek et al ., 2001).
Overall, the water-saving and economic aspects,
not the mosquito or disease control, are likely to
be the critical factors that will make farmers and
irrigation department oi cials change behaviour
and adopt AWDI in water-scarce areas. Early in
the 20th century, several studies were performed
in the Indonesian archipelago on 'intermittent
irrigation' (Takken et al ., 1990). A cycle of 9
days wet and 2 days dry in 15 ha fi eld trials in
Bali reduced the density of the malaria vector
An . aconitus by 75%. This research had direct
impact on decision making and led the local
administration to make AWDI obligatory in Bali
and Lombok despite some observed reductions
in yield under AWDI. Most studies on AWDI
have been undertaken in Asia and more research
is needed in Africa to assess the potential for
disease control and the feasibility of
implementation. This is especially important
given a scenario of increased rice cultivation in
Africa (Purevdorj and Kubo, 2005).

over a 7-10-day period, eliminating shoreline
plants and fl oating debris, and thereby
signifi cantly reducing the breeding of the local
malaria vectors (Oomen et al ., 1990). The use of
rapid reservoir drawdown to strand snails has
been investigated for both small and large
reservoirs in, among other places, Puerto Rico
with some success (Jobin, 1973). However,
implementing such interventions in large
reservoirs such as Lake Volta and Lake Kariba
seem unfeasible given the size of the reservoirs,
the complexity of the management to undertake
this operation, and the potential impact on
activities along the lake shore.

9.3.3 Changing water fl ow velocity in
natural streams and rivers

A sudden release of water from an upstream
section of a stream or canal can fl ush out an
area further downstream. The aims of fl ush
provision are to eliminate stagnant pools in an
otherwise dry riverbed and to increase water
fl ow and velocity to destroy larvae and snail
habitats. When any amount of water is released,
a turbulent small fl ood will travel down the
stream or river. Over the period between 1930
and 1950, hundreds of siphons and fl ushing
devices were installed especially in Sri Lanka, the
Philippines and India for the control of stream-
breeding mosquitoes with water releases
approximately once a week (Konradsen et al .,
2004).
In Sri Lanka, the importance of rivers and
streams in generating large numbers of the most
important malaria vector has been well
documented, and the relation between river
water fl ow and mosquito breeding was known
early in the 20th century (Konradsen et al .,
2004). In 1938, a range of experiments was
initiated to reduce the breeding of vectors
outside the rainy season or during periods of
drought when pools would form in large
numbers in the stream and riverbeds. Engineers
from the medical and sanitary services designed
dif erent types of siphons and small dams in an
attempt to fl ush a stretch of river to remove
mosquito larvae from their natural breeding
sites by the creation of strong currents and the
fi nal stranding of the larvae when the water
level falls. Flushing of streams also inhibited

9.3.2 Water-level fl uctuations in water
reservoirs

Tens of thousands of small earthen dams, and a
much smaller number of large dams, have been
constructed in tropical and subtropical parts of
Africa and Asia, which have created reservoirs
with a potential for impacting on the
transmission of mosquito- and snail-borne
diseases. In such settings, a vector control
strategy based upon water-level fl uctuations has
been applied in reservoirs. The strategy aims to
strand fl oating aquatic vegetation, and thereby
kill aquatic snails and mosquito larvae. One of
the best known cases of reservoir water level
fl uctuation is that of the Tennessee Valley
Authority in the USA, which was aimed at
malaria control. The water level was managed to
allow for slowly receding water levels of 0.3 m