Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation in Africa>
4.2 Freshwater Augmentation -- Cloud Seeding, Zimbabwe
Zimbabwe covers 390 000 km2 in area, and has a population of 10.4
million people (CSO, 1992), of whom 74.3% (or 7.73 million) are rural
(CSO, 1989). The country is situated in southern central Africa between
latitudes 150 30' and 220 30' South and between longitudes 250 00' and 330
10' East. The country falls into various land use (agro-ecological) zones
determined by rainfall, temperature and soil types as shown below. The
country has a seasonal rainfall pattern, which varies across the country,
with the highest rainfalls of over 2 000 mm falling in the Eastern
Highlands and the lowest rainfalls of around 400 mm falling in the low
areas, particularly along the area bordering South Africa. In the
middelveld, the rainfall varies from 700 to 1200 mm. Runoff is generally
8% of the mean annual rainfall (DWD, 1980). The majority of rivers are
non-perennial. An attempt to fully utilize this runoff has led to an
extensive dam construction programme. While these reservoirs are used as
sources of water for mainly urban centres, some supply water for
irrigation and mining purposes.
||% of Land Area
||Very high rainfall, above 1 000 mm; low temperatures,
below 15 C. Forestry, wattle, tea, coffee, deciduous fruit, barley and
potatoes. Intensive beef and dairy farming.
||High rainfall, 700 to 1 000 mm; warm summers, cool winters. Maize,
tobacco, cotton, winter wheat and market gardening - intensive farming.
Intensive beef and dairy farming.
||Moderate rainfall, 550 to 700 mm; high temperatures and dry spells.
Drought resistant cotton, soya and sorghum. Beef farming and breeding.
||Low rainfall, 450 to 600 mm; seasonal droughts. Irrigation of
drought resistant crops. Semi-extensive controlled grazing.
||Very low rainfall, below 500 mm; very hot. Sugar, citrus, cotton
and wheat irrigation schemes of lowveld.
||Not suitable for agricultural activity without irrigation. Some
beef production and wild life. Source: (School Atlas for Zimbabwe)
The majority of Zimbabweans live in the communal and resettlement areas
where they rely on agriculture for a living. Commercial agriculture is a
major contributor to the country's economy.
The country, therefore,
is heavily dependent on agriculture, and, as a result of recent droughts,
precipitation augmentation has become a vital component of this economic
sector. Rainfall augmentation operations are carried out during all
seasons which do not have sufficient natural rainfall.
This technique involves the beneficial modification of summer convective
rainfall to increase rainfall production efficiency. Only about 30% of the
atmospheric water vapour entering the region's storms naturally reaches
the ground as precipitation (McNaughton, 1980). The approach encourages
efficient rainfall formation through a collision - coalescence process
which is enhanced or accelerated by the addition of hygroscopic nuclei
into a storm updraft at cloud base. Augmentation provides additional water
to crops when weather conditions are favourable and is required during
periods when little or no rainfall would otherwise occur.
In Zimbabwe two methods of cloud seeding are used:
- The high level seeding method which has been used since cloud seeding
was first practised, and which involves flying into the top of a
suitable cloud and injecting it with silver iodide.
- The low level seeding method which is experimental and involves
burning flares which emit hygroscopic smoke material into the base of a
This technology is described in Part B, Chapter 1, "Agriculture."
Operation and Maintenance
Major equipment requirements are aircraft, radar, materials and skilled
personnel. In this instance, the aircraft were supplied mainly by the
farming community, with the government paying for their use, with some
additional aircraft supplied by the government. One problem was the
shortage of aircraft. The numbers of aircraft were hardly enough for a
comprehensive aerial coverage.
Aircraft and personnel are on standby during the rainy season, waiting
for the right clouds/conditions. The main rainy season in Zimbabwe is from
November to March. Rain sometimes falls in other months, mainly September,
October, April and May, and, during the rainy season, there are spells of
completely dry weather. Therefore, an operational cloud seeding
organisation would normally be inactive on about half of the days in the
rainy season, including occasions which are too wet for additional
rainfall to be required.
Level of Involvement
The private sector, in particular the farming community, assisted
governmental implementation of this technology, mainly with aircraft.
Thus, in this application, there was a high degree of community
involvement, but this could vary in other applications.
Average annual cost of Zimbabwe's national cloud-seeding operation
between 1973 and 1979 was about $10 000. The highest annual cost was $13
700 in 1978/79, which was the busiest season ever, during which chartered
aircraft were used. For the 1995/96 season, $12 500 has been set aside for
cloud seeding operations.
Effectiveness of the Technology
It was observed in Zimbabwe that the cost of cloud seeding was
significantly less than the resulting benefit from the maize yield alone.
This analysis did not take into account other crops or non-agricultural
benefits. Hence, cloud seeding is considered to more than pay for itself
In Zimbabwe, the extra maize yield from the national cloud seeding
programmes in the late 1970s was estimated at between 0.5% and 1% of the
national total (McNaughton, 1980). Pastures and grasslands also benefited.
Further, seeded rainwater was determined to be cheaper than piped water
with respect to irrigation ($0.12/km3 versus $1.76/km3 ; McNaughton,
1980). Extra benefits accruing from the cloud seeding programmes included:
(i) fog and stratus disposal around airports; (ii) hail suppression; and,
(iii) cyclone (hurricane) modification.
Experimental operations are costly, often requiring the simultaneous use
of three aircraft -- one for seeding, a second to monitor weather
conditions (such as cloud top temperatures during the five to ten minutes
after seeding), and a third to measure precipitation at the cloud base in
a rain collector. Cloud-seeding operations/aircraft have to select
suitable clouds as they arise. This selection cannot be controlled or
easily predicted in the long term. Therefore, it is difficult to supply
water when and where it is required. Also, chartered aircraft are
expensive to operate from the point of view of standing charges.
Further Development of the Technology
Currently the Zimbabwe cloud seeding fleet has aircraft based in Harare
and the Lowveld. Depending on the occurrence of potentially suitable
clouds during the height of the wet season, additional aircraft should be
made available during January or February. If maize is planted in
December, it is recommended that at least one aircraft should be on
standby until late April. While jet aircraft can cope with larger numbers
of clouds, and, therefore, reduce the number of cloud seeding aircraft
required, these aircraft are more expensive to operate. There is also a
need for a daily radiosonde data in areas with a high suitability for
cloud seeding during this period, and a need to carry out further
experiments with reference to the atmospheric conditions which permit
W. Marume, Department of Meteorological Services, Post
Office Box BE150, Belvedere, Zimbabwe.
Kamanzira, Department of Meteorological Services, Post
Office Box BE150, Belvedere, Zimbabwe.
McNaughton, D.L. 1980. Cloud Seeding in Zimbabwe and Some of its
Effects on SR52 Maize Growth. PhD Thesis, Faculty of Agriculture,
University of Zimbabwe, Harare.
Meteorological Department 1993. Reports on National Cloud Seeding
Operations. Department of Meteorological Services Reference File -
National Cloud Seeding Season 1992/93, Ministry of Transport and National
Supplies, Belvedere, Zimbabwe.