Cloud-seeding in Zimbabwe, and some of its effects on SR52 maize yield

Submitted to the University of Rhodesia in December 1979; revised version in April 1980; approved in January 1981.


Three new techniques were devised to assist with cloud-seeding experimentation: (i) an airborne rain collector, to estimate the amount of water produced by individual showers, (ii) a method whereby inclinometer compass readings measured cloud-top height, and (iii) a nomogram to derive altimeter corrections from radiosonde data.

The randomised experiments indicated that, on average, silver iodide treatment of cumulus clouds increased and prolonged their rainfall, particularly when cloud-tops were -l3°C or colder. There was some evidence that seeding effects were negligible when natural rainfall was light, and also (in heavier showers) during the first half-hour after seeding.

A comparison of distribution functions, fitted to seeded and to non-seeded populations, produced a tentative relationship between natural and seeded cloud rainfall totals. In another exercise a model "truncated ellipsoid" was constructed for every individual shower, representing its dimensions and total water content, from which point rainfalls were calculated at grid positions. Seeded and non-seeded populations were thus built up and compared, giving a tentative assessment of average rain increases produced at points on the ground.

In a computerised cumulus modelling exercise, model predictions were poor when the lower portions of clouds disintegrated during development; better results were obtained when the new, higher cloud-base was fed into the model.

A computer program was written to simulate a cloud-seeding season in Mashonaland, whereby 1000 typical seeded rainshowers were distributed at random within a rectangular area, enabling total extra rainfall to be computed at every km grid-point. Sixty percent of the area received no additional rainfall at all, because the cloud-seeding aircraft cannot attend to every suitable cloud.

Two soil water balance routines were then used to relate the apparently increased rainfall from cloud-seeding to changes in maize yield. The simpler one had been tested against extensive maize trials, and was run alongside a more complex routine which attempted to allow for numerous refinements, but which had less maize data suitable for testing. Nevertheless, the refined model's computed number of post-flowering severe stress days correlated well with 11 years of maize trials at Salisbury Research Station. However, mild stress days and earlier stress days did not correlate significantly with maize yield.

In approximately half of all rainy seasons in Mashonaland, there appears to be adequate natural rainfall, implying that cloud-seeding during these seasons is redundant as far as maize is concerned. In poor or modest rainy seasons, most places receive less than 10 mm additional rain from a cloud-seeding operation, but this can often eliminate one, sometimes two post-flowering stress days during maize growth. In a few instances, the rainfall increases are probably sufficient to extend the maize root depth slightly. However, only on very rare occasions can cloud-seeding provide sufficient extra water to bring forward a maize planting date. Nevertheless, indications are that cloud-seeding operations more than pay for themselves, although not to the extent that all possible doubt can be excluded.

David L. McNaughton

In 1982, a 'summary article' was published in the Zimbabwe Journal of Agricultural Research 20, pages 39-57, entitled "Effect of cloud-seeding on water-stress of maize", co-authored with my thesis supervisor, Dr J.C.S. Allison. [This same paper was also printed (almost simultaneously) in the Journal of Weather Modification (USA), 14, pages 23-34].


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