Cloud-seeding Effect on Water Stress of Maize in Zimbabwe

Abstract

Results of experimental cloud-seeding in Zimbabwe indicate that silver iodide treatment enhances rainfall if cloud-tops are colder than -IO°C. Still higher increases result if tops are colder than -13°C. By comparing distribution functions fitted to seeded and to non-seeded clouds, tentative relationships were built up between natural and seeded rainfalls. A cloud-seeding season in north-east Zimbabwe was stimulated by a model in which i 000 showers were distributed at random within a rectangular area, enabling total extra rainfall to be computed at every km grid-point. Less than half the area received any additional rainfall. A soil-water balance rountine was then used to relate the apparently increased rainfall to changes in maize yield. In approximately half of all rainy seasons in northern Zimbabwe natural rainfall appears to be adequate so that cloud-seeding is of negligible benefit to maize. In poor or modest rainy seasons most places probably receive less than i0 mm from a cloud-seeding operation, but this can eliminate one, occasionally two, post-flowering stress-days during maize growth. However, planting dates can hardly ever be brought forward as a result of artificial rain stimulation. Results suggest that cloud-seeding operations more than pay for themselves.