Abstract
Rain-yield findings were integrated with the average incidence of rain days and areas distribution of rain in a potential rain-modification area in Illinois to simulate regional aspects of a cloud-seeding project over a 13 000 km2 area. Potential seeding opportunities are limited because clouds cannot be effectively seeded at night, and 46% of all rain events occur at night. Further, 32% of all remaining rain events occur with severe weather warnings when Illinois law does not allow seeding. Hence, the number of candidate rain periods for modification is drastically reduced from a regional average of 31 days to only 11 days. Yield increases from the best treatment, based on all years’ performance (25% increases in rain on all days with moderate rainfalls, 2.5 mm–2.53 cm) are further reduced regionally because on 52% of the moderate rain events, 50% of the simulated project area receives less than the minimum moderate rain level, 2.5 mm, and thus has no appreciable yield gains. These various factors combine to reduce yield gains from 20% to 43% of the yield responses found in the 1987–91 field trials. The effects of the resulting crop-yield changes over the simulated project area, as calculated for varying rain-modification capabilities applied over a series of years, ranged from an average annual increase of $3.4 million to an average decrease of $2.6 million per year. The estimated annual cost of a quality cloud- seeding project over the area is $1 million; hence, regional benefits would be marginal, ±3% of the total farm income. They could be much larger if summer rainfall forecasts were sufficiently accurate to allow selection of the rain treatment best suited to the actual summer conditions, including no seeding in those summers like 1989 when natural rainfall met crop water needs. If one had advance knowledge that an Illinois summer was to be extremely hot and dry like that in 1988, could have a well-organized seeding project ready on 1 June, and had a technology that could produce a 40% increase in all summer rains, the estimated crop-yield benefit in the simulated project area would have been $25 million.
The 1987–91 field trials sampled only 30% of the growing conditions that occur in Illinois, and thus, the rain-modification results are only estimates of the possible outcomes from added rainfall. Nevertheless, they reveal clear needs for research relating to weather modification in the humid climate of the Corn Bell. First, more field trials are needed to define crop yield-rain relations in other types of growing seasons. Second, methods for seeding clouds at night must be developed if agriculturally useful increases are to occur. Since the value of choosing to modify rain in a given summer depends on the availability of an accurate forecast of summer rainfall, increased attention should be given to seasonal forecasting research.
