Abstract
Rapid growth in demand, doubling about every 12 years, led in 1961 to stringent rationing of hydroelectric power and potable water in Medellin, Colombia. After a preliminary study had shown that dry season precipitation on the city's watersheds came principally from daytime convective clouds that showered only after surpassing the freezing level, and hence might he stimulated by silver iodide, cloud seeding was undertaken in the 1962, 1963 and 1964 dry seasons.
Evaluation by the usual target-control regression procedure indicated increases of 20 to 40 per cent, nominally significant at the one per cent level. A new approach to evaluation, based on the suggestion by Thom that seeding, if it increases rainfall, should affect mainly the scale parameter of the distribution, was developed and applied to one of the targets, deriving a separate estimate of 25 per cent increase, which falls slightly short of significance at the five per cent level.
Significance is compromised by some subjectivity, though this was minimized as much as possible, and by unavoidably imperfect randomization and lack of predetermined experimental design.
The cost of operations was equivalent to the economic return from a 2 per cent increase in streamflow from the target watersheds. The indicated result overfulfilled the expectation established in advance of operations.
