By Richard Black
BBC News website environment correspondent
African farmers who use weather forecasts based on El Nino data are likely to see an increase in crop yields, according to a new study.
Accurate rainfall forecasts allow farmers to plant high-yielding varieties of maize
Researchers ran workshops in four Zimbabwean villages to inform farmers of the forecasts.
Those that used the information to choose when and what to plant saw greater yields of crops such as maize.
Details are released in the US journal Proceedings of the National Academy of Sciences.
At the beginning of their study in September 2000, the researchers, from Boston University in the US and the University of Zimbabwe in Harare, found a fair degree of scepticism among farmers about the value of weather forecasts.
During a series of workshops, they attempted to stimulate awareness of the potential benefits of using forecasts, and discussion on how farming practices could be changed to cope with abnormally wet or dry seasons.
Coming into the 2002/2003 season, the scientists predicted a lower than average rainfall because of a mild El Nino event; and broadly, this proved to be correct.
The following season, the prediction was for a return to normal, which again turned out to be largely accurate.
Just over half of the farmers - 57% - reported that they changed decisions on when and what to plant because of the forecasts, either planting at different times or choosing different varieties.
Reaping the rewards
Averaged over the two seasons, farmers who based decisions on the forecasts had a yield 9.4% higher than those who did not.
"These farmers are fairly risk-averse," Professor Anthony Patt from Boston University told the BBC News website, "and their baseline strategy is to prepare for drought, to plant quite drought-tolerant varieties and so on.
"So if they know a drought is coming, there's not a lot extra they can do.
"But in the second year they were able to take a bigger risk and plant higher-yielding varieties, and this is where they saw the benefits."
El Nino events occur roughly once in every seven years, although the pattern is far from rigid, with the mild event of 2002/3 coming just five years after a much more severe occurrence in 1997/8.
The driving force appears to be the development of unusually warm water in the eastern Pacific, which distorts the familiar pattern of ocean currents, winds and rainfall throughout the tropics, and sometimes further afield.
Through increased use of satellites and ground-based equipment, scientists are able to give notice of an El Nino season months in advance, although forecasts are imperfect.
A decade ago, researchers showed that changes in eastern Pacific sea temperatures account for 60% of the variation between annual yields of maize in Zimbabwe.
Previous studies in other parts of the world have shown that distributing drought-tolerant seeds in advance of an El Nino event can reduce its impact on yields; but the new study from Zimbabwe may be the first to show that subsistence farmers can benefit simply by using the information - so long as they believe it.
"I think one of the clear results coming out of this study is that taking the time to talk with people on a very local basis makes a huge difference," Anthony Patt told the BBC News website, "and simply broadcasting weather information isn't anywhere near so effective.
"Going out to them is resource-heavy; and I guess if you're going to commit these resources you're going to want to know that it's worthwhile, and I think that is what this paper is demonstrating."