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Reservoirs Impact Precipitation Intensity

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Reservoirs in arid regions, such as this one near Santiago, Chile, have a profound influence on local rainfall intensity, according to a new study. Wikimedia Commons/ Mauro_grade

Researchers working in Chile discover that the intensity of rainfall increases dramatically in dry areas close to large reservoirs.

February 12, 2013—Researchers hoping to gather information about design storms and return periods to aid the planning of storm-water management in Chile recently made a discovery that surprised them and led their research in a different direction: they found that the presence of large reservoirs in dry areas of the country is correlated with higher intensity rainfall in that area.

“We didn’t expect that, but we noticed that pattern and we wondered why this is,” says Pablo Garcia-Chevesich, Ph.D., one of the authors of the resulting research entitled “Inland Water Bodies in Chile Can Locally Increase Rainfall Intensity,” to be published in the February 25 issue of Elsevier’s Journal of Hydrology. “We did a more serious analysis that separated the stations located near the reservoirs from those away from them and we saw stunning results.”

The research was funded by a grant from the government of Chile. The research team was led by Roberto Pizarro, Ph.D., a professor in the forest sciences department at the University of Talca in Chile. Garcia-Chevesich is a researcher associated with the Department of Hydrology and Water Resources at the University of Arizona, and the Forest Institute, Chile.

The team placed 50 automatic rain gauges at various sites in Chile to collect data to develop intensity-duration-frequency curves to provide guidance in the design of storm-water collection systems. Pinpointing rainfall characteristics in Chile is difficult. Chile is a long, narrow country on the west coast of South America. It spans 2,880 mi north to south, with climates ranging from hyperarid to humid subtropical. The world’s driest dessert is in Chile, where virtually no rain falls in an average year. At the other end of the spectrum, extreme southern regions record 4,500 mm of perception per year.

“Chile is a good country [to study]—not only because I’m from there—but also we have too many different climates,” Garcia-Chevesich says. “Some parts of the north only get rain during an El Nino. And in the south it rains every day. You can study different climates in the same country. It’s much easier.”

The team’s research found that in hyperarid zones, the proximity of a reservoir created a sharp increase in rainfall intensity measured as mm/h. Intensity at hyperarid sites less that 5 km from a reservoir were measured to be more than 25 percent higher than at sites removed from reservoirs. The rainfall intensity near the reservoir was nearly double the intensity about 40 km away. Although the effect of reservoirs were more pronounced in hyperarid zones, greater intensity was observed in arid, semiarid, and Mediterranean zones, too.

Seventeen of the 50 rain gauges were within 5 km of a reservoir. No reservoir had more than one rain gauge. The team hopes to conduct further research soon, and will apply for research grants this spring.

“The next step is to do a more detailed analysis,” Garcia-Chevesich says, “and also include more variables. Incorporate microclimatic conditions, the topography, how big the reservoir is, how warm the water is, and things like that so that we can understand more of the effect of reservoirs on local climates.”

“Obviously, a small reservoir won’t have any effect, but the bigger the reservoir and the dryer the climate, the stronger the effect is,” Garcia-Chevesich says. “This is just for rainfall intensity, and not really the total precipitation. The rainfall intensity, in terms of storm-water management, is the most important variable.”

Garcia-Chevesich says the team hopes that further research will develop a better understanding of the effect so engineers can calculate the impact of rainfall intensity near reservoirs and use that when designing storm-water collection systems near newly constructed reservoirs.

“If we find out that there is unexpected flooding in cities close to a reservoir and we can correlate it to this, it would be necessary to redesign storm-water facilities so they can adapt to the new intensity,” Garcia-Chevesich says.

“If you are going to build a reservoir and it’s going to be a large reservoir, and you are in a dry climate, you would expect storm-water engineering [for a much] stronger intensity,” Garcia-Chevesich says. “It shouldn’t be based just on the history of precipitation and rainfall intensity. They should incorporate the reservoir into the design.”



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