For three years, Kyle Hemes of UC Berkeley and colleagues kept tabs on the heat flux and air flow above three restored Delta wetlands on Twitchell and Sherman islands, and an alfalfa field on Twitchell Island. Surface temperatures at wetlands with open water were up to 5.1 degrees Celsius cooler than the crop field during the daytime. As expected, the dark open water absorbed more solar radiation, and released the energy slowly at night. But wetland vegetation played a role as well. The tall, uneven surfaces of tule and cattail stands, and their patchy distribution, hastened the movement of heat away from the land surface. “Whereas we’re often focused on the greenhouse gas reduction benefits of restoration, the significant cooling effects associated with wetland restoration provide local climate benefits,” Hemes says. The next step, he adds, is to understand how widespread wetland restoration could have air cooling effects across an entire region. The study also suggests engineers should design wetlands that will retain patches of open water even as they age to maximize the cooling effect. While flooding wetlands can increase water loss due to evaporation, this cooling effect joins habitat creation, reversal of soil subsidence, reduction of stresses on levees, and myriad other benefits of expanding managed wetlands in the Delta. KW

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Photo: Kyle S. Hemes
 

Restoring wetlands is an extremely effective way to cool land surfaces, a study conducted in the Sacramento-San Joaquin Delta indicates. For three years, Kyle Hemes of UC Berkeley and colleagues kept tabs on the heat flux and air flow above three restored Delta wetlands on Twitchell and Sherman islands, and an alfalfa field on Twitchell Island. Surface temperatures at wetlands with open water were up to 5.1 degrees Celsius cooler than the crop field during the daytime. As expected, the dark open water absorbed more solar radiation, and released the energy slowly at night. But wetland vegetation played a role as well. The tall, uneven surfaces of tule and cattail stands, and their patchy distribution, hastened the movement of heat away from the land surface. “Whereas we’re often focused on the greenhouse gas reduction benefits of restoration, the significant cooling effects associated with wetland restoration provide local climate benefits,” Hemes says. The next step, he adds, is to understand how widespread wetland restoration could have air cooling effects across an entire region. The study also suggests engineers should design wetlands that will retain patches of open water even as they age to maximize the cooling effect. While flooding wetlands can increase water loss due to evaporation, this cooling effect joins habitat creation, reversal of soil subsidence, reduction of stresses on levees, and myriad other benefits of expanding managed wetlands in the Delta. KW

About the author

Bay Area native Kathleen M. Wong is a science writer specializing in the natural history and environment of California and the West. With Ariel Rubissow Okamoto, she coauthored Natural History of San Francisco Bay (UC Press, 2011), for which she shared the 2013 Harold Gilliam Award for Excellence in Environmental Reporting. She reports on native species, climate change, and environmental conditions for Estuary, and is the science writer of the University of California Natural Reserve System.

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