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Green roofs have been shown to reduce the stormwater runoff from buildings. In order to model the reduced stormwater effect of green roofs, reliable estimates of ET from the CAM plants is required. Greenhouse controlled environmental conditions were used to measure and predict ET from CAM plants during winter and fall/spring regimes, along with climatic parameters including air temperature, wind speed, radiation and humidity during 21-day periods without rain. The planted boxes evapotranspired 28% and 57% more water to the atmosphere than was evaporated from the unplanted boxes for the winter and spring/fall, respectively. ET for a green roof with CAM plants averaged 0.61 mm/d and 1.12 mm/d for each dry day during the winter and spring/fall season, respectively. The original Penman equation and Penman-Monteith ET models explained the observed ET rates very well with crop coefficients of 0.74 in winter and 1.97 in fall/spring.