Transpiration, evapotranspiration and evaporative fraction (EF) were measured in a temperate zone wetland dominated by Phalaris arundinacea L. under extremely high temperature and intense irradiation conditions. The infrared (IR) camera recorded no temperature extremes within the stand, indicating that the stand was capable of regulating the overheating by transpiration. The transpiration rate of individual leaves (measured by the gas-exchange method) was positively correlated with the water vapour pressure deficit of the air (R2 = 0·56, df = 1, 60, p < 0·001) reaching values of up to 7·8 mmol m−2 s−1 of leaf area. Evapotranspiration of the stand reached values of 5·3 and 5·9 mm day−1 on the 27 and 28 July 2005, respectively. The EF showed that more than one half of the available energy was consumed in the evapotranspiration processes. In the afternoon, the influence of hot air advection was obvious. A strong cooling effect of the wetland was shown. The results support the idea that wetlands sufficiently supplied with water are important in the energy and water budget of drained cultural landscapes. Copyright © 2010 John Wiley & Sons, Ltd.