Evaporation characteristics of wetlands: experience from a wet grassland and a reedbed using eddy correlation measurements

Measurements of evaporation were made from July to November 1999 using the eddy correlation method on two wetland types – wet grassland and reedbeds – in south west England. The evaporative water use of a reed bed exceeded that of the grassland wetland by 15% (or 50mm over the 5 months). The evaporation rates at both sites exceed of the Penman Potential Evaporation estimates calculated for this area. The difference between sites results from the higher roughness length of the reed bed and the lower effective surface resistance of the reed/open water assemblage. At the grassland site, a significant relationship between the surface resistance and water table level has been demonstrated. The water table at this site is managed to maintain the plant diversity and allow some agricultural access. This regime specifies a water table below the surface during the summer period, which results in higher surface resistances and lower evaporation. The results have important implications for local water resources management, especially where wetlands are maintained by pumping from rivers or groundwater.