The ratio of transpiration versus evaporation in a reed belt as influenced by weather conditions

Transpiration and various components of evaporation from a reed (Phragmites australis (Cav.) Trin. ex Steud.) belt in a north German lake were estimated from meteorological routine data by use of a two-layer evaporation model of the Shuttleworth-Wallace-type. The parameterization of the model was based on intensive measurements campaigns regarding microclimate and leaf conductance in the reed stand. The transpiration (Tr) of the reed canopy was very strong and sometimes exceeded 10 mm day(-1). The rates corresponded to local advection or downward fluxes of sensible heat in the reed belt. The annual evapotranspiration (ET) from the reed belt generally exceeded the annual evaporation from the open lake surface (E-L) Depending on meteorological conditions, ET ranged between 824 and 1324 mm a(-1) and the ratio of ET versus E-L ranged between 1.5 and 2. In 3 of 4 years of investigation, ET also exceeded annual gross precipitation, which emphasizes the general significance of reed transpiration for the water balance of lakes and wetlands. However, it was concluded that no fixed factors should be used to estimate the water consumption of reed from open water evaporation, as Tr was much more sensitive than E-L to weather conditions. (C) 1999 Elsevier Science B.V. All rights reserved.