Measurement and modelling of evapotranspiration over a reed (Phragmites australis) marsh in Northeast China

Evapotranspiration (ET), linking water, energy and carbon cycles, is frequently a major component of the water balance for terrestrial ecosystems. The accurate estimation of water loss by ET is very important for assessing water availability and requirements of terrestrial ecosystems, and making proper water resources plans, especially for wetland ecosystem management. In this study, eddy covariance technique was used to estimate the magnitude and dynamics of ET over a reed marsh in the Liaohe Delta, Northeast China. Diurnal variations of latent heat flux (LE) at different phenological stages showed similar patterns, with small nocturnal values (<30 W m−2) and peak values occurring around noon. Seasonal variations in ET had a similar trend as the seasonal patterns of leaf area index (LAI), net radiation (Rn) and air temperature (T). During the growing season (May–October) in 2005, daily ET rate ranged from 0.1 mm day−1 to 5.8 mm day−1, and total ET was 376 mm. The cumulative ET outside the growing season was 56 mm, which contributed about 13% of the annual ET in 2005. The crop coefficient, kc, for the reed marsh ranged from 0.3 to 1.4 during the growing season in 2005, with a mean value of 0.71. The regression analysis between kc and environmental variables indicated that air temperature (T), relative humidity (RH), and net radiation (Rn) were the most explanatory variables for the day-to-day variation of kc. Based on this results, a daily ET model in this reed marsh was developed from the FAO 56 approach, in which daily values of kc was a function of net radiation, air temperature and relative humidity. The model was validated using data from 2006, in which it was found that the model agreed well with the observed data, r2 = 0.79 and linear slope = 0.95.