Evapotranspiration from a freshwater marsh in the Sanjiang Plain, Northeast China

Water loss by evapotranspiration (ET) is a principle component of the hydrological cycle in wetlands. Using eddy covariance method, we measured ET from a Carex lasiocarpa dominated marsh in the Sanjiang Plain, northeast China, during the growing season (May to September) of 2005. The seasonal variation of daily ET rates and the main environmental factors influencing ET were analyzed. Penman, Penman–Monteith (PM) and Priestley–Taylor (PT) models were then used for simulating daily ET and a comparison between the estimates of the three models and the measured ET was done. The results showed that daily ET rates ranged from 0.58–4.80 mm d−1 with an average of 2.31 mm d−1 during the measurement period. The values of decoupling factor (Ω) were generally larger than 0.5 during the growing season with an average of 0.82, demonstrating that net radiation was the main factor affecting ET while the influence of vapor pressure deficit (VPD) and surface resistance on marsh ET were relatively small. ET corresponded to leaf area index (LAI) in a linear manner when LAI was less than 1. The Penman model overpredicted the mean measured ET for the growing season by 35% while the PM model underestimated the mean measured ET by 18%. The agreement between estimated ET and measured ET was best for the PT model with α = 1.01, among the three models used in this research. Given its simplicity and accuracy, the PT method was most applicable to model daily ET during the growing season at the study site. Additionally, relation between estimates by PT method and measured ET can provide prediction of ET in other, environmentally similar, freshwater marsh for which meteorological data can be obtained or estimated.