QUANTIFYING EVAPOTRANSPIRATION AND BIOPHYSICAL REGULATIONS OF A POPLAR PLANTATION ASSESSED BY EDDY COVARIANCE AND SAP-FLOW METHODS

Aims Using field data from an eddy-covariance (EC) flux tower and sap-flow sensors installed in a poplar (Populus euramericana) plantation, we investigated the magnitudes and changes of evapotranspiration (ET) under different soil moisture and climatic conditions. Our objectives were to quantify the energy partitioning and energy balance, explore the dynamic process and regulatory mechanisms on ET, and understand primary biophysical regulations, especially soil moisture. Methods An open path EC system, sap-flow sensors, soil water balance monitoring system, and microclimatic station were installed to record various components of energy fluxes and water budget at an 11-year-old poplar plantation in Daxing District, Beijing, China. We used data collected at 30-min intervals in the growing season of 2006 in this study. Important findings The overall energy closure of the study site was high (86%) during the growing season, but with notable dependence on soil water conditions. The ratio between sensible heat and net radiation (Hs:Rn) was much higher during dry conditions than that during moist conditions. With dry soils, net radiation and soil physical properties played important roles in transpiration, which was less than evaporation prior to rain events. In contrast, transpiration exceeded evaporation when the soil water content in the deep layers was more abundant following rain events. The total ET rates quantified by the soil water balance and sap-flow methods were comparable and lower than that determined by the EC method. The ratio between transpiration and ET appeared to be more dependent on net radiation and vapor pressure deficit (VPD) during wet periods than that during dry periods.