Water vapor flux features of an evergreen and deciduous broadleaf mixed forest in Mount Tianmu area

Key words: 
• forest ecology /
• water vapor flux /
• eddy covariance /
• typical evergreen and deciduous broadleaf mixed forest /
• net radiation /
• Mount Tianmu

Abstract: Evapotranspiration （ET） is an important component of ecosystem water and energy balances, and subtropical evergreen and deciduous broadleaf forests play an important role in regional water cycles and budgets. To enhance prediction of climate change impact on the terrestrial ecosystem an improved understanding of the water and energy fluxes between vegetation and the atmosphere is needed. In Mount Tianmu area of Zhejiang Province, a subtropical evergreen and deciduous broadleaf mixed forest observation tower was set up in December 2012 with a three dimensional ultrasonic anemometer, a H2O/CO2 infrared analyzer, and conventional meteorological instruments for measuring a multi-layer gradient of temperature and humidity, windspeed, and radiation. Based on data observed from July 2013 to June 2014, water vapor flux features of the ecosystem were analyzed, and the net radiation responses to water vapor flux were determined. Results showed that annual water vapor flux was positive with distinct diurnal and seasonal variations. The maximum monthly water vapor flux occurred in July with a maximum daily 0.115 gm－2s－1; the lowest was in January with a maximum of only 0.029 gm－2s－1. The maximum quarterly water vapor flux occurred in summer, and the lowest was in January. The annual total ET was 721.25 mm （about 51.5% of the annual precipitation）. The annual net radiation （Rn） for this ecosystem was 3 305.65 MJm－2. Also, Rn was the main driving force of ET and was a key factor in energy balance. Thus, water vapor flux being positive meant that the ecosystem was the source of water vapor. ［Ch, 6 fig. 2 tab. 28 ref.］