基于涡度相关观测数据的陆地生态系统蒸散年际变异及区域差异研究

Evapotranspiration (ET) is the key process of terrestrial hydrological cycle. Studying the interannual variability (IAV) of ecosystem ET and its controlling mechanisms will help us comprehensively understand the terrestrial hydrological processes, and also enable effective knowledge on how ecosystems respond to global climate change..ET has long been considered to be the most difficult process to be measured because of its invisibility and variability, which hence makes various ET models to be proposed. However, no ET models could elegantly reproduce IAV of ET yet, although most of the ET models could quite well simulate the short-term (e.g., diel and seasonal) dynamics in ET. Also, previous studies indicated that there are huge uncertainties in both magnitudes and sign among different ET IAV simulations for a give region. With the development of eddy covariance (EC) method, long-term observations of water flux data obtained from flux towers have been used to study the IAV in ET and its main influencing factors for specific ecosystems worldwide. To our knowledge, however, most of these researches only considered the effects of limited climatic factors, but less taking vegetation attributes or other biotic factors into account, which makes the ecophysiological mechanism of ET IAV not clearly explained yet..In view of the uncertainties in the ET IAV modeling and the lack of knowledge on IAV dynamics of ET, we should address some important scientific issues in future: 1) how the IAV of vegetation transpiration (T) and soil evaporation (E) result in the IAV of ET; 2) what are the relative contributions of climate effects and ecosystem responses to IAV of ET and its components (i.e., E and T); and 3) how the IAV of ET differs in trends and magnitudes for different regions/ecosystems..The accumulation of long-term eddy-covariance ET observations provides data resources and opportunity for the researches on the ecophysiological mechanisms of ET IAV. In this program, we will synthesize the long-term (longer than 5 years) eddy-covariance ET data from ChinaFLUX sites and published data from other sites in China and worldwide (60 to 80 sites), and also collect the corresponding long-term monitoring data of climatic, vegetation and soil attributes. On basis of multi-source data, we would comprehensively analyze the IAV of ET in terrestrial ecosystems and explore its ecophysiological mechanisms from the view of E and T processes, climate effects and ecosystem responses, and regional differences.

蒸散（ET）是陆地水循环的关键过程，深入研究陆地生态系统ET的年际变异规律及其控制机制有助于全面认识陆地水循环过程，促进生态系统对气候变化的响应机制的理解。鉴于目前对ET年际变异与区域差异认知的不足，未来需要重点关注如下几方面：（1）植被蒸腾和土壤蒸发的年际变异如何决定ET的年际动态；（2）气候效应和生态系统响应对ET及其不同组分年际变异的相对贡献如何；（3）ET的年际变异趋势和幅度的区域差异及其主导因素。长期联网观测数据的积累，为开展生态系统ET年际变异的生理生态学机制研究提供了数据保障和历史机遇。本项目将以ChinaFLUX的长期涡度相关观测数据为基础，整编全球不同气候区通量观测站的长期观测数据（60至80个站点），结合气候、植被、土壤属性的长期监测数据，从蒸腾和蒸发、气候效应和生态系统响应、区域差异的角度综合分析陆地生态系统ET的年际变化特征，揭示ET年际变异的生理生态学机制。

蒸散（ET）是陆地水循环的关键过程，深入研究陆地生态系统ET的年际变异规律及区域差异有助于全面认识陆地水循环过程。本研究以中国通量观测网ChinaFLUX观测台站的长期涡度相关通量观测数据为基础，构建了基于涡度相关法的中国典型陆地生态系统实际蒸散量数据集，在此基础上评估了中国区域不同气候区典型生态系统ET的年际变异规律及区域差异，分析了气候条件和生物因素对不同生态系统ET年际变异的影响，并利用改进的Shuttleworth-Wallace（S-W）双源蒸散模型对ET进行拆分，解析了植被蒸腾和土壤蒸发过程对不同气候区生态系统ET年际变异的贡献。主要结果如下：(1)中国区域不同气候区八个典型生态系统的ET表现出不同的年际变化趋势。森林生态系统中亚热带人工针叶林的年ET略有升高，温带针阔混交林和亚热带常绿阔叶林的年ET表现为降低趋势；草地生态系统中半干旱温带草原生态系统的年ET呈现升高趋势，而三个高寒草地生态系统的年ET均表现出降低趋势；暖温带半湿润旱作农田生态系统ET呈现为显著升高趋势。(2)各典型生态系统ET的年际变异主要受降水、辐射等气候因素和叶面积指数等生物因素的影响，整体来说，草地生态系统ET的年际变异大于森林生态系统。(3)温带针阔混交林、亚热带常绿阔叶林和暖温带半湿润旱作农田生态系统ET的年际变异主要受能量供给条件控制，而草地生态系统和亚热带人工针叶林生态系统的ET年际变异主要受水分条件限制。(4)模拟结果表明，草地生态系统土壤蒸发占生态系统总蒸散量的比重（E/ET）较大（53.5%~75.6%），森林生态系统E/ET比值较低（14.4%~38.5%）。不同生态系统E/ET的年际变异及区域分布主要受叶面积指数控制。(5)除亚热带常绿阔叶林和暖温带半湿润旱作农田生态系统的ET年际变异主要受植被蒸腾的年际变异控制之外，温带针阔混交林、亚热带人工针叶林生态系统和草地生态系统的ET年际变异均主要受土壤蒸发的年际变异控制。研究结果可以为准确评估我国陆地生态系统ET的时空变异和区域水资源管理提供科学依据。