陆地生态系统水和碳循环过程的研究是气候变化研究中的热点问题。地表实际蒸散是目前地表水收支（循环）中的重要参量，也是最不确定（或者误差最大）的项。生态系统水和碳循环是相互耦合的过程，水分收支状况的改变将会改变生态系统的碳源/碳汇功能。半干旱区不同生态系统地表蒸散和生态系统净碳交换量在不同时间尺度上的主要控制因子不尽相同，具有较大的年际变化。到目前为止，我国半干旱地区地表实际蒸散和生态系统净碳交换量的季节和年际变化特征的研究开展得尚不多。本项目拟利用已经得到的半干旱区退化草地、旱作农田下垫面、不同放牧强度草原下垫面的地气交换过程（涡动相关法测得的湍流通量）及辐射通量等资料，开展复杂地形条件下湍流观测、缺失资料可靠填补方法以及夜间二氧化碳通量修正方法的研究；结合有关生态和土壤特性的补充观测，分析半干旱地区实际蒸散和生态系统净碳交换量的季节及年际变化特征及其不同时间尺度下的控制因子。

The response of the ecosystem water and carbon cycles to climate change is the key scientific questions in the global change research. Evapotranspiration (ET) is a key parameter in the water cycle but still very uncertain until now. Beacause water and carbon cycles are coupled with each other, changes in water balance will produce change in carbon cycle determining whether ecosystem absor or release CO2 to the atmosphere. Due to the climate change, ET and net ecosystem carbon exchange (NEE) over different surfaces in the semi-arid areas have large inter-annual variations, and their driving factors are of interest. However, long-term studies of the seasonal and inter-annual variation in ET and NEE in the semi-aird areas of China are rare up to now. Based on the eddy covariance measurements of different grazing grassland and cropland ecosystems, this project mainly focuses on: 1) eddy covariance measurement over complex terrains; 2) gap-filling methods for long-term energy and CO2 flux datasets; 3) corrections of CO2 flux in the night-time; 4) characteristics of seasonal and inter-annual variations in ET and NEE; 5) the driving factors of ET and NEE on different time scales.