Intensified field observation of water energy and CO fluxes over semiarid area of China

Arid and semi-arid areas comprise about 40% of China's territory. Most of them are located in Northern China. These areas are important on the land -atmosphere interaction process because of their unique energy budget and hydrological processes, increasing aridity and desertification trend (Fu et al., 2002b; Huenneke et al., 2002). In addition, as mentioned in section 7.3.3, the semi-arid areas are more sensitive to climate change and land use and the extent and distribution of arid and semi-arid areas are subject to modification because of land use and water utility(Fu et al.,2002b). Higher albedo because of deforestation and desertification has been shown to cause a reduction in precipitation and evapotranspiration (ET) regionally. Changes in climate and climate variability will likely have a significant impact on these regions.

From the 1980s, land surface process studies have been important research areas in atmospheric sciences. There are many comprehensive field experiments conducted over the world in different climate areas. Some field experiments have been carried out over the arid and semi-arid region of China in recent 30 years, for example, HEIFE (Hu, et al., 1994a, 1994b), IMGRASS (Lu, et al., 2002, 2005), NWC-ALIEX (Huang, 2004, Zhang and Huang, et al., 2005, Bao and Lu, 2006), dust aerosol observations in Dunhuang (Iwasaka and Shi, et al., 2003, 2004), surface layer tur bulent flux observations over Naiman (Zhang, et al., 2001; Liu, et al., 2002) in which a great deal of observation data and research achievements were gained.

However, eddy covariance measurements of the turbulent flux in the surface layer were conducted only in the summer during intensive observation period for one or two months in most of these experiments.

Eddy covariance methods to measure directly the turbulent fluxes between the land surface and the atmosphere have been used since the late 1950s and early 1960s. Yet, only recently has the technology been available to make continuous flux measurements. There are more than 10 years continuous flux data for some sites in Ameriflux and Euroflux (Baldochi, et al., 2001). However, continuous flux measurements with the eddy co-variance method began in China only in 2002 ( Few studies have investigated year-round eddy covariance measurements of energy fluxes in semi-arid ecosystems.

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