Observational Analysis Of Land-atmosphere Interactions In A Semi-arid Region Of China Under Contrasting Climatic Backgrounds

A dataset including turbulent fluxes and micrometeorological measurements taken in a semi-arid region of Tongyu during the period of Jan.2007to Dec.2008is analyzed. The investigation focuses on major differences of water and energy exchanges between land surface and atmosphere under contrasting climate and different land use/cover types. The results indicated that exchanges of heat and water between land surface and atmosphere changed significantly under different climate backgrounds. On the whole, these variables have direct links with precipitation, which indicates precipitation be the dominant driver in controlling land-atmosphere exchanges in the semi-arid region. The precipitation amount of2008was106%higher than that of2007, for the cropland, this increased the annual average temperature, water vapor deficit and surface albedo in2008compared with those of in2007for the farmland. Meanwhile, the wetter climate background of2008resulted larger stomatal conductance of vegetations and stronger evapotranspiration and thus the mean canopy conductance of2008was17%higher than that of2007. More available soil water content and stronger evapotranspiration of the vegetation resulted the annual average latent heat flux of the farmland in2008was36%higher than that of in2007, while the sensible heat flux was11%lower. Because the Tongyu observational site is located in the semi-arid area, most of the Priestley-Taylor coefficient was less than1.0, and the annual average Priestley-Taylor coefficient of2008was12%higher than that of2007; meanwhile, the annual average decoupling coefficient of2007was lower than that of2008, which implied the latent heat flux of2007coupled more with ambient water deficit, but coupled less with net solar radiation.Though the temperature and water vapor deficit of the degraded grassland had similar changing trend compared with that of the cropland, because of the different precipitation regime during2007-2008and the severe soil erosion of the degraded grassland resulted small annual average water content in2008than that of in2007. This led to the annual mean surface albedo of2008was12%higher than that of2007. Moreover, less available soil water content of2008resulted in stomatal conductance closure and weak photosynthesis, which made the summer mean canopy surface conductance of2008was smaller than that of2007(6.56and7.86mm s-1respectively). On that background, the pattern of surface energy partitioning of the degraded grassland differed significantly from that of the farmland. Take the summer season for example, it was latent hear flux-dominated in the summer of2008but sensible heat flux-dominated in the summer of2007. Correspondingly, the annual mean Priestley-Taylor and decoupling coefficients of2008were28%and30%lower than those of2007respectively.In the next section, this paper focused on the influences of different land use patterns on surface water budget. The results showed that though the two sites were only5km away from each other, the annual total evapotranspiration of the cropland ecosystem was higher than that of the degraded grassland ecosystem by8.8%, for79.3mm (cropland) and51.1mm(degraded grassland) respectively. This indicated that in the semi-arid area, the cropland ecosystem which is involved with more human activities is confronted with more severe water supply pressure, and the land use/cover changes introduced by human activities in the semi-arid area would aggravate the aridity trend.