Energy exchange of an alpine grassland on the eastern Qinghai-Tibetan Plateau

The seasonal variability in the surface energy exchange of an alpine grassland on the eastern Qinghai-Tibetan Plateau was investigated using eddy covariance measurements. Based on the change of air temperature and the seasonal distribution of precipitation, a winter season and wet season were identified, which were separated by transitional periods. The annual mean net radiation (Rn) was about 39% of the annual mean solar radiation (Rs). Rn was relatively low during the winter season (21% of Rs) compared with the wet season (54% of Rs), which can be explained by the difference in surface albedo and moisture condition between the two seasons. Annually, the main consumer of net radiation was latent heat flux (LE). During the winter season, sensible heat flux (H) was dominant because of the frozen soil condition and lack of precipitation. During the wet season, LE expended 66% of Rn due to relatively high temperature and sufficient rainfall coupled with vegetation growth. Leaf area index (LAI) had important influence on energy partitioning during wet season. The high LAI due to high soil water content (θv) contributed to high surface conductance (gc) and LE, and thus low Bowen ratio (β). LE was strongly controlled by Rn from June to August when gc and θv were high. During the transitional periods, H and LE were nearly equally partitioned in the energy balance. The results also suggested that the freeze-thaw condition of soil and the seasonal distribution of precipitation had important impacts on the energy exchange in this alpine grassland.