Environmental and biophysical effects of evapotranspiration in semiarid grassland and maize cropland ecosystems over the summer monsoon transition zone of China

Accurate quantification of semiarid energy partitioning and actual evapotranspiration (ETc) is necessary to un-derstand the variability in regional energy and the water cycle, which is expected to intensify under climate change. The seasonal and interannual variability in surface energy and ET and its responses to environmental and biophysical factors of grassland and cropland with maize (Zea mays), and ecosystems over the East Asian summer monsoon transition zone of China, were investigated using multiyear (2003-2008) eddy covariance measure-ments. The study sites received 65-84% of the annual precipitation (Pr) during the East Asian summer monsoon (EASM; June-August), with the lowest values in the weak EASM years of 2004 and 2007. Seasonal and inter-annual variations in the portioning of net radiation to turbulent fluxes were mainly controlled by Pr via changes in soil water content (SWC) and vegetation growth. Drastic changes in the Bowen ratio, vegetation growth, and energy fluxes occurred after the onset of EASM. During the cool winter and dry spring and autumn, the sensible heat flux was the largest component of the energy balance, and the latent heat flux dominated during warm and wet periods of summer. Monthly ET was 24.8 and 25.6 mm for the grassland and cropland, respectively, and they peaked at 89.0 mm and 113.9 mm, respectively, in July. Monthly ET was positively correlated with monthly Pr, Ta, the soil water drought stress index, normalized difference vegetation index (NDVI), the surface conductance (Gs), and the Priestley-Taylor coefficient (ET/ETeq) during the growing seasons (April-September). Annual ET varied from 215 to 373 mm in grassland and from 227 to 390 mm in cropland, with the lowest value during the severe drought year of 2007. Interannual variations in ET were primarily controlled by annual effective pre-cipitation frequency, and interannual variations in summer ET were modulated by the summer monsoon dura-tion period through changes in summer Pr, SWC, and vegetation growth.