Long-term variations in energy partitioning and evapotranspiration in a semiarid grassland in the Loess Plateau of China

Estimating seasonal and interannual variations in energy partitioning and actual evapotranspiration (ET) distributions in semiarid grasslands is essential for improving the understanding of interactions between the land and atmosphere, and is useful for land surface parameterization in grasslands of the Loess Plateau. In this study, energy fluxes and ET values over a semiarid grassland site and their environmental and biological controls were investigated over multiple years (2007-2012) using the eddy covariance method. The seasonal and interannual variations in the partitioning of the net radiation (Rn) into sensible (H) and latent (LE) heat fluxes were primarily controlled by precipitation through changes in the soil water content (SWC) and vegetation growth. During relatively wet and warm periods, LE was the dominant component of the energy balance, whereas H was the dominant component of energy partitioning in dry periods. Annual ET was higher than annual P (except in 2007), with a better closure of surface energy balance during the drier years. The total annual ET and its daily maximum varied from 278.2 to 460.3mm and from 3.45 to 4.95 mm day(-1), respectively. The daily Priestley-Taylor coefficient (ET/ETeq, where ETeq is the equilibrium evaporation) decreased sharply when the SWC in the 5-cm soil layer decreased below 0.15 m(3) m(-3) during the growing seasons. The average ET for sixteen days was linearly correlated with the normalized difference vegetation index (NDVI). ET/ETeq increased nonlinearly with an increase in Gs but was insensitive to the increases in Gs greater than 6.52 mm s(-1) in the driest year (2011) and 9.17 mm s(-1) in the wettest year (2007). Additionally, there is good correlation between the threshold of Gs and the annual mean NDVI. On an annual scale, the low Gs (3.07-6.77 mm s(-1)), ET/ETeq (0.28-0.42), and higher Bowen ratio (beta) (2.07-4.19) collectively represented the overall water-limited conditions for ET in the semiarid grassland of the Loess Plateau. Our data indicate that ET and its biophysical controls vary in response to interannual changes in annual P. These results contribute to an understanding of the driving mechanisms for long-term variations in energy partitioning and biophysical controls on ET in the Loess Plateau of China.