Seasonal and interannual variation of energy balance and partition over a rain-fed cropland in the semi-arid area of Loess Plateau, northwestern China

In this study, two years' micrometeorological and energy flux observations from the Pingliang Station of Lightning and Hail Storm Research, CAS were used to investigate the seasonal and interannual variations of radiation, albedo and energy fluxes over a rain-fed cropland in the semi-arid area of Loess Plateau, with an emphasis on the influence of rain, soil moisture and agricultural production activities on the energy partitioning as well as the surface energy balance. The results revealed large annual variations in the seasonal distribution of precipitation, which along with soil freezing and thawing gave rise to significant variations in soil moisture. Radiation components had general seasonal cycles with less annual variability. Shortwave radiation fluctuated obviously with weather conditions, while longwave radiation was less affected. Surface albedo had significant seasonal and interannual variations: the maximum midday mean value (0.83) occurred after snow, then it dropped to less than 0.2 during crop growing season, after crop harvest, it fluctuated with precipitation. There was a negatively linear relationship between the albedo and the soil moisture. Large interannual differences in seasonal variations of sensible and latent heat fluxes were observed due to the interannual variations in precipitation and agricultural activities. The main consumer of available energy varied among months and years, with the monthly mean diurnal cycle of latent (sensible) heat flux peaking in summer (spring) at 251.9 (192.5) W m-2. On an annual scale, the largest consumer of midday net radiation was sensible (latent) heat flux in 2010-2011 (2011-2012), which accounted for about 35% (40%) of the net radiation. The annual surface energy budget closures were 77.6% and 73.3%, respectively. The closure was the lowest in winter and the highest in spring, which was affected by atmospheric stability and the heat storage by vegetation canopy and near surface air layer.