Assessment on surface energy imbalance and energy partitioning using ground and satellite data over a semi-arid agricultural region in north China

The processes of surface energy and water exchange are significantly necessary to the forming and evolution of regional climate. The most critical issues and challenges are 1) dynamics of surface energy balance components, 2) the surface energy imbalance problem, which is still unsolved and 3) energy partitioning in an ecosystem. This study assessed the seasonal and annual variations of the surface energy fluxes, the surface energy balance closure (EBC), the reasons behind the surface energy imbalance, and energy partitioning based on the two years (2014-2015) flux observation datasets over a semi-arid agricultural region in north China. The estimation of surface fluxes was also examined using satellite data. The EBC analysis was carried out using the ordinary least square (01.5) regression model of the dependent turbulent fluxes against the independently available energy and the energy balance ratio (EBR). The results revealed that the EBC ranged from 0.63 to 0.78 at all respects with an overall EBC of 0.74, and the daily EBC was better than 30-min averages. The identified main reasons behind this imbalance were 1) the ignorance of the heat storage terms (soil and canopy), which improved the EBC by approximately 7%; 2) the vegetation and surface heterogeneity negatively affected the EBR; and 3) the occurrence of advection broke the energy balance by decreasing EBC. The energy partitioning was investigated using the residual method and two ratios (sensible heat flux/net radiation and latent heat flux/net radiation). The results showed that the sensible heat flux was dominating during dry periods and the latent heat flux was dominating during the wet periods, and the rate of dominance was controlled by the availability of water, vegetation dynamics and weather conditions.