Seasonal and diurnal variations in moisture, heat, and CO2 fluxes over grassland in the tropical monsoon region of southern China

Abstract

In order to examine energy partitioning and CO2 exchange over grassland in the tropical monsoon environment of southern China, fluxes of moisture, heat, and CO2 in the near‐surface layer during the period from May 2004 to July 2005 were calculated using the eddy covariance method. The study site was homogenous and approximately 400 × 300 m in size. Comparison of meteorological parameters measured at this site with historical data collected at a nearby routine observatory since 1960 shows that the measurement period is representative of the climate for this area. Seasonal and diurnal variations in radiation components, energy components, and CO2 fluxes are examined. Results show that all four radiation components changed seasonally, resulting in a seasonal variation in net radiation. The radiation components also changed diurnally, except that the downward longwave radiation showed almost no diurnal variation during the period from December to April. Summer surface albedo was higher than winter surface albedo because during winter the fraction of bare soil increases and the albedo of soil is lower than that of grass in this area. The seasonal variations in both latent heat and CO2 fluxes were stronger than those of sensible heat and soil heat fluxes. This implies that both latent heat and CO2 fluxes may be more significant climate signals than sensible heat and soil fluxes. Latent (sensible) heat flux was the main consumer of available energy in summer (winter). The energy imbalance problem was encountered and the causes are analyzed.