The diurnal trends of sensible and latent heat fluxes of a subtropical evergreen coniferous plantation subjected to seasonal drought
Evidence of global climate change is well-documented,with long-term increases observed in average global surface temperature,the atmosphere′s carbon dioxide(CO2) concentration,precipitation,and runoff.SPAC system is the strongest active layer in surface energy and mass exchanges.The diurnal trends of sensible and latent heat fluxes are signatures of atmospheric and physiological processes that control biophysical fluxes at the surface.The diurnal trends of sensible and latent heat fluxes depends on temporally evolving interactions between atmospheric demand,the net radiation,temperature,and atmospheric vapor pressure deficit,and the ability of the vegetation to supply available water.Droucht can change the diurnal trends and the energy partitioning between sensible and latent heat fluxes.And that may affect the surface energy and water exchanges cycle.Continuous measurements of the influence on diurnal variation range and diurnal centroids for sensible and latent heat fluxes during seasonal drought were recorded at Qianyanzhou in 2003.The mean diurnal variation of sensible heat flux was 176 W/m2 and that of latent heat flux was 171 W/m2.The mean diurnal centroid for sensible heat flux was 11:57 and that of latent heat flux was 12:33.Seasonal drought led to increases in the mean diurnal variation range of sensible heat flux from 144 W/m2 to 321 W/m2,whereas the mean diurnal variation range of latent heat flux decreased from 324 W/m2 to 198 W/m2.The relative change between sensible heat flux and latent heat flux increased from-165 W/m2 to 76 W/m2.Temperature and water vapor pressure deficit were the major factors controlling the diurnal variation range of sensible heat flux and relative change between sensible heat flux and latent heat flux.During seasonal drought,the change in deep water content had a more significant effect on the diurnal variation range of sensible heat flux and relative change between sensible heat flux and latent heat flux,whereas the diurnal variation range of latent heat flux showed no relationship with meteorological factors.Seasonal drought weighted the diurnal centroid for sensible heat flux toward the afternoon,from 11:31 to 12:17.The relative change between sensible heat and latent heat flux changed from 1 h to 80 min.Seasonal drought had no impact on the diurnal centroid for latent heat flux.During the non-drought period,the diurnal centroid for sensible heat flux and relative change between sensible heat flux and latent heat were negatively correlated with air temperature,and positively correlated with air temperature during seasonal drought.Meteorological factors had no impact on the diurnal centroid for latent heat flux.The seasonal distribution of precipitation dominated the relative change between the diurnal centroids for sensible heat flux and latent heat flux.Precipitation may be more important during seasonal drought.It is suggested that relative change between the diurnal centroids for sensible heat and latent heat flux might be subject to the coupling between vegetation and the atmosphere.From this study,it was clear that these environmental variables,especially air temperature and water condictions regulated the diurnal trends of sensible and latent heat fluxes over the planted coniferous ecosystem.