Diurnal and seasonal variation of heat fluxes over an agricultural field in southeastern Nepal

The southeastern part of Nepal is traditionally known as the entrance of the South Asian summer monsoon system (SASM) and plays a vital role in the development of the national agro-economy. This research examined the diurnal and seasonal variation in the heat and mass fluxes. The study area lies in the agricultural fields that maintain crop cycles. Half-hourly data for a period of 54 months starting in October 2012 were obtained from the Tarahara surface flux observation system (SFOS), and the analyzed data included fast response eddy covariance measurements. The results show that all radiation components changed diurnally and seasonally, except for downward longwave radiation, which revealed weak diurnal variation during winter months. The winter surface albedo of the agricultural field was higher than the summer surface albedo and varied from 0.13 to 0.19. Latent heat release was found to be strongest during the summer season and was associated with atmospheric moisture content and precipitation. Sensible heat becomes weaker during summer and exhibits significant variation during winter months. The sensible heat flux increased rapidly with the increasing radiation flux during the pre-monsoon, which is a major energy source for driving atmospheric systems. The partitioning of energy into turbulent fluxes was also analyzed and it was found that the released latent heat consumed 67% of the net radiation; furthermore, 21% was consumed by the released sensible heat. The response of the land surface heat fluxes to the large-scale circulation system was clearly identified on the basis of the highest peak values observed in 2015 and 2016.