Abstract:

With the change of environment， the surface states of grassland has also changed， especially the near surface energy budget process changed more obvious. In order to comprehend the response of surface radiation energy balance processes to prohibited grazing and grazing in semi-arid grasslands， this paper analyzes the radiation flux observation data of typical degraded grasslands in Zhengxiangbai Banner of Inner Mongolia during the growing season （June-October） in 2020. The differences of diurnal and seasonal variation of total solar radiation， surface upward shortwave radiation， downward longwave radiation， surface upward longwave radiation， net radiation and albedo and their responses to meteorological factors， soil moisture， and vegetation conditions were studied in comparison between the prohibited grazing and grazing steppes. The indicated that the total solar radiation decreases month by month as time advances during the growing season. The surface upward shortwave radiation in the prohibited grazing area is generally smaller than that in the grazing area. The daily variation of the downward longwave radiation in each month is small， ranging from 130 to 370 W·m^-2. There is an obvious seasonal variation pattern of the surface upward longwave radiation under the prohibited grazing and grazing condition， but the little difference between them. The near surface radiation fluxes in the typical degraded grassland in Inner Mongolia has a significant unimodal diurnal variation during the growing season. The albedo of the both pastures showed a "U" shaped daily variation pattern. The Albedo of grazing pasture in the growing season was significantly higher than one of prohibited grazing pasture. The Piecewise Structural Equation Modeling analysis revealed that the radiation components （surface upward shortwave radiation， downward longwave radiation） and the vegetation index （normalized difference vegetation index） had a positive and highly significant effect on the net radiation； while Albedo and another radiation component （surface upward longwave radiation） had a significant negative effect on the net radiation in the prohibited grazing area. In the grazing area， the surface upward shortwave radiation and the downward longwave radiation had a highly significant positive effect on the net radiation； while Albedo and the surface upward longwave radiation had a negatively significant effect on the net radiation. Therefore， in the study， the vegetation status was the strongest factor affecting the near surface radiative energy balance process in typical degraded grasslands in Inner Mongolia under different grazing prohibition conditions.

Key words: semi-arid steppe, radiation flux, diurnal variation, growing season variation, Piecewise Structural Equation Model