Surface Temperature Changes And Biophysical Effects On The Loess Plateau In The Context Of Ecological Restoration

As one of the important factors affecting climate change by human activities,land use mainly affects the climate system by changing the surface biogeophysical process and biogeochemical process.The impact of land use on the climate system through biogeochemical processes has been extensively researched,but the understanding of biogeophysical processes is still insufficient,which is not conducive to accurate assessment of regional climate.This study focuses on the Loess Plateau area that has experienced large-scale and high-intensity human activities.Using remote sensing products,reanalysis data and other multi-source data,we first analyze the transfer of cultivated land from 2000 to 2015 and the seasonal changes in surface albedo and evapotranspiration.Secondly,the generalized additive model is used to study the relationship between surface albedo and land use structure,and the response of surface albedo to leaf area index,vegetation coverage and forest density is further discussed.Then,based on Pearson's correlation,the seasonal variation of latent heat flux,sensible heat flux,net surface radiant flux and their relationship with surface temperature are analyzed.Finally,based on the biophysical parameters of cultivated land and forest and grassland,combined with energy redistribution factors,the radiation and non-radiation effects of returning farmland to forest and grassland are estimated,and the non-radiation effect index(NRFI)is used to quantify the contribution of non-radiation effects in the change of surface temperature rate.The research results show that:(1)21.15%of the cultivated land in the Loess Plateau has changed,mainly as cultivated land converted to woodland and grassland,20.93%of forest land has changed,mainly converted to grassland and cultivated land,and 16.67%of grassland has changed.Mainly transformed into cultivated land and forest land.Surface albedo shows obvious spatial heterogeneity,evapotranspiration has obvious seasonal variation characteristics,and summer evapotranspiration is the highest.(2)The response of the surface albedo to the leaf area index is generally positive,and the response to the leaf area index shows a large difference between land types.The response of deciduous broad-leaved forest surface albedo to vegetation coverage is negative,while the response of coniferous and broad-leaved mixed forest surface albedo to vegetation coverage has regional differences,and the overall response of shrubland surface albedo to vegetation coverage is positive.The response of surface albedo to forest density is different due to geographical distribution.Land use structure plays an important role in the adjustment of surface albedo.(3)The spatial correlation between latent heat flux and surface temperature shows a negative correlation in the northern and central parts of the Loess Plateau,and a positive correlation in the southern,western and eastern parts of the Loess Plateau.The spatial correlation between sensible heat flux and surface temperature is positive correlation in the northern part of the Loess Plateau,and negative correlation in the southern,western and eastern parts of the Loess Plateau.The spatial correlation between the net radiant flux and the surface temperature and the positive and negative correlation between the latent heat flux and the surface temperature are basically the same.The inter-annual seasonal variation of surface temperature is characterized by the highest surface temperature in summer,generally higher than 24?,followed by spring,and surface temperature in autumn is higher than that in winter.The latent heat flux in summer is the highest(73.61 W/m2),and the latent heat flux in winter is the lowest(7.61 W/m2).Spring is similar to summer and higher than autumn.The sensible heat flux is higher in spring and summer,and fluctuates greatly.The change trend in winter is opposite to that in autumn,and the fluctuation is greater in spring and summer.The net surface radiant flux changes more slowly,with the highest in summer,followed by spring,and higher in autumn than in winter.(4)The biogeophysical difference between crop land and forest and grassland is that the surface albedo of cultivated land is greater than that of forest and grassland,and the evapotranspiration is similar to or slightly greater than that of forest and grassland.Cooling effect,but the impact of land use changes in this area on surface temperature changes is dominated by non-radiative effects,during the study period,the surface temperature in the growing season dropped by 0.17?,and the contribution rates of non-radiative effects from May to September were 79.1%,56.1%,60.6%,77.9%and 74.7%.The overall contribution rate during the growing season is 69.7%,and the change in surface temperature has different sensitivity to energy redistribution factors,surface albedo,and soil heat flux.Energy redistribution factors play a decisive role in the changes in surface temperature,making the growth season the surface temperature drops by 0.21?.This research can not only provide a theoretical basis for accurately assessing the impact of land use change on regional climate,but also has important significance for responding to or mitigating the impact of climate change in the Loess Plateau.