Estimation Of Gross Primary Productivity In Heihe River Basin By Remote Sensing And Analysis Of Driving Factors

The terrestrial ecosystem carbon cycle is an important part of the global carbon cycle,and gross primary productivity(GPP)is a key point of the terrestrial ecosystem carbon cycle,which can reflect the level of productivity of the terrestrial ecosystem and is also an important parameter to characterize the carbon exchange flux between biosphere and atmosphere.Satellite remote sensing technology is playing an increasingly important role in regional vegetation structure and ecosystems,providing the technological means for spatial expansion for GPP estimation by providing regional and even global observation capabilities.Heihe river basin is the second largest inland river in northwest China,in recent years,under the influence of climate change and human activities,the ecological environment of the region has been deteriorating.Therefore,it is of great significance to conduct GPP estimation by remote sensing in Heihe river basin and quantitatively distinguish the contribution of climate change and human activities to GPP.Under this background,paper with VPM model and MODIS GPP algorithm as tools,based on multi-source data such as remote sensing images and meteorological data,estimating the GPP of terrestrial ecosystem in Heihe river basin.And analyze the spatial and temporal distribution characteristics of GPP,the correlation between climate factors such as temperature,precipitation and photosynthetic active radiation with GPP and the uncertainty of model,and quantitatively analyze the impact of climate change and human activities on GPP.The main research results of this paper are as follows:(1)Based on VPM model and MODIS GPP algorithm to estimate GPP of Heihe river basin,and by Dayekou Guantan flux data and sun-induced chlorophyll fluorescence(SIF)data validate the result of the estimation.The results show that the estimation accuracy of VPM model is higher than that of MODIS algorithm in the south of Heihe river basin where vegetation types are abundant and vegetation growth is good.From the perspective of vegetation types,the estimation results of VPM model of farmland,grassland and sparse vegetation are also better than that of MODIS algorithm,and the estimation accuracy of farmland is the best,followed by grassland and sparse vegetation is the worst.(2)The GPP in Heihe river basin is generally between 0?1000gCm-2a-1,spatially,GPP is more south than north.The vegetation in the middle and lower reaches is sparse and there is a large area of gobi and desert.Except in areas covered by farmland in the middle reaches and the banks of the lower reaches,the GPP is generally below 50cm-2a-1.From the perspective of seasonal distribution characteristics,the vegetation in Heihe river basin has the highest GPP in summer,followed by spring and autumn,and the lowest in winter.The interannual change of GPP in Heihe river basin generally shows an upward trend of fluctuation,which indicates that the ecological environment of Heihe river basin is improving gradually.(3)There is a significant correlation between GPP and climate factors such as temperature,precipitation and photosynthetic active radiation in Heihe river basin,among which,it is mainly positively correlated with temperature and photosynthetic active radiation and negatively correlated with precipitation.Different calculation methods of model parameters such as FPAR(Fraction of Photosynthetically Active Radiation,FPAR),water stress factor Ws and temperature stress factor Ts will affect the results of the model estimation and increase the uncertainty of the simulation results.Among them,changing FPAR has a greater impact on GPP,Based on EVI(Enhanced Vegetation Index),FPAR factor has a greater impact on the northern part of the basin,resulting in the absence of GPP data in this region.Changing the calculation method of Ts has little influence on the estimation results of GPP.The Ws spatial distribution calculated based on Vapor Pressure Deficit(VPD)is opposite to the Ws spatial distribution calculated based on Land Surface Water Index(LSWI),while the estimated GPP spatial distribution is close to the value range,and Ws has little impact on GPP.(4)This paper was carried out to distinguish the impacts of climate change and human activities on GPP quantitatively,and extracts the vegetation types of Heihe river basin by grassland,farmland and sparse vegetation,then evaluates their responses to climate change and human activities respectively.The results show that the vegetation restoration area(73.5%)in Heihe river basin was larger than the vegetation degradation area(26.5%).Among the vegetation degradation areas,the degradation area caused by human activities is 7.9%,the degradation area caused by climate change is 9%,and the degradation area caused by the combined action of the two is 9.6%.Therefore,in recent years,the fluctuating growth state of GPP in Heihe river basin ecosystem is mainly affected by human activities,followed by climate change.