Variations Of Water And Carbon Fluxes In The Dryland Of North China

The dryland in northern China has a vulnerable eco-environment,where the water and carbon cycles are sensitive to climate change and human-induced land use/cover change(LUCC).During the past several decades,the significant increase of air temperature and the expansion of human activities have been changing the land use/cover types in this region.Therefore,studying on the spatial and temporal variations of water and carbon fluxes in the dryland ecosystem is vital and necessary,as well as its responses to environmental changes.This study will provide the scientific understandings for the eco-environmental protection in the typical dryland of north China,especially in Inner Mongolia and northern Loess Plateau.Two typical Sandy Land(Mu Us Sandy Land and Horqin Sandy Land)located in northern Loess Planteau and Inner Mongolia have been selected for field observations,and eddy covariance(EC)techniques were used to observe the water and carbon fluxes.Based on the observations,firstly,we analyzed the diurnal and seasonal patterns of water and carbon fluxes,as well as the main environmental controlling factors.The results showed that the evapotranspiration(ET)rate and carbon sequestration ability at the Yulin station that located in the Mu Us Sandy Land is higher and more significant than those at the Tongliao station that located in the Horqin Sandy Land.Regarding the seasonal variation,the dynamics of ET at Yulin station and Tongliao station are both controlled by radiation.Gross primary productivity(GPP)and ecosystem respiration(Reco)are mainly influenced by temperature,while net ecosystem exchange(NEE)is controlled by vegetation phenological change.The extreme water and temperature conditions have obvious effects on carbon fluxes,especially the extreme dry water condition.Secondly,with the LUCCs that resulted from human activities during 2011-2016 in Yulin station,this study quantitatively analyzed the impact of human-induced LUCC on water and carbon fluxes,respectively.The results showed that both ET and normalized ET were increased in the land degradation and vegetation rehabilitation processes,and the increase of normalized ET was faster and more obvious in the vegetation rehabilitation process.Human activities explained 84.5%,59.2%,and 81.6% of the inter-annual variability of GPP,Reco,and NEE.This study indicates that human-induced LUCC will accelerate the ET rate in this region and reduce the carbon sequestration capacity,which may lead to deterioration of the vegetation growth environment and more vulnerable ecosystem.Thirdly,this study collected data of water and carbon fluxes from other nine flux stations that are located in the typical dryland of north China.Together with the large scale remote sensing data(including surface temperature(LST)and vegetation index(NDVI))and meteorological data(including short solar radiation(Rs)and wind speed(Ws)),the support vector regression(SVR)method was used to develop an upscaling model.The gridded water and carbon fluxes were estimated by SVR method,and the special and temporal patterns of water and carbon fluxes were analyzed in the dryland of north China.The results showed that the SVR model could simulate ET and NEE with reasonable accuracy at all flux stations,with the determination coefficients of 0.76 and 0.64,respectively.The annual mean values of ET and NEE during 2001-2015 were 204±6.3 mm and-63±7.2 g C/m2/yr,respectively.In the typical dryland of north China,ET and NEE are gradually decreasing from the southeast to the northwest.