LUCC-driven Changes In Gross Primary Production And Water Consumption In Northern China

Northern China has experienced large scale soil erosion,soil degradation and desertification over the last several decades.In order to solve the environmental and ecological problems there,several national-scale policies have been vigorously implemented.With greening in the world,the vegetation coverage in China has increased significantly.The afforested area increases by about 25%,including afforestation accounts for about 42% and cropland about 32%.The increase of afforested area and cropland with high water consumption,along with the unreasonable utilization of water resources,lead to the great challenge of water resources management in northern China.Gross primary productivity(GPP)refers to the transformation of green plants into their own organic matter through photosynthesis,a key indicator to evaluate the carbon cycle in the ecosystem.Actual evapotranspiration(ET)and water storage(WS)are the key variables in water consumption driven by land use and land cover change(LUCC).Therefore,it is important to understand how they respond to LUCC,which is not only conducive to the scientific management and prediction of water resources,but also important for the carbon cycle.To estimate the impact of LUCC on gross primary productivity and water consumption,this study uses the 500 m resolution remote sensing data product,generated by PML-V2,a GPP-ET coupled biophysical model that is driven by MODIS remote sensing data together with Global Land Data Assimilation System(GLDAS)climate forcing data.Along with other data,including precipitation,runoff,Gravity Recovery and Climate Experiment data(GRACE)data,and leaf area index,this study uses Mann-Kendall with the Sen trend method and water balance method to explore the temporal and spatial characteristics of LUCC-driven impact on GPP,ET and water storage change in northern China,and further quantifies the LUCC-driven impact in different regions and analyzed the main influencing factors.The main conclusions are summarized as follows:(1)Land use and land cover change have taken place dramatically in northern china,with cropland,forest and shrubland increased by 0.80%,0.52% and 0.02%,others and grassland decreased by 0.81% and 0.53% from 2003 to 2017,respectively.In Inner Mongolia Plain,main change reflects by the fact that the other vegetation type is converted to grassland and cropland,while in Northeast Plain the conversion is mainly from grassland to forest and cropland.Leaf area index increases significantly in the Northeast Plain and the Loess Plateau,but decreases in the Inner Mongolia Plateau.The emissivity has little change in the north of China,but the reflectance decreases across the research area,except for the Northeast Plain.(2)LUCC-driven mean annual change in GPPrs and ETrs is spatially heterogeneous.The GPPrs and ETrs in Inner Mongolia Plain and Changbai Mountain decrease while in Loess Plateau and Northeast Plain increase from 2003 to 2017.Cropland has the highest GPP change of 184 gCm-2 y-1 in the Loess Plateau,followed by the Northeast China Plain(128 gCm-2 y-1)and the Inner Mongolia Plateau(82 gCm-2 y-1).The highest ET increase of 45 mm y-1 occurs in shrubland in the Loess Plateau,followed by the increase of 20 mm y-1 in the Northeast China Plain.GPP and ET increase by 164 TgC y-1 and 13 km3 y-1 in northern China driven by LUCC respectively,with the trend significant increase in the Loess Plateau and the Northeast Plain.Cropland has the highset GPP volume change of 49 TgC y-1 in the Northeast Plain and grassland has the highest ET volume change of 3.7 km3 y-1 in Inner Mongolia Plateau?(3)Driven by LUCC in four regions(the Inner Mongolia Plateau,the Northwest desert,the Loess Plateau,and the Northeast Plain),GPP and ET both increase in 2003-2017.Their changes in the Loess Plateau and Northeast Plain are higher than those in the Inner Mongolia Plateau and the Northwest desert.An important finding of this paper is that GPP and ET have increased faster during the period of 2010-2017,compared to the period of 2003-2009.The main reason is because the increase in LAI is faster for the post-2009 period,indicating that the recent LUCC change has a greater impact on GPP and ET.(4)ET increase in the Yellow River Basin is mainly caused by the increase in the area of forest and cropland and LAI increase.Based on water balance estimates together with GRACE data,this study investigates the water storage trends in the YRB from 2003 to 2016.Our results indicate that the decadal WS has decreased noticeably in the YRB.The WS has decreased by 5.6 mm y-2 in the YRB,for which vegetation change contributed 1.94 mm y-2(about 34.6%).(5)LUCC has caused dramatic LAI change that becomes a main driving factor for changes in GPP,ET and WSC.For the same land use type,there exists the strong correlation between trend of GPP and trend of LAI across the four regions,with r > 0.95 for all land use types.The water storage change driven by LUCC mainly takes place in the middle and lower reaches of the Yellow River Basin,where LAI has increased strongly.