The Spatial And Temporal Variations Of Water Use Efficiency In China's Terrestrial Ecosystems Simulated Using Remote Sensing And A Process-based Model

The carbon cycle and water cycle of terrestrial ecosystems are interactively linked at various spatial and temporal scales.The temporal and spatial variations of water use efficiency(WUE),which acts as an indicator of the extent of coupling between carbon and water cycles,and underlying driving forces have always been the focus of global change study.As one of the most crucial and sensitive region in global carbon cycle,the carbon sequestration by the terrestrial ecosystems in China is limited by water availability.The interactions between carbon and water cycles are one of hot topics of terrestrial carbon study in China.Since 2000,China had been hit by several climatic extremes and experienced tremendous land cover type changes.The human intervention interacted with climate change would inevitably alter the terrestrial carbon and water cycles in China.Studying the effect of these natural and anthropogenic disturbances on the temporal and spatial variations of WUE is of great scientific importance.In this study,MODIS reflectance and land cover products were used to retrieve 8-d 500 m leaf area index(LAI)products covered China during the period 2000 to 2010.Then this LAI data,meteorological data,soil data and land cover data were employed to drive the process-based Boreal Ecosystem Productivity Simulator(BEPS)to investigate the temporal and spatial variations of WUE in China's terrestrial ecosystems during the period from 2000 to 2010.The BEPS model was first calibrated and validated using carbon and water fluxes observed using the eddy-covariance technique at typical flux sites,forest NPP measured at different regions and statistical hydrological data.Then it was used to simulate the daily gross primary productivity(GPP),Net primary productivity(NPP),Evapotranspiration(ET)and WUE(WUE=NPP/ET)at daily time steps from January 1,2000 to December 31,2010.Finally,the spatial and temporal patterns of WUE in China's terrestrial ecosystems were analyzed and the key underlying driving factors were indentified for the period of 2000 to 2010.The main conclusions of this study could be drawn as follows:(1)Variations of vegetation LAI in China from 2000 to 2010The MODIS reflectance and land cover data were input into an inversion model based on the 4-scale geometric optical model to retrieve 8 day 500 m LAI products in China during the period 2000 to 2010.Validation in 9 forest and grassland areas using measured LAI and TM/ETM data indicated the accuracy was above 70%.The LAI product exhibited higher quality over forest than grassland.Although the national annual mean LAI showed upward trend in most area of China from 2000 to 2007,it did not show obvious increased trends from 2000 to 2010.Vegetation LAI in Jiangxi,Fujian,eastern of Hunan,and northern of Guangdong decreased obviously during the past 11 years.Annual mean vegetation LAI was positively correlated with annual mean temperature(AMT)in most areas of northeastern China but negatively correlated with AMT in most parts of southwest China.The increase of annual total precipitation induced the decrease of forest LAI in northern of northeastern China,southeastern China and coastal area of southern China,but increase of forest LAI in central south and north China,and cropland and grassland LAI in vast north part of China.(2)Carbon and water simulated ability of the BEPS modelValidations showed that the BEPS model was able to simulate the GPP,NPP,and ET well in terrestrial ecosystems of China.It successfully captures the seasonal and interannual variations of tower-based GPP measurements at Changbaishan(CBS),Qianyanzhou(QYZ),Yucheng(YC),and Haibei(HB)sites for the period from 2003 to 2006.The comparison of simulated GPP against tower-based GPP at these 4 sites produced a linear regression correlation coefficient of R2=0.02.The relative prediction errors(RPE)of estimated annual GPP were mostly in the ranges of ±10%.This model is also able to reproduce spatial variability of NPP in 491 forest plots distributed in different regions of China.Validation using using tower-based ET at CBS,QYZ,YC,HB,and Xilinhot site(XLHT)showed that the BEPS model was able to simulate ET in China's terrestrial ecosystems,explaining 66%of the variations of observed daily ET at 5 ChinaFLUX sites.The BEPS model could explain the 92%variations of inferred basin-level ET in 10 major river basins of China(p<0.0001).(3)Spatial pattern of WUE and main affecting factorsDuring the study period,estimated national totals of annual NPP,annual ET,and annual mean WUE of China's terrestrial ecosystems averaged at 2.74 Pg C yr-1,3.49×103 km3 yr-1,and 0.79 g C kg-1 H2O,respectively.The WUE of cropland was higher than that of forest and grassland.Simulated annual mean WUE exhibited a distinguishable spatial variation,in association with climate conditions,soil and vegetation types.Simultaneously,the annual mean WUE in northeastern China was the highest(1.25 g C kg-1 H2O),while Inner Mongolia were the lowest(0.33 g C kg-1 H2O).In a certain range,the increase of precipitation could make annual mean WUE raise.However,the continuing increase of precipitation would lead annual mean WUE decrease.In detail,the WUE of most vegetation types would decrease with the increasing temperature.There were no obvious relation between the radiation and annual mean WUE.The annual mean WUE obviously increased with the increase in areas with low LAI and slowly increased or even declined with the increase of LAI in areas with high LAI.(4)Temporal variations of WUE and main driving factorsDuring the recent 11 years,the national total NPP increased at a rate of 6.2 Tg C yr-2(p=0.33).NPP increased in areas north of the Yangtze River but decreased in most areas of southern China.Meanwhile,ET increased in most regions.The increasing rate of national annual total ET was 14.29 km3 yr-2(p=0.19).WUE increased in most northern regions and decreased mostly in southern regions.The national mean WUE slightly decreased at a rate of 0.0015 g C kg-1 H2O yr-1(p=0.50),mainly caused by the annual change of ET(p=0.008).The increase of WUE in the northern regions was mainly caused by the increase of NPP,while the decline of WUE in regions south of Yangtze River was mostly induced by the decrease of NPP and increase of ET.WUE was negatively correlated with temperature in most areas except the southern part of north China,central of Inner Mongolia,central and north parts of Tibet Plateau,and north part of southwest China.WUE was positively correlated with precipitation in most regions of China.In the past decade,the increase in precipitation in north China caused WUE to increase.The increase in temperature caused WUE to increase in Tibet Plateau and to decline in central China and southwest China.The response of WUE to drought was related to with drought severity and land cover types.