Temporal And Spatial Variation Of Vegetation Water Use Efficiency And Its Influencing Factors

Under the influence of global climate change and human activities,the water and carbon cycle of terrestrial ecosystems are constantly changing.The vegetation water use efficiency(WUE),which measures the carbon-water coupling of vegetation,reflects the photosynthetic carbon sequestration process and evapotranspiration The relationship between characteristics is of great significance to improve the understanding of the regional vegetation water carbon utilization and the rational use and allocation of water resources.Especially in arid and semi-arid areas,the improvement of vegetation WUE not only regulates the regional ecological environment,but also has a significant impact on changes in agricultural output caused by water deficit.In the Hexi area,which contains a series of zonal landscapes of mountainous-oasis-desert,water resources are very tight and the ecological environment is fragile.The area faces water only if there is an oasis.Anhydrous land will become desertified and people will even be forced to emigrate Therefore,it is particularly important to study the change of vegetation WUE and its influencing factors,and to investigate the countermeasures for improvement.This study uses Hexi region as the research area.First,based on remote sensing data,meteorological data,land use data,DEM data,social statistics data,etc.,the improved CASA model is used to estimate the NPP,and the vegetation WUE is calculated based on the MOD16-ET data.To analyze the temporal and spatial changes of vegetation NPP,ET,and WUE in the region from 2000 to 2016;secondly,to separate the climate change and human activity dominance zone of WUE using partial differential decomposition method and analyze the influence factors of WUE;finally,for human activities Minqin County,a typical district under the leadership of the People's Republic of China,designed optimization schemes to enhance the vegetation WUE in typical districts,reduce unnecessary waste of water resources,and use more of the saved water resources for ecological environment restoration and regional green sustainable development.The specific research conclusions are as follows:(1)From 2000 to 2016,the vegetation NPP in the Hexi area showed an increasing trend,and vegetation evapotranspiration showed a decreasing trend.The coupled WUE change of the vegetation showed an upward trend.Specifically,the vegetation NPP in Hexi area increased at a rate of 1.33 gC·m-2·a-1,and the vegetation NPP decreased from southeast to northwest.The vegetation NPP change in the west was more unstable than that in the east.Obviously,the vegetation ET decreases at a rate of 1.19mm·a-1,and decreases spatially from south to north,which has the same trend as the vegetation coverage of the underlying surface.After coupling,it was found that the vegetation WUE fluctuated in Hexi area,with an average value of 0.83gC·m-2·mm-1,and the vegetation WUE increased most significantly in the desert oasis area.Vegetation WUE decreases from southeast to northwest,and the change trend of vegetation WUE in most areas is positive,mainly distributed in the north of Qilian Mountain and adjacent to the corridor oasis area.Vegetation WUE in a few areas showed a significant reduction trend,mainly distributed in Liangzhou-Minqin Oasis and Ganzhou-Linze Oasis.(2)The vegetation cover of Hexi Oasis shows an increasing trend,which is mainly caused by the growth of cultivated land,and the phenomenon of cultivated land expansion in the region is obvious.The specific performance is as follows:the oasis vegetation is mainly grassland and arable land,which account for 69%and 26%respectively,and the changes are the most obvious.The increase in arable land within the oasis vegetation area of Hexi is the largest,with an increase of 1400 km2,Which increased by 12.05%,grassland contributed the most to its growth;shrubs in oasis vegetation had the largest decline,with a decrease of 65 km2,mainly converted to grassland,followed by cultivated land.(3)Based on climate change and human activities,the impact on the vegetation WUE in the Hexi Oasis was separated.The results showed that climate change mainly manifested as a promotion effect on the vegetation WUE in the Hexi Oasis.The inhibitory effect of human activities is greater than that of promotion,and its contribution to the oasis vegetation WUE is-0.04.The negative effect area is mainly distributed to the east of the corridor oasis,and the vegetation cover is mostly farmland.The negative contribution of human activities to WUE in Liangzhou-Minqin Oasis and Ganzhou-Linze Oasis area is significant.After further selecting 8 climate and human factors to analyze the vegetation WUE,it was found that the impact of human factors is greater than climate factors.The total agricultural,forestry,animal husbandry output value,crop planting area,and number of livestock have the most significant effects on vegetation WUE.The correlation coefficients are 0.814,0.793,0.723,that is,the related agricultural and animal husbandry measures led by human activities have a significant impact on the vegetation WUE.(4)Research on countermeasures to increase vegetation WUE in typical areas of WUE reduction caused by human activities-Minqin County.Since the underlying vegetation in this area is dominated by farmland,the most economic and effective crop planting structure adjustment method is selected for vegetation WUE.Promotion and optimization scheme design.Constraints were limited through government announcements,and four types of optimization models based on multi-objective economical water-saving,water-use-efficiency-improving,comprehensive,and ecological-optimizing models were constructed.There were 14 optimization schemes,respectively in water-saving effects and carbon sequestration.Effect,water efficiency,economic efficiency,water use efficiency to achieve different optimization effects.The best water-saving effect can save 26.9%of water resources,the maximum economic benefit can be increased by 49.72%,and the carbon sequestration effect can be increased by up to 85.74%.Provides program selection and data support for crop planting structure adjustment and vegetation cover regulation under different optimization goals,saving more water resources for regional ecological environment restoration.