Responses Of Soil Water Use Efficiency To Meteorological Drought In Xinjiang

In the context of global warming,frequent drought events will seriously affect the carbon and water cycle of the ecosystem.Soil water use efficiency（SWUE）is an important parameter to characterize the coupling relationship between carbon cycle and water cycle in ecosystem.It’s also an effective index of ecosystem water use efficiency（WUE）.Therefore,understanding the temporal and spatial evolution of soil water use efficiency and revealing its response mechanism to climate change is the key to correctly understand and predict the global carbon and water cycle process and the response and adaptation mechanism of ecosystem to climate change.Drought is one of the most important climatic features in Xinjiang.Due to the dual effects of climate change and human activities,drought disaster is becoming more and more serious in Xinjiang.Therefore,the in-depth analysis and discussion of SWUE in Xinjiang will play a very positive guiding role in the scientific management of Xinjiang ecosystem.This study calculated soil water（SW）,soil water use efficiency（SWUE）and meteorological drought index（AI）in Xinjiang from 2001 to 2020 based on gross primary productivity（GPP）and land cover type data of MODIS remote sensing data,soil water reanalysis data of ERA5,precipitation（P）and potential evapotranspiration（PET）data of CRU.In this study,the Mann-Kendall trend test was used to investigate the spatio-temporal evolution of SWUE.This study analyzed the spatio-temporal variation characteristics of meteorological drought,and used Pearson correlation coefficient method to explore the relationship between SWUE and meteorological drought.AIC Akakike information criterion and Pearson correlation coefficient method were used to discuss the time-delay effects of the response of SWUE to annual and monthly meteorological drought.The main conclusions are as follows:（1）From 2001 to 2020,the average annual GPP（0～1095.04 g C/m²）,SW（11.05～1916.5 mm）and SWUE（0～1.59 g C/mm）were higher in mountainous areas,but lower in plain basins,because of the humid climate and higher vegetation coverage in mountainous areas.Within the year,in summer,the climate was hot and rainy,vegetation growth was vigorous.In winter,the climate was cold and little rain,vegetation was lifeless.So GPP,SW and SWUE were the highest in summer and the lowest in winter.The interannual GPP(2.911 g C/m²·y^-1)and SWUE(0.0068 g C/mm·y^-1)showed an increasing trend,while the SW(-1.2116 mm·y^-1)showed a decreasing trend.The vegetation restoration and the consumption of more water resources were the main reasons leading to the opposite trend.（2）From 2001 to 2020,drought events in a few years led to a large inter-annual variation of AI.The average annual AI ranged from 0.17 to 0.33.Within the year,in summer,the temperature was high,and the potential evapotranspiration was larger,so the drought was the most severe.In space,compared with mountain area,plain basin had single landform and fragile ecological environment,so mountainous area was more humid,plain basin was more arid.SWUE was obviously correlated with drought.Spatially,drought had a stronger effect on vegetation growth than on soil water evaporation in most areas of Xinjiang,so SWUE decreased with the intensification of drought,among which SWUE and AI were significantly positively correlated in 25.23%of the regions.The SWUE of farmland and grassland had the most significant correlation with drought.However,the study also found that the drier the area,the higher the SWUE,the more significant the rise,and the weaker the correlation with meteorological drought.Compared with the humid area,the opposite characteristics were presented,because the vegetation in the long-term drought environment had stronger drought tolerance and was not easily affected by dry and wet changes.（3）The response of SWUE to meteorological drought in Xinjiang has time-delay effect.At the annual scale,due to the lingering effects of drought,such as loss of carbohydrate reserves,depletion of deep groundwater,vegetation death,and increased risk of pests and pathogens,14.61%of the regions in the study area had legacy effects on the response of SWUE to the previous year’s drought,among which the forest ecosystem response to drought has the strongest legacy effects.On the monthly scale,the adaptation mechanism of vegetation to the environment,the time required for nutrient transport,the water storage capacity of soil,and the water transport time in soil lead to the lagged effect of drought.The mean lagged time was 1.48 months.Spatially,because precipitation at high altitudes is stored as snow,the lagged time in mountainous areas is longer than that in plain basins.