Analysis Of The Impact Of Atmospheric And Soil Water Stress On Vegetation During Sudden Drough

In the context of climate warming,the water stress of frequent flash droughts have a great impact on the ecosystem.In addition,human activities such as irrigation would alleviate water stress,which raises an urgent need to understand the impact of flash droughts on vegetation.Previous studies found that atmospheric and soil drought affected vegetation productivity through different mechanisms,but they often occurred together and were difficult to separate their impacts.For flash droughts,which develop rapidly due to strong land-atmosphere coupling,their impact on vegetation was explored by several case studies,but the main water stress factors were unclear.Therefore,this study decoupled the soil and atmospheric water stress on vegetation during flash droughts and hot flash droughts(accompanied by high temperature and strong radiation)using long-term global FLUXNET observation data,remote sensing vegetation products,and hydrometeorological reanalysis.In addition,the land surface model was used to simulate and analyze the impact of irrigation on flash drought and its ecological effects.The main conclusions are as follows:(1)The water stress on the gross primary productivity(GPP)was dominated by soil water deficit during flash droughts,but the atmospheric water stress increased significantly during hot flash droughts.In this study,decoupling the atmospheric and soil water stress on GPP during flash droughts at the hourly scale showed that soil and atmospheric water stress were dominant for 46% and 6% of 35 FLUXNET stations,respectively,while the response of the remaining stations was not significant.For hot flash droughts,37% and 49% of the stations were dominated by soil and atmospheric water stress respectively,which showed a significant increase in the impact of atmospheric drought.Specifically,non-forest vegetation responded faster to soil water stress than forest vegetation.In eastern China where flash droughts occur frequently,it was found that soil water stress on GPP during flash droughts was significantly greater than atmospheric water stress based on vegetation remote sensing and atmospheric reanalysis data,especially in humid regions.Compared with other regions,the atmospheric water stress in the North China Plain is more significant.Further analysis found that the regions dominated by atmospheric water stress increased from 8% during flash droughts to 36% during hot flash droughts,which is equivalent to the regions dominated by soil water stress.(2)Vegetation light use efficiency was more dominated by atmospheric water stress than soil water stress,and water use efficiency increased during flash droughts in most regions.For light use efficiency,the observational data from global FLUXNET stations indicated that 80%of the stations were dominated by atmospheric water stress,while 14% of stations were dominated by soil water stress during flash droughts,and the impact of atmospheric water stress on forests was particularly significant during the recovery of flash droughts.In addition,nonforest vegetation was subjected to greater soil water stress than forest vegetation.Further,using remote sensing data and reanalysis data,the impact of flash droughts over eastern China was investigated.The results showed that atmospheric water stress was the dominant factor for the significant decline in light use efficiency during flash droughts,accounting for 55% for the study region.As for water use efficiency,during flash droughts and hot flash droughts,most areas in eastern China showed an increasing trend,which may be due to a decrease in stomatal conductance,leading to a decrease in transpiration,thereby increasing water use efficiency.The water stress on water use efficiency is similar during the two periods,possibly due to the further decrease in vegetation stomatal conductance caused by the increased vapor pressure deficit during hot flash droughts,which leads to a similar decrease in GPP and photosynthetically active radiation.(3)Irrigation effectively alleviated flash droughts in the North China Plain and further reduced its impact on crops.In this study,the land surface model CLM5.0 was used to simulate the differences in the characteristics of flash droughts under irrigation and non-irrigation conditions,and their impacts on vegetation productivity over eastern China.The results showed that irrigation decreased temperature,increased evapotranspiration,and increased surface air humidity in northern China.Irrigation reduced the frequency of flash droughts by 15% and decreased the intensity by 21%.In addition,the area with the greatest impact is located in the North China Plain with the largest proportion of crops.Irrigation could reduce the frequency of flash droughts in the North China Plain by 76% and the duration of flash droughts by 8%,and significantly alleviate the negative impact on vegetation 68%.Irrigation had reduced water stress in East China and North China,mainly by reducing atmospheric water stress in East China and soil water stress in North China.