Drought Monitoring In The Loess Plateau Based On Solar-induced Chlorophyll Fluorescence

With the intensification of climate change,the frequency,duration and extent of droughts were becoming more and more severe.Thus,it was of great significance to ensure the sustainable development of agricultural production in China,through rapid and accurate monitoring of the occurrence of drought,and exploring the responses of vegetation photosynthesis to drought and the influences of meteorological factors on vegetation photosynthesis.Remote-sensing-based SIF(solar-induced chlorophyll fluorescence)signals had the potential for early monitoring and accurate assessment of regional vegetation photosynthesis under drought conditions.In this study,the Loess Plateau was selected as the study area,and the standardized precipitation evapotranspiration index(SPEI)and soil moisture were used to describe the drought conditions in the Loess Plateau.The GOSIF dataset that was used to represent vegetation photosynthesis,vegetation index,and the gross primary production(GPP)were used to analyzed the ability of SIF signals to monitor the change of vegetation activity during a typical drought event in 2007 in the Loess Plateau.Then,the GOSIF data were used to explore the SIF potential to detect the spatiotemporal variations of drought-affected GPP.Finally,this study investigated the responses of vegetation photosynthesis to drought and the influences of meteorological factors on vegetation photosynthesis in vegetation growing season(April to October)during 2001-2017 in the Loess Plateau.The main conclusions were drawn as follows.(1)There was a certain lag and underestimation by the vegetation index in the monitoring of drought effects on vegetation activities in the Loess Plateau.The SIF signals could monitor the drought effects on vegetation in the Loess Plateau more quickly and accurately than the vegetation index in terms of both temporal and spatial changes.The SIFyield could eliminate the effect of photosynthetically active radiation(PAR)and was more sensitive to the change of moisture conditions like precipitation.The SIF signals could be used as an important indicator to monitor the occurrence and dynamic changes of regional droughts.(2)There was a strong linear relationship between GOSIF and GPP at the sites with flux towers.For different vegetation types,the R~2 values were:deciduous broadleaved forest(0.94),evergreen needleleaf forest(0.91),meadow(0.86),cropland(0.85),and rainforest(0.67).In the Loess Plateau,the spatial distributions of GOSIF and VPM GPP were consistent.There was a significant linear relationship between GOSIF and VPM GPP(R~2=0.88),and the linear relationship would weaken with the increase of drought degree.The SIF signals could more accurately capture the changes of vegetation GPP under drought stress than the vegetation index like EVI(Enhanced vegetation index),and hence could be used to estimate vegetation GPP under drought stress.(3)About 87.8%of total areas of the Loess Plateau had a significant positive correlation between SIF and SPEI in the vegetation growing season.Vegetation photosynthesis in semi-arid areas was more sensitive to drought(r=0.49)and about 97.7%of the semi-arid areas had significant correlation between SIF and SPEI.Vegetation photosynthesis in sub-humid areas was less sensitive to drought(r=0.34)and about 84.5%of the sub-humid areas showed significant correlation between SIF and SPEI.Different vegetation types had different photosynthetic responses to drought.Among them,grassland had the highest sensitivity to drought with SPEI time scale of three to four months,while forest had the lowest sensitivity to drought with SPEI time scale of three to ten months.(4)There was a significant correlation between meteorological factors and SIF signals in the Loess Plateau.Temperature was an important meteorological factor affecting the photosynthesis of vegetation.The proportion of pixels with a significant positive correlation between temperature and vegetation SIF was about 98.9%in the Loess Plateau.The proportion of pixels with a significant positive correlation between precipitation and SIF was about 99.2%.The influence of precipitation on vegetation photosynthesis increased with the increase of aridity.The effect of precipitation was the greatest in arid regions.The proportion of pixels with a significant positive correlation between PAR and SIF was about 72.8%,which was mainly distributed in the southern semi-humid areas of the Loess Plateau.The impacts of meteorological factors on vegetation photosynthesis were largely determined by the differences in drought resistance of different ecosystem types.Temperature and precipitation were the most important meteorological factors affecting vegetation photosynthesis in grassland,cropland,and desert ecosystems,while PAR was more important than precipitation in forest ecosystem.