Research On Drought Grade Calibration Of Temperature-vegetation Dryness Index In Desert Steppe

Scientific and reasonable drought grading standard is quite necessary for drought monitoring.Temperature-vegetation dryness index,TVDI,which is widely used for drought monitoring because of its more significant correlation between soil relative humidity in numerous drought monitoring remote sensing indices.However,the current drought grading standard of TVDI is still scarce for the quantitative remote sensing based necessary and sufficient empirical research basis,so which has some problems such as irregularity and arbitrariness etc.On the other hand,the desert steppe in Inner Mongolia is a quite valuable ecologicalenvironmental resources,but which also belongs to extremely fragile ecosystem due to its arid climate biotope and under the coupling action with unreasonable grassland resource utilization mode and behavior,which is facing the increasingly severe threat of desertification and drought.The basic scientific data is needed for ecological-environmental remote sensing monitoring urgently.Based on the above,this paper aims to establish a reliable TVDI-based drought grading standard through drought grade calibration research for TVDI based on Landsat 8 remote sensing images in desert steppe region,so as to provide a scientific basis for drought monitoring in desert steppe.The TVDI models were built up to combined with field based soil relative humidity through different scale levels such as study area scale,image scene scale and cross-ecosystem scale respectively by using Landsat 8 images in the study area.Which is located in desert steppe in middle part of Siziwang Banner,Inner Mongolia.Then the best model was selected and corresponding TVDI was generated.At the final stage,drought grade calibration of TVDI for desert steppe and corresponding analysis were conducted according to the TVDI data in the study area,field based soil relative humidity in the study area and soil relative humidity based drought grade criteria which were selected for this research.The main research conclusions of this thesis are as followings:(1)The best matching result for field based soil relative humidity in generated TVDI data set which based on three scales and four schemes(the cross-ecosystem scale includes two types of model,i.e.one is polygon based model and the other is cloud mask based model)is result of image scene scale and the next are results of the cross-ecosystem scale,polygon based model,and the third and last is result of the study area scale and the cross-ecosystem scale,cloud mask based model respectively.Among them,results of the study area scale and the cross-ecosystem scale,cloud mask based model are closest for fitting effect.It can be seen that when the TVDI model is established for drought monitoring of the study area,as the modelling approach,the smaller scale of the image scene scale is better than the larger scale of the cross-ecosystem scale.(2)The grading scheme of drought in desert steppe based on TVDI values in study area is as bellow: no drought,0-0.73;light drought,0.73-0.78;moderate drought,0.78-0.83;severe drought,0.83-0.89;extreme drought,0.89-1.(3)The overall drought situation is relatively serious in the study area.The total area of moderate drought,severe drought and extreme drought occupy 98.8% of the total area of study area.As the spatial distribution pattern of drought situation,the severe drought pixels were distributed in the western part and central part of the study area,while the northeastern part and southeastern part were relatively light.Drought situation in the study area was mainly characterized by extreme drought,followed by severe drought and less moderate drought.The areas with less drought degree are mainly concentrated near the northwest-southeast valley.Although there are some pixels which belong to severe drought and extreme drought grade along the valley area,however the regional overall drought degree was lighter obviously.In particular,the southeastern area of Honger Soum is located at the outlet of the mountainous area,coupled with the relatively low terrain,the rainwater in the valley eventually flows through here,the soil moisture conditions are relatively better,and the degree of drought is reduced,so it is also a large and concentrated area with lighter drought in the study area.