| Soil moisture is an important indicator of drought.The karst area of Guizhou has deep gully,large topographic fluctuation and poor accessibility.The traditional soil moisture measurement method is time-consuming and labor-consuming,which is difficult to meet the needs of regional monitoring and comprehensive analysis.The inversion of soil moisture by satellite remote sensing technology has the advantages of high efficiency,continuity and good timeliness,which can effectively meet the above needs.Soil and water loss is serious and drought occurs from time to time in karst area of Guizhou.There are few reports on the literature of using remote sensing technology to retrieve soil moisture in this area.Using remote sensing method to carry out drought monitoring and research in this area has important guiding significance for the study of drought resistance in karst area.In view of this,this thesis selects Bijie City,Guizhou Province,a typical karst area as the study area,and uses the MODIS remote sensing image data of the annual vegetation growth period(March to September)in the study area from 2014 to 2018 to carry out the research on the inversion of soil moisture and the temporal and spatial evolution characteristics of drought in karst area.There are three potentially applicable soil moisture inversion models TVDI,VSWI and OPTRAM based on MODIS optical remote sensing in the study area.They are evaluated in combination with meteorological drought index SPEI(Standardized Precipitation Evapotranspiration Index).The results show that the OPTRAM model is suitable for soil moisture inversion in the study area.Based on the soil moisture inversion results of OPTRAM model,the variation characteristics of soil moisture and drought in Bijie City at different time and spatial scales during the study period were analyzed.Finally,combined with the interpolated meteorological data,altitude data and vegetation enhancement index EVI,the effects of these factors on soil moisture changes in the study area,karst canyon area and karst plateau area are discussed under the influence of single factors and multiple factors,the main influencing factors were explored by multiple regression analysis,and its multiple linear regression model was obtained.The main research contents and conclusions are as follows:(1)Study on the applicability of the models.Based on previous studies,this thesis selects three MODIS optical remote sensing soil moisture inversion models TVDI,VSWI and OPTRAM with potential application value in this area,and evaluates the applicability of the three models combined with SPEI.The results show that OPTRAM model is more suitable for soil moisture inversion in the study area.(2)Based on the remote sensing inversion results of OPTRAM model,the temporal evolution characteristics of soil moisture and drought in the study area were studied.The time scale of this thesis is divided into interannual time scale and monthly time scale:on the interannual time scale,the drought occurred in the study area in 2014,2016 and 2017.On the inter monthly time scale,July is the most affected by the drought,followed by August,and then May and September.In June,due to abundant precipitation and other reasons,the scope of drought is the smallest.(3)Based on the remote sensing inversion results of OPTRAM model,the spatial evolution characteristics of soil moisture and drought in the study area were studied.In this thesis the spatial characteristics of soil moisture and drought distribution are analyzed from five situations:the whole study area,three-level different terrain spatial range,county,different altitude range and different karst landform area.In the whole study area,the spatial distribution pattern and statistical data of the mean value of soil moisture for many years show that the drought is mainly distributed in Weining County,Hezhang County,Jinsha County and the border of Bijie City.In the three-step terrain spatial range of the study area,the three terrain spatial ranges in the study area are affected by the arid climate to a certain extent,in which the third step drought distribution range is the largest and the second step drought distribution range is the smallest.From the county level,the study area governs 8 areas,and qixingguan district and Weining County are greatly affected by drought.From different altitudes,the range from 800m to 1200m,2100m-2200m and 2600m-2700m is the most affected by drought.In different karst landform areas,it shows that the impact of drought in Karst canyon area is greater than that in Karst Plateau area.(4)Analysis of influencing factors of soil moisture in the study area:analyze the influence in different geomorphic areas(the whole study area,karst canyon area and karst plateau area)from the influence of single factors and the comprehensive influence of multiple factors.In terms of the influence of single factor,in the whole study area,karst canyon area and karst plateau area,the correlation between temperature,precipitation,altitude and vegetation enhancement index EVI and soil moisture is 0.6304,0.7012,0.635,0.4142,0.8058,0.288,0.6450,0.4347,0.4611,0.6338,0.2953 and 0.0308 respectively.Under the comprehensive action of multiple factors,the main factors affecting soil moisture in the whole study area,karst canyon area and karst plateau area and their influence degrees are as follows:temperature,precipitation and vegetation enhancement EVI,and the order of influence degree is precipitation>temperature>vegetation enhancement index EVI;Precipitation,altitude and vegetation enhancement EVI,and the order of their impact degree is altitude>precipitation>vegetation enhancement index EVI;Temperature,altitude and vegetation enhancement EVI,and the order of their impact degree is altitude>temperature>vegetation enhancement index EVI. |
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