Drought Monitoring In Ejina Oasis Based On Optical Remote Sensing

In Northwest China,drought is one of the most common natural disasters.As the last green ecological barrier in the Northwest,the Ejina Oasis in the Inner Mongolia Autonomous Region plays an important role in maintaining regional ecological balance,conserving water sources,and preventing sand and soil.However,this place has a high frequency of drought and a long duration,making it an extremely severe drought area.Soil moisture is an important part of the surface environment and the main carrier of the global material cycle.It promotes the energy exchange between the landing site and the atmosphere and maintains the global water and heat balance.At the same time,soil moisture is an important indicator for assessing drought conditions,an important content of ecosystem research in arid areas,and a major factor in assessing regional environmental stability.Therefore,the study of soil moisture has become one of the hot research issues in Ejina Oasis.Due to the frequent occurrence of natural disasters and over-exploitation of human beings,the vegetation area in the oasis has been reduced,the surface desertification and salinization have intensified,water resources are scarce,and the ecological environment is fragile,which seriously affects the sustainable development of the regional economy.The ecological and environmental problems of the Ejina Oasis have attracted widespread attention from the Chinese government and scholars from all walks of life.Researchers have carried out a lot of work on vegetation growth and environmental changes in the area,but there are still gaps in the research on regional drought monitoring.Therefore,this article takes soil moisture research as the starting point to monitor the drought situation in Ejina Oasis.Traditional drought monitoring methods are time-consuming,costly,and have lag,which are not conducive to real-time drought monitoring.With the continuous development of remote sensing technology,optical remote sensing has been used by many scholars in large-scale drought monitoring.Among them,Landsat 8 is the most widely used optical remote sensing data.There are a variety of drought monitoring methods based on optical remote sensing,including spectroscopic drought monitoring models and vegetation index-based drought monitoring models.However,the principles of various drought monitoring models are different,the models have their own strengths,and the scope of application is also different.Therefore,to achieve accurate monitoring of regional droughts,the choice of models is particularly important.This paper takes Ejina Oasis as the research area and uses Landsat 8 remote sensing images on August 24,2017.Based on the above two types monitoring methods,a variety of regional drought monitoring models are established,and the models are compared and selected to determine the drought status of Ejina Oasis.Conduct monitoring research.Among them,the spectral drought monitoring model includes three spectral models:Soil Moisture Monitoring Index(SMMI),Perpendicular Drought Index(PDI)and Modified Perpendicular Drought Index(MPDI).The vegetation index-based drought monitoring models include the Normalized Difference Drought Index(NDDI),the Vegetation Supply Water Index(VSWI)and the Temperature-Vegetation Drought Index(TVDI).First,calculate the required parameters of the model,including soil line,vegetation coverage,Vegetation Index(VI)and Land Surface Temperature(LST),etc.Then,use the above parameters to construct six drought monitoring models.For TVDI,choose five different vegetation indices,including Normalized Difference Vegetation Index(NDVI),Ratio Vegetation Index(RVI),Soil-Adjusted Vegetation Index,(SAVI),Modified SAVI(MSAVI)and Optimized SAVI(OSAVI)were selected,and the vegetation index with the best performance was used to participate in the construction of TVDI;Measured data synchronized with image data,compare the monitoring accuracy of different methods on the drought status in the study area,analyze the advantages and disadvantages of different methods,choose the best monitoring method,explore the drought degree and its spatial pattern in the study area,and analyze the main factors affecting the drought status Elements to provide decision-making support for maintaining the regional ecological environment.The main conclusions of this paper are as follows:1.The three monitoring models of spectroscopy,SMMI,PDI,and MPDI,have a good correlation with the measured soil moisture.With the increase of soil moisture,the three drought indexes show a decreasing trend,and there is a significant negative relationship with soil moisture.relationship.Among them,MPDI has the best fitting effect,R~2=0.4741.SMMI came in second,with R~2=0.4563.The PDI effect is the worst,R~2=0.4508.2.Among the three monitoring models based on vegetation index,VSWI has a positive correlation with soil moisture,that is,the larger the value of VSWI,the higher the volumetric water content of soil and the more humid the soil.There is a negative correlation between NDDI and soil moisture,that is,the larger the value of NDDI,the lower the soil moisture content and the drier the soil.The regular features obtained by TVDI are consistent with NDDI.Among the fitting relationships between the results obtained by the three methods and the measured soil moisture,NDDI has the worst fitting effect,with R~2of 0.2017.The fitting effect of VSWI is average,R~2is 0.3193.TVDI has the best fitting effect,with R~2of 0.52.The above results indicate that the NDDI and VSWI monitoring methods have low applicability in this study area.The model with the highest fitting accuracy in the spectroscopy method is MPDI,and the model with the highest fitting accuracy in the vegetation index-based model is TVDI.At the same time,the accuracy of TVDI is better than that of MPDI.Therefore,TVDI is the most suitable drought monitoring model for this study area.3.The regression analysis results of the TVDI models of the five preferred vegetation indices show that the determination coefficients of TVDI and soil moisture corresponding to SAVI and RVI are relatively low,both below 0.5,indicating that the TVDI corresponding to SAVI and RVI are not applicable to this study area.The coefficient of determination between TVDI and soil moisture corresponding to MSAVI is at an intermediate level,0.5078.The coefficients of determination of TVDI and soil moisture corresponding to OSAVI and NDVI are better,both reaching above 0.52.Among them,OSAVI performs best in the five vegetation indices.The TVDI model based on OSAVI is the best model for drought monitoring in the study area.4.From the results of drought grades,in the entire study area,humid areas accounted for3.42%,normal areas accounted for 3.43%,mild drought areas accounted for 9.02%,arid areas accounted for 25.86%,and severe drought areas accounted for 58.28%.The study area is dominated by severe drought areas,and the drought phenomenon is severe.The spatial distribution characteristics of drought in the study area show that the oasis is the center,extending from the inside to the outside,and the degree of drought is increasing.At the same time,the degree of drought in the oasis is lower than that in the surrounding areas and the spatial distribution of drought in the entire study area is uneven.5.The main factors affecting the spatial distribution of regional droughts include land use types,topography,climate and vegetation.Land use types have a greater impact on drought conditions.Different land use types have different drought conditions and different distribution conditions.The degree of drought is water,woodland,grassland,sand,and bare soil in descending order.Topography is one of the factors that affect the drought,but because the overall terrain of the study area is relatively flat,the impact on the drought is not obvious.Climate has a greater impact on the distribution and changes of drought,and the correlation is obvious,that is,precipitation decreases,surface temperature increases,soil moisture decreases,and drought increases.There is a clear correlation between the degree of vegetation coverage and the trend of drought changes.The smaller the vegetation coverage,the more severe the drought.