| Soil moisture is an important factor of material and energy flow and interconnection in a certain range formed by soil,vegetation and atmosphere.It has a strong influence on the evapotranspiration and transport of surface soil water and the cycling of carbon and nitrogen elements.Soil moisture can determine the growth state and distribution pattern of vegetation in an ecosystem in a region.Soil moisture is one of the indispensable indexes and parameters in many fields such as hydrology,ecology and agriculture.The distribution of soil moisture in the topsoil is affected by many factors such as precipitation and topography,so it shows the characteristics of uneven spatial distribution.With the help of remote sensing,it becomes a new breakthrough point to acquire regional soil moisture distribution characteristics in a large range and rapidly.This paper takes the Qinghai-Tibet Plateau as the study area and uses the arid vegetation index method to retrieve soil moisture.Firstly,the MOD11A1 data from March 2000 to February 2020 were compensated by the historical mean method and the spatial distribution pattern of LST over the Qinghai-Tibet Plateau was analyzed.After that,Normalized Difference Vegetation Index(NDVI)of the Qinghai-Tibet Plateau from 2000 to 2019 was analyzed by means of spatial trend analysis based on MOD09 GA data of the same period.Spatial distribution of normalized vegetation index(NDVI).Then,NDVI and LST were combined to obtain the monthly dry and wet edge fitting equations and fitting coefficients of the Qinghai-Tibet Plateau from March 2000 to February 2020.Then,spatial trend analysis and future trend analysis were used to analyze the TVDI(Temperature Vegetation)of the whole Qinghai-Tibet Plateau from 2000 to 2019 Spatial distribution characteristics of temperature and vegetation Dryness Index.Finally,soil moisture of the Qinghai-Tibet Plateau was retrieved based on the measured soil moisture data and TVDI distribution results,and the inversion results were verified by combining the mean relative error and root mean square error of evaluation parameters.There is a positive linear correlation between the soil moisture obtained from inversion and the soil moisture used for verification.The results showed that:(1)For the MOD11A1 land surface temperature product data of the Qinghai-Tibet Plateau,the historical average method can compensate the pixel value of the remote sensing image without pixel relatively close to the real situation.After using the historical average method to compensate the data,it is found that the effective utilization rate of pixels has been greatly improved,the image information basically reflects the real ground object information,and the compensation results meet the needs of remote sensing images.It shows that this method has strong adaptability and rationality.(2)The statistical data showed that the average NDVI of the four seasons from 2000 to 2019 presented summer > autumn > spring > winter,and the vegetation cover change of each season was significantly different.Among the vegetation cover changes over the Qinghai-Tibet Plateau in the past 20 years,the improved vegetation accounted for 30.79%,the degraded vegetation accounted for1.74%,and the stable vegetation accounted for 67.47%.This indicates that the vegetation coverage of the Qinghai-Tibet Plateau has been stable in the past 20 years,and the vegetation coverage of some areas has improved.(3)The spatial variation trend of TVDI over the Qinghai-Tibet Plateau in the past 20 years was mainly not significantly increased,accounting for 32.77% of the total pixel,and not significantly decreased for 30.61%.In the future trend analysis based on Hurst index,it is found that the average value of Hurst index is 0.4108,and the area between 0 and 0.5 of Hurst accounts for 85.14%.The results indicate that the variation trend of vegetation drought over the Tibetan Plateau in the next 10 years is generally opposite to that during 2000-2019.Combined with the previous TVDI variation trend,it is easy to find that the vegetation drought index of the Qinghai-Tibet Plateau in China will show a trend of a large area reduction in the future.(4)Using the measured data of the site combined with the TVDI results to construct the model to perform the soil moisture distribution pattern of the Qinghai-Tibet Plateau.It was found that the soil moisture results of the inversion and the measured soil moisture of the verification site had a high correlation,and the correlation coefficient R2 reached 0.7728.The results showed that the TVDI combined with the measured data of the station can accurately reflect the distribution pattern of soil water in the Qinghai-Tibet Plateau.On the time scale,the distribution pattern of in winter > spring>summer >autumn,in the surface soil water on the Qinghai-Tibet Plateau was presented.The spatial distribution pattern showed that the northern Qaidam Basin was the low value region of the Plateau soil moisture and showed obvious zonal distribution characteristics in the southeast and southwest of the Plateau.The high value of surface soil moisture mainly occurred in the great bend of Yarlung Zangbo River and the Amir Plateau in the northwest of the Plateau. |
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