| Drought is one of the most serious meteorological disasters in Tibet,which occurs to different degrees every year.It has a great impact on agricultural and animal husbandry production.As an important ecological barrier in China,the quality of the ecological environment in northern Tibet is not only related to the social and economic development of the region,but also plays a crucial role in maintaining the quality of the ecological environment in the middle and lower reaches of the Yangtze River.Therefore,it is of great practical significance to find an objective,dynamic and real-time drought monitoring method for northern Tibet.In the way,the rules and trend of the spatio-temporal change of drought could be explored precisely.This has very important practical significance for accurate drought monitoring and disaster prevention and reduction in northern Tibet.Based on 16 d synthetic MODIS NDVI/EVI and LST data from March to August of 2002to 2017,this paper calculated the TVDI and analyzed the spatial and temporal change characteristics of drought in northern Tibet.Based on the percentage of monthly precipitation anomaly and the TVDI index,by using the fuzzy mathematical method,the the monitoring standard of remote sensing drought level in northern Tibet was established.Then,the temporal and spatial distribution characteristics of the drought level intensity in this area were analyzed.In the end,combining meteorological data and MODIS TVDI,a linear regression model for predicting drought in the north of Tibet was established.The main conclusions are as follows:(1)The TVDI index calculated based on the product data of MODIS vegetation index(NDVI and EVI)and surface temperature(LST)has a good correlation with the measured soil moisture content at a depth of 20 cm.Among them,the TVDI value calculated by enhanced vegetation index(EVI)is better than that by normalized vegetation index(NDVI).During the early period of plant growth from March to May,there was a positive correlation between LST and EVI.In the circumstance,TVDI was not applicable to drought monitoring in northern Tibet.From June to August,LST and EVI were negatively correlated,and the TVDI_E index could be calculated to monitor the summer drought in northern Tibet.The calculation results of TVDI_E from 2002 to 2017 showed that,the drought in northern Tibet was the most severe in 2009 and the least severe in 2012.During the year,the drought gradually worsened from the beginning of June to the end of the month.Then,it gradually eased from the early July.The average annual low value of TVDI_E in the study area was mainly distributed in the sub-humid climate region.It located in the southeast and northern region,as well as in the mountain and river valleys in the western region.High value areas were widely distributed in the middle and western regions of the study area.(2)By using fuzzy mathematics and referring to the classification standard of meteorological drought based on precipitation anomaly rate,the classification standard of drought based on MODIS TVDI_E can be established in northern Tibet.Considering the differences in climate zones,the classification criteria for the five grades of non-drought,mild drought,moderate drought,severe drought and special drought in the sub-humid zone were(0?0.37),(0.37?0.55),(0.55?0.69),(0.69?0.83),(0.83?1).The corresponding standards for semi-arid areas were(0?0.52),(0.52?0.71),(0.71?0.82),(0.82?0.91),(0.91?1).The comparison of the results of remote sensing and meteorological indicator drought level monitoring showed that the monitoring results of the two indicators were mostly consistent.The grade difference was mostly 1,while 2 or more difference was rarely.It showed that this standard could be used effectively for monitoring the drought degree in the region.The monitoring results of drought degree in northern Tibet from 2002 to 2017showed that,the summer drought is mainly lightly and moderately in recent years,while the severe and extrem drought is relatively infrequent.The areas with high frequency of moderate drought and above were mainly in the southwest and central part of the study area.(3)MODIS TVDI_E from June to August in northern Tibet has a significant negative correlation with the preliminary precipitation(the first 8 days to 48 days).While has a significant positive correlation with the average temperature.Then,the preliminary precipitation and temperature indexes which have the greatest correlation with TVDI value could be selected.On this basis,the linear regression models of annual summer drought TVDI prediction in this area can be established.To verify the accuracy of the models,using the GPM satellite precipitation data and CPC global daily temperature data to calculate the predicted TVDI value,whch are compared with the actual MODIS TVDI value.The results show that,about 90%of the difference between the predicted value and the actual value range from-0.1 to0.1.It indicates that the overall fitting effect of the model is good.Further more,the prediction results of drought degree by the model are verified by the observations data of nine national meteorological stations.The results of the comparison show that,the predicted drought level of TVDI is highly consistent with the observed data.The results further reveal that the model can be used to predict the summer drought level in northern Tibet. |
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