TEMPORAL AND SPATIAL CHANGES OF DROUGHT IN BEIJING-TIANJIN-HEBEI REGION BASED ON REMOTE SENSING TECHNOLOGY

Drought is an extremely complex natural disaster phenomenon. Sustained drought will lead to the aggravation of water shortage, food production reduction, land desertification and ecological crisis, which will have a great impact on social and economic development, industrial and agricultural production and ecological environment. In recent years, human activities have intensified, the global climate has been warming, and the frequency and intensity of extreme events such as drought have been continuously strengthened, which has been widely concerned by scholars at home and abroad. The Beijing-Tianjin-Hebei region, as the political and cultural center of China, is seriously affected by the drought, which has a certain impact on the social economy of the region. Therefore, it is of great practical significance to carry out remote sensing drought monitoring and understand the temporal and spatial characteristics of drought. Based on MOD13A2 and MOD11A2 in MODIS products, this paper constructed the NDVI-Ts feature space to obtain the Temperature Vegetation Dryness Index (TVDI) in Beijing-Tianjin-Hebei region, and analyzed the spatial and temporal variation characteristics of drought in Beijing-Tianjin-Hebei region from 2001 to 2014 with TVDI as the drought index. The results showed that: (1) the NDVI-Ts feature space in the Beijing-Tianjin-Hebei region showed that when the value of NDVI rose from 0 to 1, the maximum temperature of the surface gradually decreased, while the minimum temperature gradually rose. Both of them had a linear relationship with NDVI, and their spatial shape was similar to a triangle, indicating that the study of drought in Beijing-Tianjin-Hebei was effective by using this spatial feature; (2) the Beijing-Tianjin-Hebei region experienced drought every year in the period of 2001-2014, and the drought was more serious in 2001, 2004, 2008, 2009, 2011 and 2012; (3) The drought in the Beijing-Tianjin-Hebei region was widespread, and the severely arid regions were mainly concentrated in the southern and central regions. In terms of drought intensity, the most frequent occurrences in the period of 2001-2014 were Xingtai, Hengshui, Shijiazhuang, and Handan, and the annual drought in 14 years; the least number of occurrences was Chengde, a total of 4 times; the number of occurrences was second only to Chengde, Tangshan, Zhangjiakou, Qinhuangdao, a total of 7 times. Our results indicated that the Beijing-Tianjin-Hebei region was prone to drought and the drought degree was severe. * Corresponding author The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W10, 2020 International Conference on Geomatics in the Big Data Era (ICGBD), 15–17 November 2019, Guilin, Guangxi, China This contribution has been peer-reviewed. https://doi.org/10.5194/isprs-archives-XLII-3-W10-747-2020 | © Authors 2020. CC BY 4.0 License. 747


INTRODUCTION
As one of the most complicated climate disasters that have the most serious impact on human society, drought has the characteristics of high frequency, long duration and wide range of influence, which is an important problem that plagues the development of today's society (Liu et al, 2019). As the main producing area of wheat in China, the Beijing-Tianjin-Hebei region is a temperate monsoon climate with uneven annual precipitation distribution, increased demand for water resources and increased pollution, resulting in frequent drought disasters in the Beijing-Tianjin-Hebei region, seriously affecting residents' living, social and economic development. Environmental improvements have also caused economic losses to some extent (Nait-Ali et al, 2017;Centritto et al, 2011). Therefore, the analysis of the spatial and temporal changes of drought in the Beijing-Tianjin-Hebei region is of great significance for alleviating the loss of drought disasters, rationally planning limited water resources and ensuring its economic and social development.
Goward and Hope used NOAA-AVHRR data to find that the Normalized Difference Vegetation Index (NDVI) and Surface Temperature (Ts) vary with soil moisture (Goward, Hope, 1989); Price and Carlson et al. The scatter plots showing NDVI as the abscissa and Ts as the ordinate are generally triangular (Price, 1990;Carlson et al, 1994); Nemani, Moran, etc. further discover that the feature space of Ts-NDVI is trapezoidal (Nemani et al, 1993;Moran et al, 1994); eventually Sandholt et al. The Temperature Vegetation Dryness Index (TVDI) was proposed and used as an important indicator for monitoring drought by remote sensing (Sandholt et al, 2002). TVDI has been widely used as soon as it is proposed. Qi et al. used

Overview of the study area
The Beijing-Tianjin-Hebei region is China's "Capital Economic Circle", located in the northern part of the North China Plain, between 36°05'~42°37'N, 113°27'~119°50'E ( Fig. 1). Tangshan 5 Qinhuangdao 6Handan 7 Xingtai 8 Baoding 9 Zhangjiakou 10 Chengde 11 Zhangzhou 12 Langfang 13 Hengshui) The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W10, 2020 International Conference on Geomatics in the Big Data Era (ICGBD), 15-17 November 2019, Guilin, Guangxi, China It is bordered by the Yanshan Mountains in the north, the North China Plain in the south, the Taihang Mountain in the west, and the Bohai Bay in the east. The terrain is higher in the northwest and north, and the terrain is flatter in the south and east. From the northwest, the Yanshan-Taihangshan mountain system gradually transitions to the plain in the southeast, showing the topographical features of the northwest high and southeast low (Zhang et al, 2017). From the perspective of geomorphology, the area contains a variety of geomorphological features, but still dominated by plain landforms, along the coast of the shoal and wetlands. The Haihe River Basin spreads in the form of fan-shaped water system in the Beijing-Tianjin-Hebei region.
The climate is typical of temperate semi-humid and semi-arid continental climate, and the spatial distribution of annual precipitation is affected by factors such as atmospheric circulation and geographical regions. The distribution is uneven, mainly due to the large precipitation on the side of Yanshan and Taihang Mountain. The precipitation in the plain area is generally higher than that in the dam area, and the precipitation in the east is significantly larger than that in the west. format is HDF and the spatial resolution is 1km.

Data preprocessing:
(1) Due to the large amount of data in this study, the data is processed in batch processing. First, the MRT toolbox is used to perform format conversion, projection coordinate conversion, and the like on the downloaded data MOD13A2 and MOD11A2, and the coordinates used in the study are the WGS84 coordinate system. In ENVI 5.3, the administrative vector boundaries of the Beijing-Tianjin-Hebei region are used to tailor the data required for the study area. (2) In the interannual variation analysis, this study averages the NDVI and LST data from April to October every year to prevent image outliers from affecting the processing results.

Research methods
The Temperature Vegetation Drought Index (TVDI) was first proposed by Sandholt. By constructing the NDVI-Ts feature space, the dry-wet edge equation is calculated, and finally TVDI ( Fig. 2) is obtained. The NDVI-Ts feature space is approximated as a triangle (Sandholt et al, 2002). The specific formula is: TVDI= Ts-Ts min Ts max -Ts min (1) Ts min =a 1 +b 1 ×NDVI,Ts max =a 2 +b 2 ×NDVI (2) Where: Ts represents the surface temperature of any pixel; represents the corresponding wet edge; represents the corresponding dry edge; represents the coefficient of the fit equation of the dry and wet edges in the feature space, respectively. TVDI=1 on the dry side and TVDI=0 (Sun et al, 2014) on the wet side.
When calculating TVDI, we can determine the dry and wet edges based on the NDVI value of the pixel, and then determine the TVDI value by the location of the pixel's surface temperature in the feature space (Liu et al, 2013). The closer TVDI is to 0, the higher the soil moisture and the lower the degree of drought; the closer TVDI is to l, the lower the soil moisture and the higher the degree of drought .  (Table 1), which better reflects the local drought conditions (Wang et al, 2013). Based on this, this study will sample the partitioning criteria in subsequent analysis.

Analysis of the characteristics of the arid space in
Beijing-Tianjin: Global warming has been established in the past 100 years (Bachmair et al, 2016;Nail, 2018;Wilhite, 2000).
Under this background, the aridification trend in the region is becoming more and more intense, and the aridification in year by year, indicating that the drought is in the Beijing-Tianjin-Hebei region. The impact is mitigating. The terrain in the north of Chengde is descending from the northwest to the southeast, and the northwest is located in the Inner Mongolia Plateau. Therefore, the climate varies greatly from north to south, and the meteorological elements are three-dimensionally distributed, making the climate diverse.
The summer is mild and thunderstorm; the autumn is cool, the temperature difference between day and night is large, the frost damage is heavy, the summer is cool, the rainfall is concentrated, and there is basically no hot period, so there is basically no drought; the border of southeast of Qinhuangdao in Tangshan, Tianjin, low terrain, Yanhai, Most wetlands, so severe droughts rarely occur.
Understanding the characteristics of regional drought is a multidisciplinary research process. It is a natural process of