Study On Spatio-Temporal Variations Of Drought In China Based On Remote Sensing

The fundamental goal of this thesis was investigating the spatio-temporal variability of vegetation cover and their relations with precipitation and temperature over China using NOAA/AVHRR data during the 20 years period from 1982-2001, and the value of NDVI was taken as a tool for drought monitoring.For this research Microsoft Excel, SPSS ver.10.0, ERDAS ver.9.1 and ArcGIS ver.9.2 environments were used to manage and manipulate thematic data, and to process satellite images and tabular data. And three kind’s data were used:- NDVI data were obtained from the Global Inventory Monitoring and Modeling Studies (GIMMS) group, derived from the NOAA/AVHRR land data set, at a spatial resolution of 8 km and taken at 15-day intervals, for the period January 1982 to December 2001.- Meteorological data were obtained from Global Historical Climatology Network GHCN V2 data in National Climatic Data Center (NCDC). Monthly mean temperature (204 images for the 17 years 1982-1998) and monthly precipitation (240 images for the 20 years 1982-2001); which were generated from 680 climate stations distributed across China.- DEM data-SRTM DEM with spatial resolution 90m (64 images), were downloading from:http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp1- Lagged correlation analysis was used to study the effects of vegetation cover on climate in Summer in different zones of China. The results show that there are positive correlations between Normalized Difference Vegetation Index (NDVI) in Spring and climate (temperature and precipitation) in Summer in most zones of China, and the lagged correlations show a significant difference among geographical zones. The stronger correlations between NDVI in previous seasons (Winter and Spring) and climate in Summer occur in three zones (Mid-temperate zone, Warm-temperate zone and Plateau climate zone), and this implies that vegetation changes have more sensitive 2- Lagged correlation analysis and statistical analysis were used to investigate the relationships between NDVI and climate factors (temperature and precipitation) in six climatic zones of China, and also investigate the relationships between NDVI and climate conditions in different sub-regions.The results show that there are positive correlations between NDVI and climatic factors in most zones of China; and the more significant correlations occur in three zones (Plateau climate zone, Warm-temperate zone and Mid-temperate zone). Both of the temperature and precipitation are important to the vegetation change in all zones, but the temperature influence is slightly larger than the precipitation; while precipitation plays key role in Plateau climate zone. The highest correlation between NDVI and temperature with value 0.929 occurs in subtropical zone; while the highest correlation between NDVI and precipitation with value 0.889 occurs in Mid-temperate zone, which indicates the sensitivity of vegetation cover to precipitation in the arid and semi-arid zones of north China.In different sub-regions, temperature and precipitation play an important role to the changes of vegetation cover especially in low mountain area (200-1000m). The temperature dominates in most of sub-regions while precipitation plays a more important role in high mountainous area (>3000m) as in Plateau climate zone. Furthermore, the time series of NDVI demonstrate a positive trend from 1982 to 2001, which means that the vegetation biomass present on land surface is increasing. This trend is strongly correlated to increased precipitation and temperature from 1982 to 2001.3- NDVI was used to investigate the vegetation variations in China during the period 1982-2001. And GIS was used to examine the relationship between precipitation and the NDVI in China, and the value of NDVI was taken as a tool for drought monitoring. The results show that in the study period, China’s vegetation cover had tended to increase, compared with that in the early 1980s. The agricultural regions (He Nan, He Bei, An Hui and Shandong) and the west of China marked by increasing, while the eastern coastal regions marked by decreasing. The correlation between monthly NDVI and monthly precipitation/temperature in the period 1982-2001 is significantly positive (R2=0.80, R2=0.84 respectively); indicating the close coupling between climate conditions (precipitation and temperature) and land surface response patterns over China.4- Climate-related increases in terrestrial vegetation activity in the northern regions of the Northern Hemisphere have been identified by recent satellite based studies. However, evidence for this increase from ground observations is very limited. A time series NDVI for the growing season (April to October) from 1982 to 2001, along with historical climate data, were used to analyze year to year variations of vegetation activity and to explore the relationship between the NDVI and climatic variables in growing season. The magnitude of the mean NDVI in growing season for the 1980s and the 1990s is not significantly different. The increase in NDVI corresponds to an increase in temperature on the national scale, while regional variations in NDVI appear to be related to precipitation. The NDVI trend shows a large spatial heterogeneity, possibly associated with changes in regional climate, land use and vegetation types. Our study suggests that agricultural practices caused an increase in NDVI in some regions, and rapid urbanization on the east coast resulted in a sharp decrease in NDVI since the 1980s.5- NDVI Anomaly Index and Drought Index were used to express the agricultural drought and meteorological drought, respectively. On the basis of the precipitation and temperature data and NDVI data of AVHRR from 1982 to 2001 in China, the values of Drought Index and NDVI Anomaly Index were calculated. Examination of NDVI time series reveals two periods; (a) 1982-1989, marked by low values below average NDVI and persistence of agricultural drought with a significant large-scale drought during the 1982 and 1989; and (b) 1990-2001, marked by a wetter trend with region-wide high values above average NDVI and a maximum level occurred in 1994 and 1998. The meteorological drought was different from year to year but so clearly in western, northern and northeastern of China (Xin Jiang, Inner Mongolia, Gan Su, Hei long jiang, Ji lin and Liao ning).