Spatial And Temporal Pattern Changes Of Vegetation And Water Resources And Their Correlation With Climate In Tibet From 2000 To 2020

Climate is an important environmental factor that affects the changes of vegetation and water resources all the time.As a sensitive area,vegetation and water bodies are important and major components of grassland ecosystems and wetland ecosystems in the Tibetan Plateau region,and we need to keep an eye on vegetation and water body changes and their relationship with climate at all times.The area of Tibet Autonomous Region occupies half of the Qinghai-Tibet Plateau,and the Tibet Autonomous Region is chosen as the study area.In this study,the MODIS(MOD09A1)reflectance products of TERRA remote sensing satellite are used to extract the infrared band,near-infrared band and green light band,and construct NDVI and NDWI indices to quantitatively describe the vegetation and water bodies to reflect the changes of vegetation and water resources.The spatial and temporal distribution characteristics of NDVI and NDWI in Tibet from 2000 to 2020 and their change trends are studied,and their relationships with precipitation and temperature are analyzed.It is mainly found that NDVI and NDWI in different regions have different degrees of response to climate,which can improve the understanding of the ecological impact of climate change and provide reference values and significance for ecological environmental protection in alpine regions.The main conclusions of this study are as follows.(1)In terms of the characteristics of NDVI changes during the growing season and NDWI changes during the season of abundant precipitation.The trend of vegetation cover in Tibetan region has roughly experienced a sharp increase,a sharp decrease and a small fluctuation during 2000-2020:2000-2002 was a period of significant increase in vegetation cover.2003-2009 was characterized by a small change and was a relatively stable period,with the NDVI index fluctuating up and down around the value of 0.33.2010-2011 was a period of sharp increase in vegetation cover,with the NDVI index rising to 0.34.From 2011 to 2014,the vegetation cover in Tibetan region experienced a continuous decrease,and the NDVI value decreased from0.34 to 0.32.From 2014 to 2020,the vegetation cover continued to increase,and in2020 it increased to the highest NDVI value of 0.36 during the study period.The NDWI of water resources in Tibet showed small fluctuations from 2000-2005 and 2011-2020,fluctuating around the NDWI values of 0.13 and 0.18,respectively.2005-2006 was a period of sharp changes,with the NDWI changing from 0.12 to 0.28.The following years showed small fluctuations,but in 2010-2011 the NDWI values decreased sharply.In the following years there were small fluctuations,but in 2010-2011 the NDWI value dropped sharply,from 0.27 to 0.18.(2)In the analysis of the spatial variation of vegetation NDVI and water resources NDWI.The trends were classified into three categories according to the significance test results:slight degradation(?-0.0005,?1.96),stable and unchanged(-0.0005??0.0005,?1.96),slight improvement(?0.0005,?1.96),and significant improvement(?0.0005,?1.96).The ratio of slightly degraded area of vegetation covers 23.4%of Tibetan area,mainly distributed in the eastern part of Nagqu city.There are seasonal and spatial differences in the response levels of different vegetation types to temperature and precipitation.27.2%of the area ratio of NDVI is stable and unchanged,and 29.9%of the area ratio of slight improvement.The ratio of area with significant improvement was 19.6%,which was mainly concentrated in the northern part of Ali region and Nagqu city.The trend of NDWI in the whole region during the growing season showed that the improved part was greater than the degraded performance,which was scattered in the Tibet Autonomous Region.The statistical results in Table 1 show that the proportion of NDWI change area is 13.8%for slightly degraded area,54.6%for stable unchanged area,19.3%for slightly improved area,and12.3%for obviously improved area.The stable unchanged area accounted for the largest proportion,and the proportion of improvement was greater than the proportion of degradation.(3)In terms of spatial and temporal changes in the distribution of precipitation during the growing season in terms of temperature and precipitation abundance.The distribution of spatial pattern changes of precipitation,the precipitation near the Himalayas within the range of Ali region and Shigatse region is a decreasing trend,and the precipitation in most areas of Changdu city shows a decreasing trend.Within the Ali region,the increasing trend of precipitation in the southeast direction is more significant.Precipitation is more increased in the eastern part of the city of Shigatse.Precipitation in the central part of Nagqu,the south-central part of Lhasa,the whole Shannan city and most of the whole Linzhi city show an increasing trend.Within the Tibetan region,the area with increasing trend of precipitation is 950,000 KM~2,and the area with decreasing trend of precipitation accounts for 220,000 KM~2.The distribution of the changes in the spatial pattern of temperature shows that most of the areas showing an increasing trend in temperature are concentrated in the southeast direction of Ali region and the central and southern areas of Nagqu city.In the whole area of Lhasa city,the temperature trend shows an increasing trend.In the eastern part of the Rizkazar city area and close to Lhasa city,the temperature also shows an increasing trend.Most of the areas in the western direction of Shigatse show a decreasing trend.Some areas in Shannan and Linzhi are decreasing in temperature,while a few areas near Lhasa show a slight increase in temperature.The temperature in the northwest,northeast and southeast of Changdu City shows an increasing trend,while the temperature in the northwest of the city shows a decreasing trend.In the Tibet Autonomous Region,the area showing an increasing trend of temperature is 1.15million KM~2,and the area showing a decreasing trend of temperature is 31,000 KM~2.(4)The correlation analysis of NDVI with precipitation and temperature.The distribution of correlation coefficients between NDVI of vegetation and monthly average precipitation in the growing season,in the whole Tibetan region,the distribution of areas with negative correlation between vegetation and precipitation,except for some sporadic distribution,is more concentrated in the eastern part of Nagqu city,most of Changdu city,and the southeast of Ali region,totaling 320,000 KM~2,accounting for 41%of the whole study area.The positive correlation areas are mainly concentrated in the northern part of Ali and Nagqu cities,the southern part of Shannan and Linzhi cities,and the western part of Shigatse city,with a total of 450,000 KM~2 of positive correlation between NDVI and precipitation in Tibet,accounting for 59%of the whole area.The correlation coefficients of NDVI and monthly mean temperature of vegetation in the growing season are distributed,and there are very obvious regional differences in the correlation between NDVI and temperature,with large areas of negative correlation distributed in the central,western and south-central parts of Tibetan region,with a total area of 430,000 KM~2,accounting for 51%of the whole study area.The positively correlated areas are also more concentrated and distributed in the northern part of Ali region and Nagqu city,with an area of 420,000 KM~2,accounting for 49%of the area of Tibetan region.(5)Regarding the correlation analysis of NDWI with precipitation and temperature,the distribution of the partial correlation coefficients between NDWI and precipitation,the negatively correlated areas are concentrated in the eastern end of Nagqu City and Linzhi City,and the rest are scattered in the Tibetan region.280,000KM~2 are negatively correlated with precipitation,accounting for 37%of the area.The positive correlation is concentrated in the northernmost part of Ali and Nagqu,and the northwestern part of Shigatse,and the positive correlation between NDWI and precipitation in the whole study area is 490,000 KM~2,accounting for 63%.The positive correlation between NDWI and temperature is concentrated in the central and western part of the Tibetan region,the central part of the Ali region,the central part of the Nagqu city,the central and eastern part of the Shigatse city,and the northern part of the Shigatse city.The negative correlation between NDWI and temperature is concentrated in the central and western part of the whole Tibetan region,Ali region,the central part of Nagqu city,the central-eastern part of Shigatse city and the north-central part of Shannan city.The areas with negative correlation with temperature account for a total of 390,000 KM~2,accounting for 48%.Due to the differences of climate and geographical environment in Tibet,the distribution of correlation coefficients between NDVI and NDWI and climate also indirectly shows that vegetation and water resources in different regions have different degrees of correlation to climate.