Study Of NDVI And Its Relationship With Precipitation Over The Lancang-mekong River Basin From 2000 To 2017

The Lancang-Mekong River Basin(LMRB)flows through China,Vietnam,Myanmar,Thailand,Cambodia and Laos,and plays an important role on agriculture,energy,manufacturing,food security and ecosystems of these six countries.Therefore,in this paper the correlation between vegetation and precipitation in LMRB was studied,which is conducive to grasping the characteristics of the regional ecological environment changes and its response to precipitation.Furthermore,it is of great significance for monitoring the stability of LMRB,early warning of disasters,and Mitigating degradation processes.Based on MOD13A3-NDVI and ERA-Interim daily precipitation data from 2000 to 2017,three aspects were studied in this paper:(1)the spatio-temporal changes of vegetation coverage and precipitation,(2)the correlation between vegetation and precipitation of LMRB,(3)the time lag effect of response of NDVI and precipitation.The maximum value composites(MVC),mean value method,trend line analysis,and correlation analysis were applied in this paper.The main conclusions are as follows.(1)In this paper,the spatio-temporal change characteristics of NDVI in LMRB during the 18 years were studied.The areas with NDVI value greater than 0.4 account for 96.58%,so the vegetation coverage in LMRB is generally very good.The distribution of NDVI in the upper basin shows typical latitudinal zonality,while in the lower basin it is distributed according to landforms.The vegetation coverage in more than 90% of LMRB is basically stable.However,significant degraded areas and improved areas of vegetation mainly occur in the lower basin.The vegetation growth of LMRB has significant seasonal characteristics.The vegetation coverage rate is higher in summer and autumn,while lower in spring and winter.The NDVI peak value of the upper basin appears in August,while it appears in October in the lower basin,so it is delayed for nearly two months in the lower basin.And NDVI increased most in July,August and September during the growing season.(2)The spatio-temporal change characteristics of precipitation in LMRB during the 18 years were also studied.The precipitation in LMRB shows a downward trend.From the central to the north and south,the annual precipitation decreases gradually.The strong precipitation center is located at the junction of Yunnan,Myanmar,Laos and Thailand.PCI gradually increased from south to north of LMRB,and the difference of monthly distribution of annual precipitation is gradually narrowing.The precipitation centroid alternates ahead and delays along the northwest and southeast.The rainy season in the upper basin is from May to September,while it is from April to October in the lower basin.The precipitation decreased in spring and summer,while that increased in autumn and winter.During the 18 years,the monthly distribution of precipitation in LMRB gradually became more balanced,and the rainy season is delayed to varying degrees.(3)The correlation between vegetation and precipitation as well as the time lag effect were studied.The proportion of the area where NDVImax is positively correlated with annual precipitation in LMRB is 57.16%.The area where NDVImax is negatively correlated with PCI is relatively large.In 61.91% of LMRB,vegetation was positively correlated with PC.There is a significant positive correlation between annual precipitation and NDVI in grassland and woodland areas,while there is a significant negative correlation in savanna and cultivated land areas.The delay of precipitation centroid is conducive to the increase of NDVImax in forest and dry land areas,while it is more favorable for the growth of NDVImax in grassland and savannah areas when precipitation centroid advanced.The correlation coefficient between NDVImax and precipitation was 0.504 during the 18 years,and there was a significant positive correlation between NDVImax and precipitation.NDVI of grassland is sensitive to seasonal precipitation,while NDVI of woodland,wetland and cultivated land is less sensitive to precipitation.NDVI of spring and summer has the highest correlation with precipitation in the same period,NDVI of autumn is more sensitive to precipitation in two months ahead,and NDVI of winter has higher correlation with precipitation in the same period and three months ahead.