Study On Dynamic Change Of Vegetation In Heihe River Basin Based On Wavelet Multi-scale Transformation

Based on MODIS NDVI and EVI data from 2001 to 2015,the NDVI,EVI,temperature and precipitation time series data of Heihe River Basin were multi-scale decomposed by wavelet transform in combination with temperature and precipitation data.The sequential variation and spatial differentiation of NDVI and EVI in Heihe River Basin can be analyzed by linear trend analysis method on the regional scale.The wavelet coefficients and statistics can be obtained with the wavelet transform method that selecting the optimal wavelet function and the best decomposition scale to decompose the sequential data of Heihe River Basin.Studying on the characteristics of vegetation dynamic change with the correlations and lagging response characteristics between vegetation changes and climatic factors in the Heihe River Basin,which is helpful to explore the vegetation sensitivity that responses to the climate change in Heihe River Basin and evaluate the resilience of ecosystems to global climate change.The results of this study show that:(1)The NDVI trend of the Heihe River Basin is mainly stable and unchanged,accounting for 88.68%of the total Heihe River Basin.The area of NDVI variation trend increases by 190430.4km~2;The area of NDVI variation trend decreases by6120.07km~2.The area where the trend of NDVI increases is 184310.3km~2 larger than the decreased one.The trend of NDVI in Heihe River Basin is increasing,and the vegetation status of Heihe River Basin is developing in a good direction.EVI is more suitable than NDVI for the upper reaches of the Heihe River Basin in the study on vegetation dynamic monitoring in Heihe River Basin.The trend of EVI in Heihe River Basin is basically consistent with NDVI,but the area where there is a slight decrease in the trend of EVI is 4679.5km~2 larger than NDVI.The area in which EVI in the upper reaches of the Heihe River Basin increases significantly is 1631.39km~2more than NDVI.The area of EVI reduction in the lower reaches of the Heihe River Basin is 5538.13km~2 more than the NDVI reduction area.(2)According to the wavelet transform method and the monthly scale analysis:The increase in the upper and middle reaches of the Heihe River Basin is faster than the rate of increase in NDVI,EVI reaches the peak of the time slightly above the peak of NDVI;The increasing trend and decreasing trend of EVI in the lower reaches of the Heihe River Basin are equally gentle and the trend is not obvious.The time of reaching the peak is basically the same.The time when NDVI rises in the upper reaches of the Heihe River Basin is relatively late.And the downstream NDVI peak is in August,which is later than in upstream and middle reaches.The EVI in upper,middle and lower reaches partly attains a minimum in 1,2,11 and 12 months and reaches a maximum in 6-9 months.The overall trend is the same as NDVI's.The upper reaches of the Heihe River Basin reached the maximum in August,and the middle and downstream temperatures reached their peak in July.The lowest values were in January.The maximum value of upstream,middle and downstream precipitation occurred in June.The temperature curve of the upper reaches,the lower reaches and the middle reaches of the Heihe River Basin presented a wavelike change,and the overall trend is not obvious.The overall trend is increasing as the upstream precipitation is the highest and the variation curve of the precipitation is volatile.The fluctuation of the middle and lower reaches is smaller and the trend is not obvious.(3)The correlation coefficients of NDVI and precipitation in the upper reaches,middle reaches and downstream of Heihe River Basin are 0.65,0.61 and 0.73respectively.The correlation coefficients of EVI and precipitation in upper reaches,middle reaches and lower reaches of Heihe River Basin are 0.61,0.72 and 0.65respectively.The upstream,middle and downstream NDVI of Heihe River Basin And the correlation coefficient of the EVI and the precipitation in the upper reaches of the Heihe River Basin were 0.67,0.45,0.42 respectively.The correlation among EVI,temperature and precipitation in the upper reaches of the Heihe River Basin was slightly greater than which among NDVI,temperature and precipitation,EVI is relatively sensitive to high vegetation coverage area,and is more suitable for vegetation monitoring research in the upper reaches of Heihe River Basin.The hysteresis effect of vegetation on temperature and precipitation in different areas of Heihe River Basin is different.The lag time of vegetation and precipitation in the upper reaches is 2 months in the early stage and 1 month in the early stage.The lag of the vegetation and precipitation in the middle reaches is one month earlier.The response of the downstream vegetation to the current precipitation is the most sensitive and there is no lag.The correlation coefficient between the upstream vegetation and the current temperature is the largest and without lag.The lag of the vegetation and temperature in the middle reaches is one month earlier.The response of the downstream vegetation to the temperature is not sensitive with the lag 3months.