| Based on the comprehensive meteorological drought index(CI)calculated from 97 meteorological stations and the MODIS enhanced vegetation index(EVI)data from 2000 to 2017 in the Dongting Lake Basin,we analyzed spatial and temporal variations of drought intensity and EVI during growing seasons(from April to October).We also analyzed spatial distributions of drought intensity and EVI in spring,summer and autumn of typical seasonal dry and wet represent years as well as the correlation between drought intensity and EVI.Combined with the vegetation type data were applied to analyze the spatial and temporal patterns of the EVI(natural and artificial vegetation)and extreme drought intensity during growing seasons over the recent 18 years and to examine the sensitivity of natural vegetation and artificial vegetation to extreme drought.We aimed to answer the question whether variations in EVI can reveal large-scale meteorological drought events,and to explore the potential of EVI as an indicator of dry and wet conditions in this basin,and in order to provide a scientific guidance for regional agricultural development and ecological protection.The results showed that:(1)The lowest value of EVI occurred in 2001,2005 and 2011,while the highest value of drought intensity also appeared in these three years.The variation trends of drought intensity and drought stations ratio in spring,summer and autumn were consistent,the maximum drought intensity in spring,summer and autumn appeared in 2011,2013 and 2009,respectively.For drought intensity,high values were mainly distributed in summer and autumn,while low values were present in winter and spring.For EVI,the maximum and minimum values were present in summer and winter,respectively.The drought intensity,EVI and forest coverage that have been corrected for inter-annual and seasonal effects during the growing seasonshowed significant inter-annual fluctuations.The year with high drought intensity was consistent with that with low forest coverage.(2)In spatial,the extreme drought intensity during growing season was ranked as that in autumn > in summer > in spring,and the hengshao area was the high value area of drought intensity in all seasons.The EVI varied seasonally in an order of that in summer > in spring > in autumn.EVI value is higher in the western and eastern edge of the watershed,and lower in the middle.The EVI frequency chart was appeared “unimodal structure”.On the whole,the change trend of vegetation mainly increased,only the summer showed the greatest trend of vegetation decreases.The increasing trend of vegetation in spring,summer and autumn accounted for 71.45%,48.73% and71.56% of the watershed respectively.The seasonal vegetation with higher coefficient of variation were mostly distributed in Dongting lake plain.(3)The spatial distribution of high drought intensity matched well with that of low EVI in typical seasonal drought represent years.The correlation coefficients between drought intensity and EVI in spring,summer and autumn of dry represent years were statistically significant(P<0.1,0.01 and0.1 for spring,summer and autumn,respectively).The spatial distribution of drought intensity and EVI anomalies did not match in spring,summer and autumn in typical seasonal wet represent years.No significant correlations were found between drought intensity and EVI anomalies during these time periods.Our results suggested that growth of vegetation can well reflect large-scale meteorological drought in the Dongting Lake Basin.(4)According to the high-resolution vegetation type data and combined with the cultivation practice in this basin,the vegetation in was divided into natural vegetation and artificial vegetation.The results show that the inter-annually maximum value of extreme drought intensity and the minimum value of EVI for both natural vegetation and artificial vegetation regions appeared in 2011.The maximum EVI of natural vegetation occurredin July,while the maximum EVI of artificial vegetation occurred in August.The sensitivity of artificial vegetation to extreme drought was higher than that of natural vegetation.However,the sensitivity of both vegetation types to extreme drought varied with vegetation growth stage,with the most significant correlation between the EVI and the extreme drought intensity in August.The extreme drought intensity and EVI can depict well the spatio-temporal evolutions of drought in spring,summer-autumn in 2011 and summer drought in 2013.(5)The multi factor linear regression model of EVI with temperature and precipitation of different vegetation types was better,climate change could explain 45% and 41% of EVI changes of natural and artificial vegetation.The correlation coefficients and partial correlation coefficients of natural and artificial vegetation EVI time series with temperature were statistically significant at p values of 0.01 in spring and autumn.By contrast,artificial vegetation was more sensitive to hydrothermal conditions.There was time lag in the response of natural and artificial vegetation EVI to precipitation in the growing season,and the lag time of both the two to precipitation amount is about 1 month. |
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