Temporal And Spatial Variation Of Extreme Climate Indices And Its Impact On Green Cover In Yellow River Basin

The Yellow River Basin is located in the arid and semi-arid areas of northern China.This area is not only a sensitive area of climate change,but also a fragile area of ecosystem change.With the global warming,extreme climate events occur frequ ently and their intensity increases.Vegetation is easily affected by extreme climate change.Assessing the impact of extreme climate on vegetation change is therefore critical to maintaining ecological protection in the Yellow River Basin.By using the meteorological data of maximum and minimum temperature and precipitation and the remote sensing data of Net Primary Productivity(NPP)the normalised difference vegetation index(NDVI),this paper studies the temporal and spatial changes of vegetation and extreme climate,and analyzes the response between vegetation and extreme climate in the Yellow River Basin.The results demonstrated that:(1)From 1961 to 2020,the extreme low temperature indices and diurnal temperature range in the Yellow River Basin showed a significant decreasing trend,while the extreme high temperature indices,extremal indices of extreme temperature indices and growing season length showed a significant increasing trend.The change trend of extreme precipitation indices is weak,that is,it shows a weak upward trend,and the significance test is not significant.The most stations of extreme climate indices were consistent with the change trend of the overall.In terms of Mann-Kendall test,the mutation point of extreme climate indices mainly occurred in the 1980 s and 1990 s,and that of extreme precipitation indices mainly occurred in the 1910’s.Spatially,the extreme low temperature decreased greatly in the northwest,the extreme high temperature and extremal indices of extreme temperature indices increased significantly in the west of Ningxia,and the extreme precipitation indices in the southeast showed a downward trend.(2)From 1986 to 2018,the vegetation in the Yellow River basin increased significantly,and the growth rates of NPP and NDVI were 3.58 g Cm-2a-1 and 0.13%a-1.The seasonal variation of monthly mean vegetation for many years is obvious.NPP and NDVI shows "single peak" in July and August respectively.On the seasonal scale,NPP and NDVI show an increasing trend in four seasons.In NPP,the largest increase rate in summer is1.89 g Cm-2a-1.The growth rate of NDVI the largest increase rate is autumn(0.17%a-1).In the Yellow River Basin,the NPP and NDVI of land use types such as cropland,forest and grassland showed an upward trend,and the fastest growth rate of cropland in NPP was4.11 g Cm-2a-1,the fastest growth rate of forest in NDVI was 0.16%a-1.Spatially,the mean values of NPP and NDVI show a distribution pattern of high in the southeast and low in the northwest and the areas with an obvious increase in the change trend are mainly located in the Shaanxi-Gansu-Ningxia region.Therefore,the vegetation in the Yellow River Basin showed a significant improvement trend in the past 33 years.(3)Based on the analysis of the response of vegetation to extreme climate events,in the interannual variation,NPP and NDVI were negatively correlated with extreme low temperature indices,the extremely low value of extremal indices,diurnal temperature range and consecutive dry days.And NPP and NDVI were positively correlated with extreme high temperature indices,the extremely high value of extremal indices,growing season length and extreme precipitation indices.In most of the 82 stations,the NPP and NDVI of 60% of the stations was negatively correlated with extreme low temperature indices,in which NPP and extreme low temperature indices became significantly and strongly negative correlation in the west of the Yellow River basin.NPP and extreme high temperature and other indices were mainly positive correlation in the west and negative correlation in the east.NPP and NDVI were strongly correlated with extreme precipitation indices in the central and northeastern regions.In the annual variation,each month NPP and NDVI were negatively correlated with cold days and cold nights,and positively correlated with warm days,warm nights and Maximum TX.NPP and NDVI with the daily range,max 1-day precipitation amount and max 5-day precipitation some months have a positive correlation and some months have a negative correlation.In most of the 82 stations,the correlation between NPP and extreme temperature indices was similar.The correlation between NDVI and extreme temperature indices is the same in most stations from January to April and October to December,while the correlation from May to September is opposite to that in other months.NPP and NDVI are positively correlated with extreme precipitation indices from April to October,and negatively correlated with most stations from January to March,November and December.Spatial correlation between NPP and extreme climate indices in each month and sea son most stations in the western region showed a positive correlation.Time lag cross-correlation analysis and cross-wavelet methods showed that most of the responses of NPP and NDVI to extreme temperature indices in the Yellow River Basin were in the same period or lagging 6 or 7 months,while NPP and NDVI were in the same period with extreme precipitation indices.NPP and NDVI mostly have a resonance period of 8-16 months with extreme climate indices.Compared with the extreme climate events in the relevant bulletin,it is found that there is no obvious extreme climate event in the year when the vegetation grows better,and there is a lar ge drought in the previous year.When there are many extreme climat e events in a year,the vegetat ion growth in this yea r is poor.The study shows that the extreme climate events in the Yellow River Basin are increasing and the vegetation continues to improve.Low temperature and little rain are not conducive to the growth of vegetation,and the adverse effects on vegetation lag for about half a year;appropriate warming and precipitation promote the growth of vegetation in the same period.This study is expected to provide a basis for ecological environment and the sustainable development of the Yellow River Basin.