Study On Hydrological Drought Characteristics And Its Influence On Vegetation In Haihe River Basin

Based on the monthly runoff data and normalized vegetation index(NDVI)data from 31 hydrological stations in Haihe River Basin,the standardized runoff index SRI was calculated and the variation trend of runoff and SRI was analyzed.Meanwhile,a multivariate joint analysis of drought duration,intensity and intensity peak in the study area was conducted by using twodimensional and three-dimensional Copula functions.The combination probability,return period and spatial distribution of drought characteristic variables in the region were deduced.By analyzing the trend and spatial characteristics of NDVI data in the basin,the temporal and spatial change rules of vegetation were discussed.Based on the correlation analysis between SRI and NDVI,the influence and lag effect of hydrological drought on vegetation were discussed.The main conclusions of this paper are as follows:(1)The annual runoff and SRI changes in the Luanhe River Basin in the southern and northeastern parts of the Haihe River Basin show a significant decreasing trend,and there is a risk of worsening hydrological drought in this region.The severe drought occurred mainly in the central and southern part of the basin,the moderate drought occurred mainly in the southwest,and the mild drought occurred mainly in other regions.The spatial distribution characteristics of the three characteristic variables of drought duration,drought intensity and peak drought intensity were similar.The return periods were shorter in the northeastern region and longer in the southern region,which indicated that hydrological drought was prone to occur in the northeastern region,but not in the southern region.(2)The two dimensional and three dimensional joint probabilities increased from northwest and south to east.Drought events with duration or intensity or peak value exceeding the mean level were more likely to occur in northeast than southwest.The probability of co-occurrence is smaller than that of joint occurrence,and the probability of occurrence of drought events with duration and intensity exceeding the average level of the basin is relatively small.Two dimensional and three dimensional conditional probabilities are larger in Southwest China,but smaller in Northeast China.In Southwest China,when one variable exceeds the average drought level of the basin,the probability of the other two variables being smaller than the maximum of the basin is greater,while the probability in Northeast China is relatively smaller.(3)The return periods of drought events with longer duration,greater intensity and peak intensity were longer.With the increase of univariate return periods,the multi-variate joint return periods and co-occurrence return periods became longer.The two-dimensional and threedimensional joint return periods of most sites are smaller than the univariate return periods,while the co-occurrence return periods are larger than the univariate return periods.Therefore,the multivariate joint and co-occurrence return periods can be used as the threshold values to analyze the drought return periods.(4)NDVI showed a significant upward trend at higher elevations,and the vegetation coverage increased in this region,but the NDVI trend was not significant in the lower reaches of the basin and in the southwest of the basin.The NDVI of different seasons was significantly different,with the maximum value in summer and the minimum value in winter.The correlation between NDVI and SRI was strong in spring and winter,when the vegetation was susceptible to hydrological drought in the same period.At the same time,the response of vegetation to hydrological drought has a certain lag time,the lag time is 5-12 months,the lag time is relatively long in the north of the basin,the lag time is relatively short in the south.