Spatio-temporal Variability Of Dryness And Wetness And Its Response To Climate And Land Surface Changes

Faced with high risk of dryness/wetness change,the regional and global water cycle process is constantly affected by climate change and land surface mutual feed system.Based on the standardized wetness index(SWI)which accounted for the joint effects of climate variability and land surface change,this article comprehensively evaluated the applicability of SWI and the spatio-temporal characteristics of dryness/wetness changes in China.The joint probability model of SWI and vegetation(the Normalized Difference Vegetation Index,NDVI)was established on the basis of Copula function and Bayesian conditional probability model to quantitatively evaluate the spatio-temporal distribution characteristics of vegetation vulnerability.By using the decomposition method and the sensitivity method based on Budyko hypothesis,we also quantitatively revealed the effects of climate change and human activity on dryness/wetness changes in Yellow River basin and Beijiang basin.The main conclusions of this paper were as follows:1)The comparative analysis between the Standardized Precipitation Evapotranspiration index and SWI showed that 12-month SWI(SWI12)had good applicability in China.SWI12 showed significant drying trends in Northern China(Songhua and Liaohe River Basin,Haihe River Basin and western Yellow River Basin)and western Yangtze River basin due to decreasing P and increasing PET and in the Pearl River Basin due to weak surface water storage capacity,while significant wetting trends occurred in Continental Basin and most areas of Huaihe Basin due to increasing P.In the past 25 years,the percentage of drought area in China indicated by SWI12 was normally between 20% and 40%,while the number could be up to 60% in maximum.There were obvious differences in drought characteristics between northern China and southern china.Drought characteristics showed lower drought frequency,longer drought duration and stronger drought severity in northern China than that in southern China.2)NDVI showed high(>0.3)vegetation coverage from May to September in most regions of China and low vegetation coverage from October to April in northern China.In summer,NDVI showed decreasing trends in many regions(e.g.eastern China in June,the Yangtze River Basin in August),while in other seasons NDVI showed increasing trends in most regions.SWI positively correlated with NDVI for each month in more than 80% of China regions,which reflected that dryness/wetness changes can inhibited vegetation in most regions.The correlation between SWI and NDVI was strong and the response time of vegetation to drying/wetting variability was short in spring and summer in Northern China,while the correlation between SWI and NDVI was strong and the response time of vegetation to drying/wetting variability was short in autumn and winter in Southern China.Under drought stress,vegetation losses were more likely to occur in summer and autumn than in spring and winter,and in northern basins than in southern basins.With the increase of drought grades,the vegetation loss probability increased in the same vegetation loss scenario,and the probability of the large vegetation loss scenarios decreased along with increasing severity of loss under the same drought stress.The regions with high vegetation vulnerability were mainly distributed in Northern China and Northwestern China in summer and autumn,and in Northern China and Southwestern China in spring and winter.(3)The simulation runoffs derived from the two separation methods based on Budyko hypothesis and the observed runoff had highly fitting precision,and the Beijiang River Basin(0.76)performed better than the Yellow River Basin(0.48)indicated by Nash-Sutcliffe efficiency coefficient.The quantitative separation results showed that the dryness/wetness changes in the Yellow River Basin were mainly affected by climate while the dryness/wetness changes in the Beijiang River Basin were more likely affected by human activities.Human activities prolonged the drought duration and increased the drought severity,leading to graver dry trends in the Yellow River basin and Beijiang River basin.The two basins were more likely to affected by climate change with more severe drought.The impact of climate change increased with the increase of dryness/wetness grade in the Yellow River Basin,while the impact increased with the increase of dryness grade in the Beijiang River Basin.