Study On The Influence Of Dewfall On Drought Assessment And The Response Of Vegetation To Drought

As a natural disaster with complex formation mechanism and frequent occurrence,drought seriously affects the social economy,water supply security and ecosystem.Precipitation,soil water content and evapotranspiration are common hydrometeorological variables in drought analysis,but dewfall plays an important role in ecosystem health in arid and sub-arid areas.In this paper,the daily dewfall was calculated from 1975 to 2018.Combining with the actual observed precipitation,an improved drought index SPI-cor(standardized precipitation index-correction)based on SPI(standardized precipitation index)was constructed,and the response relationship between vegetation and drought was analyzed.The main research contents and results are as follows:(1)Dewfall calculation and analysis of its temporal and spatial distribution characteristics.Based on GLDAS-2 data set,the daily dewfall of 523 meteorological stations in China from 1975 to 2018 was calculated by Penman-Monteith equation,and its temporal and spatial distribution characteristics were analyzed.According to the analysis results of moving average(MA)method and ensemble empirical mode decomposition(EEMD)method,dewfall has obvious periodicity on an annual scale.Dewfall in the all arid,semi-arid,sub-humid and humid areas is mainly concentrated from April to August.From high to low,the dewfall values is successively sub-humid,arid,semi-arid and humid area.The proportion of dewfall in precipitation gradually decreases from arid area to humid area.The annual average dewfall are high in northeastern and northwestern and low in central and southern China;except for the northwestern China where the proportion is high and gradually climbs from the outer edge to the northwestern inland,proportion of dewfall in precipitation rises from south to north.(2)Applicability analysis of dewfall in drought assessment.The spatial applicability of SPI and SPI-cor was analyzed from 3 aspects: different climate zones,weather stations and drought events.From arid area,sub-arid area,semi-humid area to humid area,the difference between SPI and SPI cor gradually decreases,that is,the impact of dewfall on drought assessment decreases in turn.When dewfall as a supplement to precipitation has an impact on drought grade evaluation,the impact is an average drought grade.The inconsistency between SPI and SPI-cor grades is concentrated in arid,sub-arid and semi-humid areas,but the SPI and SPI-cor grades of most stations in humid areas are consistent.By comparing with the drought records in China,SPI-cor is more suitable in the arid,sub-arid and semi-humid areas in the central and northwest of China,while SPI is more suitable in the north and northeast of Inner Mongolia,and there is little difference between SPI and SPI-cor in the humid areas in the south.(3)Study on the response of vegetation to meteorological drought.The correlation between vegetation and drought and the response lag time to drought were analyzed on multi-year scale and 16 day scale.On the multi-year scale,VCI and SPI-cor are mainly positively correlated,and the negative correlation are mainly in the arid and semi-arid areas of northwest China.The lag time of VCI to SPI-cor is mainly 0～2×16 days,and the lag time in northern Xinjiang,southwestern China and northern Inner Mongolia are2～6×16 days,6～10×16 days and 5～8×16 days,respectively,and the lag time in eastern Inner Mongolia and western of northeast China is 7～10×16 days.On the 16 day scale,the positive correlation between VCI and SPI cor was mainly positive,and the negative correlation was mainly in the central region.With regard of every 16 days,the correlation between VCI and SPI-cor are mainly positive,and the negative correlation mainly appears in the central China.The fluctuation of the area proportion of the positive correlation fluctuates little in the year except for the sudden change around April 23.The average lag time of VCI response to SPI-cor is 5×16 days,which waves greatly from January 1 to February 18,and then becomes relatively stable.According to arid and humid areas,altitude characteristics,geography,soil,vegetation and land use,the response of vegetation to meteorological drought at multi-year scale and 16 day scale were further discussed.