Spatial And Temporal Characteristics Of Drought And Its Influencing Factors In North China

With the change of global climate and regional ecological environment,drought has become one of the most serious natural disasters on earth,which has a wide space,a long duration and the most serious impact on social economy and environment.The economic loss caused by drought disaster accounts for 50% of all natural disasters in China,which is the first natural disaster in China.North China is not only the political,economic and cultural center of China,but also an important commodity grain production base of China.This area belongs to the warm temperate semi-arid and semi humid monsoon climate area,with uneven spatial and temporal distribution of precipitation,great interannual changes,and the frequency of drought ranked first in China.Since the 1980 s,the drought in North China has been increasing.Since the late 1990 s,drought has become an important limiting factor for the stable increase of grain production in the region.Therefore,it is of great significance for agricultural production and economic development in North China to study drought monitoring,develop appropriate drought monitoring model,and analyze timely and accurately the temporal and spatial characteristics of drought and its influencing factors.Compared with traditional observation methods,remote sensing drought monitoring technology has the advantages of wide observation range,continuity of observation time and large amount of information,which has become an important means of drought monitoring.On the basis of systematic analysis and review of the research status and methods of remote sensing drought monitoring at home and abroad,the suitability of CWSI(crop water stress index),VSWI(drainage supply water index)and TVDI(temperature drainage index)drought monitoring models in North China is comprehensively compared and analyzed,The most suitable CWSI model was selected to analyze and discuss the spatial and temporal characteristics of drought and its influencing factors in North China from 2001 to 2018.The main conclusions are as follows:(1)The comparative analysis of CWSI,VSWI and TVDI in North China in 2012 shows that as a whole,CWSI is better than VSWI and TVDI in describing the spatial distribution and annual change of soil moisture in North China.CWSI is more suitable for monitoring the growth season drought in North China.(2)The annual drought monitoring results of CWSI in the growth season of North China in the past 18 years show that the drought is mild on the whole,and the drought is obviously slowing down.From 2009 to 2010,it was the turning point from drought to wetness in the growing season of North China.Before and after that,the drought changed from mild drought to non drought.Although the drought was alleviated,it was still serious in some areas.(3)CWSI in North China showed a regular fluctuation trend in the growth season,and the average change of 8-day over the years showed a double peak curve as a whole,the peak value was in the 113~120 days and 273~280 days,and the impact degree of drought was serious drought and mild drought respectively,and the valley value was in the 209~216 days,showing no drought.There are three stages in the growth season of CWSI,the 161~168 days is the turning point from drought to wetness,the 249~256 days is the turning point from wetness to drought,and the 281~288 days is the turning point from drought to wetness.The drought in the three periods of growth season is generally shown as early growth season > late growth season > middle growth season,but there are great differences in different regions.(4)The analysis of the drought stability,variation rate and the change of the center of gravity of the drought prone areas in North China shows that the drought stability gradually decreases from southwest to northeast.In Shanxi Province,Yanshan Mountains,meadow steppe area of Inner Mongolia and Hulunbuir City,except for most areas in the northwest,the change of drought in growth season is mainly improved.In the northwest of Xilinhot,Ulanchab,Datong and Shuozhou,in the north of Chifeng and hehaipingyuan,the change of drought in growth season is basically unchanged,and only the areas with worsening interannual change in growth season account for 0.25% of the total area indicates that the drought in North China has generally developed well in the past 18 years,and the drought in most areas has been alleviated in varying degrees.In the past 18 years,the center of gravity of the drought prone areas in the growing season in North China has obviously moved to the west,slightly to the south,and generally to the southwest.(5)Among the factors that affect the spatial change of CWSI in the growing season of North China,soil type and vegetation type are the dominant factors;temperature and precipitation are the important factors that affect the spatial change of drought;altitude,slope,river density and city distance are the secondary factors that affect the spatial change of drought;road distance and population density are the single factors that affect the spatial change of drought The spatial change of drought is not affected by slope direction and land average GDP.Although individual population density and average land GDP have little influence on spatial change of drought,their interaction with temperature,precipitation,soil type and vegetation type will greatly increase their influence on spatial change of drought,which is shown as a non-linear relationship.Each indicator factor has its suitable range.In this range,drought is not easy to occur.The temperature is less than 12.69 ?,the precipitation is more than 380.84 mm,the altitude is 200 m ~ 1000 m,the slope is more than 25 °,the vegetation type is swamp vegetation,the soil type is eluvial soil and hydrogenic soil,the river density is 1.62km/km2 ~ 4.32km/km2,and the urban distance is more than 383 km.(6)Among the climatic factors that affect the change of CWSI time(interannual)in the growing season of North China,the influence of precipitation on the drought is stronger than that of temperature,and there is a certain lag distance between the drought and temperature in the growing season,the lag distance between the drought and temperature is 10 months,and the lag distance between the drought and precipitation is 7 months.In terms of the spatial difference of the influence of human activities on the interannual change of drought in North China,the area where the drought slows down during the growing season accounts for 96.40% of the total area,and the area where the drought aggravates only accounts for 3.60% of the total area.For different vegetation types,under the influence of human activities,the drought in growing season shows a trend of slowing down,and the sequence of slowing down range is: Grass > crop > shrub > coniferous forest > swamp > broad-leaved forest > meadow > grassland > desert.