| The Wei River Basin tends to get warmer and drier as a result of ongoing climate change and human activity,and the rapid rise of society and economy has further worsened the tense water resource situation,greatly hindering the high-quality development of the basin.It is common to describe and measure the occurrence and development process of drought events in the basin from different perspectives,such as hazard and vulnerability.However,the impact of drought events depends not only on hazardous factors but also on the status of the vulnerable population,which has an important influence on the development and range of action of the drought events.Based on collecting and organizing the average temperature and precipitation data from the Wei River Basin from 1915 to 2020 and the soil moisture data from 1950 to 2020,this study calculated the meteorological drought index SPEI and the agricultural drought index SSMI and interpreted their temporal trends,variations,and periodic patterns.The spatiotemporal evolution characteristics of the frequency of occurrence of these two types of droughts(causing and bearing drought attributes)were explored.Furthermore,by integrating the SPEI and SSMI,which represent causing and bearing drought attributes respectively,a meteorological-agricultural comprehensive drought index MADI-3 was developed and applied to analyze the characteristics of drought evolution in the Wei River Basin.Finally,the MADI-3 comprehensive drought index was applied to analyze the spatiotemporal evolution characteristics of drought in the Wei River Basin,and the spatiotemporal continuity identification method of drought events was used to identify the occurrence and development process of drought month by month.Based on the research method of spatiotemporal characteristics of drought,the spatiotemporal dynamic evolution characteristics of drought were extracted from multiple time scales,such as month-by-month,single-field,and multifield.The main conclusions obtained are as follows:(1)The spatiotemporal evolution characteristics of meteorological drought index with disaster-causing attributeOver the past 100 years,the Wei River Basin has tended to become warmer and drier,with a trend of increasing meteorological drought.The trends in different seasons manifested by grid points representing the dominant factors were different.Spring showed a significant increase,while summer and autumn did not significantly intensify,and winter did not significantly lighten.The annual average SPEI showed two obvious turning points in 1941 and 1986,while the seasonal average SPEI showed significant differences in variability.From a long-term perspective,the small cycles of drought and wetness change in the Wei River Basin are around 3 years and 10 years,respectively,while the large cycle is around 30 years.In terms of occurrence frequency,the annual meteorological drought occurrence frequency in the Wei River Basin is about 32.08%,of which the frequencies of moderate drought and severe drought are 17.92% and 10.38%,respectively,and those of extreme drought and exceptional drought are both 1.89%.The areas with the highest frequency of exceptional drought occurrence are mainly concentrated near Xianyang City,and those with a high frequency of severe drought occurrence are from Qingyang City to Dingxi City.The highfrequency area of moderate drought occurrence is in the southern part of Yulin City and Dingxi City,and mild drought is more likely to occur in the middle and lower reaches of the basin.The occurrence frequency of meteorological drought at the seasonal scale is about30.42%.Among them,severe drought is easy to occur in summer(6.60%),and moderate drought is easy to occur in spring(10.38%),while the frequency of mild drought occurrence is relatively stable in all seasons(about 18.40%).In addition,the drought trend in the Wei River Basin is consistent with the global trend of aridification,showing a continuous intensification.It can be seen that the aridification trend caused by global warming has objectively affected the northern part of China’s Loess Plateau,and previous studies have shown that climate warming will drive a dual change in temperature and precipitation in this region,and the trend of aridification in the future may be more apparent.(2)The spatiotemporal evolution characteristics of agricultural drought index with disaster-bearing attributeOver the past 70 years,the agricultural drought in the Wei River Basin has not significantly decreased.There were differences in the trend of the dominant grid points in the four seasons,with no significant decrease in spring and summer,no significant increase in autumn,and a significant decrease in winter.The annual average SSMI had a clear turning point in 1971 and 1995,while the seasonal average SSMI showed significant differences in variability.From a long-term perspective,the variation in agricultural drought in the Wei River Basin is driven by small cycles(8-14 years)and large cycles(around 27 years),and the current stage is relatively humid.In terms of frequency,the occurrence of agricultural drought in the Wei River Basin at annual and seasonal scales is approximately 32.39%,which is roughly equivalent to the frequency of meteorological drought.Among them,the frequency of light and moderate droughts is 23.94% and 5.63%,respectively,while the frequency of severe droughts is roughly 2.82%.The regions with the highest frequency of extreme droughts are located near Xianyang and Dingxi cities,but the corresponding frequency of severe droughts is relatively low.The high-frequency area of moderate droughts is mainly concentrated in the north of Yan’an City to the east of Tianshui City,while the frequency of light droughts is higher in Qingyang City,the southern part of Yan’an City,Tongchuan City and the eastern part of Xianyang City.The frequency of agricultural drought at the seasonal scale is around29.58%.Among them,severe droughts are prone to occur in summer and autumn(4.23%),while moderate droughts are prone to occur in winter(16.90%).The frequency of light droughts is highest in autumn(22.54%).There is a certain time domain asynchrony between agricultural drought and meteorological drought in the Wei River Basin,and the distribution patterns of different scales or levels of drought also show significant spatial heterogeneity.The evolution of drought in the basin is driven by disaster-bearing and causative factors,which may lead to even more complex changes in the drought situation,and further exploration is needed through comprehensive drought indices.(3)The spatiotemporal dynamic evolution characteristics of the comprehensive drought index with dual attributes of disaster-causing and disaster-bearingBased on the meteorological drought index SPEI and agricultural drought index SSMI,the meteorological and agricultural comprehensive drought index MADI-3 with both disastercausing and disaster-bearing attributes was constructed by the entropy weight method.The applicability verification results showed that the index can objectively reflect the changes of SPEI-3 and SSMI-3,and has good spatial correlation.Based on the MADI-3 index,a total of98 drought events occurred in the Wei River Basin from 1950 to 2020 were identified on a monthly basis,with a drought month proportion of approximately 38.4%.The central part of the basin had the most concentrated monthly drought points,and correspondingly,the monthly drought intensity was higher.The range of monthly drought intensity for 326 months was 0.02～1.15,with an average intensity of 0.27.The comparative analysis of typical historical drought events showed that the description of drought occurrence and development process in the Wei River Basin using the drought spatiotemporal continuity discriminant method based on MADI-3 is appropriate.Droughts in the Wei River Basin are concentrated in the first half of the year and the timing of their occurrence may gradually shift earlier.The intensity,extent,and rate of spatialtemporal migration of droughts all increase gradually.From the changes in the latitude and longitude coordinates of the spatial-temporal concentration points of droughts,it can be seen that drought events in different periods have different development directions,but there is an overall tendency towards northeast migration.Additionally,the ratio of drought intensity to drought area has a logarithmic relationship,while there is a linear relationship between drought intensity and duration for the 98 drought events analyzed.When analyzing the development process of multiple drought events,it was found that summer drought and autumn drought were more severe in the 17 events of multi-seasonal drought. |
