| Standardized precipitation index (SPI) of different temporal scales were calculated with daily precipitation data collected in 92 meteorological stations in Inner Mongolia area during 1970 and 2014. According to the results, the annual and seasonal variation characteristics of frequency,influence range and intensity of drought in Inner Mongolia were analyzed. Using the run theory,the characteristic variables of drought duration and drought severity were separated from the monthly scale SPI sequence. Thus,the temporal and spatial variation of drought duration and severity in Inner Mongolia were investigated. Based on the determination of distribution function of these two variables,the probability of different drought duration and severity grades and their corresponding recurrence period are calculated respectively. The joint statistical model of drought duration and drought severity in Inner Mongolia was established through introducing the Copula method, and then the joint probability and joint recurrence period of different duration and severity are researched. This paper explores the characteristics of drought change in Inner Mongolia in the past 45 years, as well the temporal and spatial transitional property of drought events. Results show that:(1)The drought frequency of different grades in east and medium areas of Inner Mongolia are relatively similar, while the grades are higher in the west past. The drought frequency distribution in different seasons is similar to the situation of annual scale, and their occurrence frequency is also similar in each season. However, of which the area of drought zone in winter is larger. The annual trend of drought station proportion and intensity are similar, which shows the trend of early decreasing and later increasing. What’s more, the increasing trend is more significant after 2000. From the perspective of different seasons, the station proportion and severity in spring have declined and which in summer have increased. The influence range of drought in autumn is exploring no matter the intensity is slightly decreasing. However, the trend in winter shares the opposite tendency. Obvious differences in the responses to climate change in the east, medium and west of Inner Mongolia have been studied significantly. The influence range and intensity of annual drought in east and medium areas are increasing, while the west regions are decreasing. From the perspective of different seasons, the spring drought in Inner Mongolia has eliminated; summer drought and autumn drought have increased in east and medium areas, while decreased in the west. Overall, drought tendency are enhanced in the east and medium part of study areas, but they are nearly eliminated in the west.(2) The trend of the annual drought duration and drought severity is similar to each other,which are relatively lower before the 1990s and higher after that. The whole trend of drought is enhanced. To spatial distribution, the average annual drought and severity distribution are relatively similar, including the top in medium regions, the second place in east regions, and lowest in west regions.-According to the trend coefficient of these two variables, the up-trend of the drought duration and the intensity is mainly distributed in the east and west, while down-trend occupies the major land in the west. According to the probability of different duration grades,drought types in Inner Mongolia are mainly within a month or within a season,while the inter-seasons type is relatively less,and the type of over half a year is the least. The major drought situation of west regions belongs to the type within a month,while for east and medium regions, the inter-seasons type and the within a month type hold the dominant position,however, the medium have more drought events of inter-seasons type and over half a year type.According to the probability of different severity grades, drought types in Inner Mongolia are mainly slight and moderate , followed by severe and extra severe type. With the increase in drought grades, the high-value areas have contracted in the west and gradually extended to the medium and east regions. The recurrence period of different duration and severity all increased with the increase of the threshold. Both of their recurrence period are longer in east and medium regions ,and shorter in west regions.(3) Drought events in Inner Mongolia are mostly lasting less than 4 months and the severity values are less than 2. From the view of spatial distribution, the drought type with shorter duration and less severe mainly occur in the west, while the drought with a long time and high severity happen mainly in the medium and east part. Among these six major drought types, the average return period of slight drought within a month is the shortest and the most frequent.Followed as the sequence: moderate drought within a month, extra severe drought within a month, moderate drought within a season. The average return period of extra severe drought within a season is the longest and rare. The response period of each grade drought is longer in west regions, contrary to the medium and east regions. With the increase of drought severity, the regional divergence is becoming larger significantly. |
PDF Full Text Request