| Drought is a natural phenomenon that stands for a serious shortage of wa ter supply to its demand, which is one of the most serious natural disasters around the world. In recent years, drought has become a serious environmental problem that impact on human under the background of global warming, which has aroused general concern of the scientific community. Currently, there is an urgent need of studying the law and causes of spatial-temporal variations of drought, and explore the response strategies and approaches to solve the negative effect of drought, thus monitor, make early warning and adjust the balance of water supply.In this research, standardized precipitation evapotranspiration index(SPEI) and meteorological data of 126 lattices in Q ilian Mountain Area during 1961~2014 were used to analyze spatial- temporal variation characteristics of drought in each regions of Qilian Mountain Area in different seasons, decades, intensity and time scale. And the key reasons of drought in Q ilian Mountain Area were studied in both aspects of atmospheric circulation and geographical environment / global warming.The main conclusions are drawn as follows:1. According to the analysis of SPEI variation in different time scale in Q ilian Mountain Area and the comparison of drought disaster record, we found SPEI1 and SPEI3 of short time scale were sensitive to both short time precipitation and temperature variation, the large fluctuation of value show the frequency and capricious of drought and flood. In Q ilian Mountain Area, there is a large proportion of major disaster year and high frequency of persistent drought of half- year- long, especially during 1994~2003 and after 2008. The recorded drought disaster in this area showed obviously aggravating trend since the 1990 s. In addition, there was once continuous drought for several years or even more t han ten years in Q ilian mountain area, such as 3a continuous drought of 1961~1963, 1990~1992 and 1994~1996, and 6a continuous drought of 1997~2002. The value of SPEI24 and SPEI48 maintains negative, which is in accordance with the records. In the eastern region, extreme drought month increased significantly after 1980 s, and the frequency of great drought also increased sharply in the 1990 s. In the middle region, extreme drought frequency of one- month-scale and three-month-scale was significantly lower than the eastern region, except for 1996~2001. Drought of year scale was concentrated. In the western region, monthly drought frequency was more concentrated since 2000; yearly drought occurred mainly around 2000. We use SPEI of different time scale from 126 gr id points to compare with the statistics of Chinese meteorological disaster encyclopedia, and the results show that SPEI can well accord with records of drought in Q ilian Mountain Area, and characterize the drought changes in this area in recent 54 a.2. In the aspect of time, yearly SPEI of different regions in Qilian mountain area showed a downward trend in recent 54 years. The value of SPEI of Q ilian mountain area, eastern region and western region declined in a speed of 0.089/10 a, 0.128/10 a, 0.119/10 a and 0.064/10 a. After mutation testing and accumulative anomaly detection, a significant mutation of Qilian Mountain Area in 1994 was found; the SPEI of eastern area mutated in 1997; in middle area it reduced suddenly in 1990; mutations in western region was uncertain, indicating that drought mutation of Q ilian mountain area mainly occurred in the 1990 s. In the early spring, decline speed of SPEI was 0.179/ 10 a, among which the most serious periods were in 1970 s and after 1990 s. In the autumn, the eastern, middle and western region had a same spring drought change trend of getting worse as the whole area. Value of SPEI in Q ilian mountain area, eastern region, middle and western region decreased for the speed of 0.193/10 a, 0.073/10 a, 0.263/10 a and 0.128/10 a. In the aspect of drought intensity, medium drought events in Q ilian mountain area increased significantly since 1990 s, and occurred least frequently in 1980 s. Medium drought events occurred most frequently in the western region. Moderate drought event occurs most frequently in the western region; there was few extreme drought in 1960 s, 1970 s and 1980 s, but spurted since 1990 s, and the peak was at the beginning of the 21 st century of 399 times of drought. After researching the drought changes during 1961~2014 in Q ilian mountain area, we see that the climate in this area change from moist to dry with a significant dividing line of 1990 s, and there is a worse tendency of drought in this area.3. In the aspect of space in Q ilian mountain area, drought aggravates most significantly in the eastern region and alleviate from east to west, indicating a need of concern about drought trend in the eastern region. There is also an obvious tendency of drought aggravate in the spring that 122(about 97%) grid points showed the same tendency, and alleviate from middle to west and east. In the summer, drought condition in the eastern region was slight, but it may be related to the growing effect of west wind. The middle and western region showed an alleviating tendency of drought condition. Drought in autumn cannot be ignored that almost all grid showed an aggravating tendency, and the western and eastern region showed a worse degree of drought, especially in Chaidamu mountain and Tuergendaban mountain in the western region.4. In the aspect of atmospheric circulation affecting drought in Q ilian mountain area, ENSO events intensity was significantly negative correlated to SPEI of eastern area, that ENSO events were enhanced when SPEI decline and weakened when SPEI rise. SPEI variation in the middle region and the western region was positively correlated with ENSO events with hysteresis. NAO event intensity was highly correlated with annual SPEI in the Q ilian mountain area, that SPEI rise when NAO events enhance and showed obvious downward trend when NAO events weaken. There is significant hysteresis of SPEI and NAO event in the middle and the western region. Variation of AO event and NAO event intensity has a consistent effect on SPEI changes in different regions in Q ilian mountain area, while AO events show a stronger effect. SPEI in Q ilian mountain area was in significantly low value when AO was weak, and in significantly high value when AO was strong.In the aspect of space, drought index in Q ilian mountain area was slightly correlated with ENSO in the long sequence of time, and has a clear division line of each region. It was all negative correlated in the western region and almost positively correlated in eastern and middle region; correlation with clear spacial relation in the eastern region was more obvious. Drought in Q ilian mountain area had a complicated relation with NAO, that was negatively correlated in eastern region and strongest positive correlated in middle and western region in the west of Qinghai lake, which mean the west of Q inghai lake is also the division line of influenced area of west wind Asia and monsoon Asia. AO index and NAO index was consistent with drought space correlation in Q illian mountain area, while AO was more strongly correlated with SPEI with a lager influential area to the east than NAO, and the correlation of AO and drought in Q illian mountain area was most complicated. The research shows that AO and NAO had a stronger effect than ENSO to drought in Q ilian mountain area in recent 54 years, which was rela ted to strengthening of the west wind and weaken of monsoon. This relation can be proved by 500 hpa spatial distribution of water vapor transportation: west wind had stronger influence to drought in Q ilian mountain area than monsoon; the influence of platea u climatic region should not be ignored that the west-east direction terrain of Q ilian mountain area can not keep water vapor brought by strong west wind, and drought in this area kept aggravating.5. In the aspect of geographical environment / global warming, because of continental humid and semi humid climate of Q ilian mountain area, climate factors like temperature and precipitation show significant horizontal difference and vertical zonality, which lead to unbalance of precipitation in Q ilian mountain area. In addition, this region located in alternating zone controlled by west wind and monsoon, and drought occurred frequently because of changeable force of the west wind and monsoon. Of course, the influence of global warming can not be ignored that it cause not only temperature rising around the world but also large change of climate by influencing atmosphere, land, and biosphere, thus the change of drought in Q ilian mountain area becomes more complicated. |
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