| The regional drought is an increasingly serious problem in the background of global warming. In recent decades, drought events frequently occurred in dry-farming areas of Gansu Province. There are many kinds of drought index, but the applicability of different drought index is very different in different time and different areas. Using temperature and precipitation data of35meteorological observation stations located in dry-farming areas of Gansu Province from1961to2010,4kinds of drought index (K, SPI, CI, and MCI) were calculated. First of all, according to both relative soil moisture at0-20cm depth and drought events in weather disasters, the optimal drought index was determined compared with the distribution of drought and flood during Apr.-Sep., the main stage of crops growth in dry-farming areas. Then, making use of the most appropriate drought index, spatial and temporal characteristics of droughts in the region was investigated. Further more, a regression equation is established between relative soil moisture and air temperature as well as precipitation based on50-year observational data. Finally, the drought trend in the future85years (2015-2100) in the region is projected using the5model outputs in the CMIP5(Coupled Model Intercomparison Project) under conditions of RCP4.5and RCP8.5. The study shows that:(1) the4drought indices can generally present the drought conditions similar with that of soil drought. For drought intensity monitored by the four indices, K index is the strongest among the4indices, followed by CI, MCI and SPI. K index has the highest false alarm rate while SPI index has the most number of missing report on heavy drought among the four indices. Both SPI and CI indices have a better response to rainfall while K index just respond to the heavy rain. In the middle part of the research area, MCI and CI index often present a consistent result while MCI, CI and SPI indices usually have a good consistent result in southern part and eastern part. In summary, K index is the most appropriate for the middle part, MCI index is the most appropriate for the other part of our research area, and MCI index can be used for the whole research area with better results. (2) Drought occurred with high frequency in spring and summer while in autumn with the lowest frequency. The drought with a large area often easily occurred in summer and autumn. The drought area has increased in each season during the past50years. In spring, each degrees of drought can easily occur and become severe. In summer, light or moderate drought occurs easily and severe drought rarely occurs. In autumn, drought rarely occurs and the degree of drought often is slight if occurring. Droughts usually occur in the northern part of the middle region and southern part of the south region of Gansu. The duration of drought for each season has increased in the latest50years except autumn. The proportion of drought stations for each season has also increased except summer. The annual average of drought intensity has increased in summer while decreased in other seasons in the past50years.(3) Relative soil moisture at0-20cm depth has a good positive relationship with monthly precipitation and a significant negative correlation with air temperature. The increase of soil water usually lags behind that of rain. In recent20years, the variation of soil water is consistent with that of precipitation. There is no definite relationship between relative soil moisture and air temperature, however, the increase of air temperature will intensify existing drought. An equation of linear regression is built based on the data of relative soil moisture, air temperature and precipitation, showing that it can make a good predict of soil drought in the context of both range and intensity.(4) Based on the outputs of5models in CMIP5under conditions of RCP4.5and RCP8.5, trend for temperature and precipitation were detected. The results show that averaged annual precipitation between2015and2100will increase significantly in the research region with the rising amplitude increasing from north to south and decreasing from west to east. However, Precipitation will decrease gradually from2015to2055, and begin to increase in2055. At the same time, temperature will generally rise in the northeast part while decline in other part of research region.(5) Under conditions of RCP4.5and RCP8.5, relative soil moisture will remain in more than65%during2015-2100, though accompanying with a declining trend. From Apr. to Sep., soil drought will generally experience a process of development, mitigation and remove. Drought has a low possibility of occurrence during2015-2100based on MCI index. In general, drought will frequently occur from June to August. If drought occurs, it will appear in middle and east of Gansu province as well as the southern part, but the severe drought often occur in the northeastern regions of east and middle parts of Gansu province. |
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