Trend of drought and its relationship with extreme precipitation in Huaihe River basin over the last 60 years

The trend of drought and its relationship with extreme precipitation events in Huaihe River basin over the last 60 years were analyzed using the observations at 28 meteorological stations during the period from the start of their observation(mostly started in 1951—1958) to 2011.According to the results of standard precipitation index(SPI) and humidity index(HI) at different time scales(from 1 month to 12 months),at 1-month and 3-month scale,there is a general tendency of drying in the spring(especially in April) and wetting in the summer(especially July and August);at 12-month scale,no trend is observed in SPI in the west and middle part of Huaihe River basin,but a widespread increase in HI is observed in western part;at 12-month scale,there is a general tendency of drying in eastern part with respect of both SPI and HI.There is significant decrease in potential evapotranspiration over the whole basin,and due to the decrease in potential evapotranspiration,HI exhibits a general tendency of increase which indicating the increase of the proportion of humid area in Huaihe River basin.There is a significant decrease of the number of rainy days(precipitation amount is greater than 0.1mm),resulting in a general tendency of more intense precipitation than the past.It is found that precipitation intensity is negatively related with SPI,as a result,the possibility of meteorological drought occurrence is increased.