Analysis of the spatio-temporal patterns of dry and wet conditions in the Huai River Basin using the standardized precipitation index

• There is a prevailing trend of drying spring/autumn and wetting summer/winter.
• The trends in spring and summer have been mostly insignificant.
• The trends in autumn and winter are significant at more than 30 rainfall stations.
• Significant transitions in SPI series mainly occurred in the middle to late 1980s.
• There is a significant increase in the number of severely wet years since 1990.

Located in a transition zone between the northern and southern climates in China, the Huai River Basin is prone to extreme events such as drought and flood. Based on the daily precipitation data at 134 stations between 1961 and 2013, this paper analyzed the spatial and temporal patterns of the dry and wet conditions in the Huai River Basin through the statistical analysis of the rainfall stations' annual and seasonal standard precipitation index (SPI) series. Annual SPI series exhibited a decreasing trend at 86 stations and an increasing trend at the remaining stations. None of the increasing trend was significant, while the decreasing trend was significant at two stations at 5% significance level (α = 0.05) and one station at 10% level. Seasonal-wise, there has been a prevailing trend of drying in spring and autumn, and wetting in summer and winter. The trends in the spring and summer SPI series have been mostly insignificant, while those in autumn and winter, significant (α = 0.10) at over 30 stations. The Pettitt test results indicated that the significant transitions (α = 0.10) in the autumn and winter SPI series mostly occurred in the middle to late 1980s. Comparison of the average number of dry and wet years between the two sub-periods of 1961–1984 and 1990–2013 suggested a significant increase (α = 0.05) in the average number of severely wet years across much of the basin. Overall, significant changes have already occurred in the dry and wet conditions of the Huai River Basin, which could have profound impacts on the food and water safety situation of the region.