Abstract: The inter-annual variation of heterogeneous annual precipitation distribution over 1961-2015 was studied by using the precipitation concentration degree (PCD) and precipitation concentration index (PCI) previously defined by other researchers. Results revealed that precipitation concentrated in some seasons, indicating that the distribution of precipitation in Huaihe basin was quite uneven at annual scale. Annual PCD showed an increasing trend with apparent undulation since 1990. According to Mann-Kendall analysis on the data of monthly and seasonal PCI, most months in which the PCI values were averagely positive (from May to August) displayed an increasing trend, while most months in which the PCI values were averagely negative (in April and October) exhibited an decreasing trend, with only one month(November) changing in the opposite direction. The trend of seasonal PCI was similar to that of monthly PCI: in spring and autumn when PCI were averagely negative, PCI showed decreasing trends; in summer when PCI was averagely positive, an increasing trend; in winter, the change of PCI was insignificant. In general, the trends of seasonal and monthly PCI at annual scale exacerbated the polarization of precipitation distribution: wet periods are tending to be wetter and dry periods are getting dryer.

Key words: Huaihe River Basin, heterogeneity, precipitation concentration degree, concentration index, polarization