Abstract: In pursuance of investigating the variation characteristics of rainfall amount in Poyang Lake area, the daily rainfall dataset (including rainy days and rainfall amount) of Duchang hydrometric station, located at middle Poyang Lake, with a 60-year common record length of the daily rainfall from 1953 to 2012, is classified into different levels according to rainfall classification standard. The variation characteristics and distribution features of rainy days and rainfall amount are examined with methods of trend and catastrophe test, Hurst index, precipitation concentration degree (PCD) and precipitation concentration period (PCP). Results reveal that: (1) The number of average annual rainy days is 134, of which light rainy days, moderate rainy days, heavy rainy days and torrential rainy days account for 70.9%, 18.66%, 8.2% and 2.98%, respectively; during the past decades, the total annual rainy days, light rainy days and torrential rainy days exhibit a significant decreasing trend and is likely to continue. Total annual rainy days changed abruptly (reduced by 14 days) in 1978. In spring and summer, heavy and torrential rainy days occupy a high portion, whereas in autumn and winter, light and moderate rainy days are dominant. (2) The average annual rainfall amount is 1 335.66 mm, of which light rain, moderate rain, heavy rain and torrential rain account for 20.74%, 29.50%, 27.35% and 22.41%, respectively; the annual rainfall amount exhibits a significant and lasting decreasing trend and changed abruptly (declined by 301.26 mm) in 1976. (3) The precipitation concentration degree (PCD), which varies greatly due to distinguishable distribution feature of rainy days and precipitation in different years, has an average of 0.4; the precipitation concentration period (PCP) is mainly from mid-March to mid-June, of which from mid-April to mid-May accounts for 71.43%. The decrease of rainy days is mainly caused by declining light rainy days, but annual rainfall amount is affected by rainfall of all levels. PCD, closely related to the ratio of heavy rain and torrential rain to annual precipitation, is significantly positively correlated with the uniformity coefficient of precipitation.

Key words: graded rainy days, graded rainfall amount, precipitation concentration degree, variation characteristics, Poyang Lake area