Background There are mainly two kinds of weather systems affecting precipitation in southeast coastal areas of China. Frontal precipitation is dominant in the first flood season (before June 30), while the precipitation is mainly caused by tropical weather systems such as typhoons in the second flood season (after June 30). The extreme precipitation feature is significantly different between the first and the second flood seasons, which may lead to different possibility of flood and soil erosion in different flood seasons. Therefore, it is of great significance to understand the hazard of extreme precipitation in different flood seasons.

Methods In order to reveal the difference in the hazardof extreme precipitation between the first and the second flood seasons, Fujian province was taken as an example. Fujian province located in the southeast coastal areas of China, backed by The Eurasian continent and facing the Pacific Ocean in the east, special geographical environment resulted in a sensitive region of frequent precipitation. The extreme precipitation indices covering RX1day, RX5day, SDII, R50 and R95p were selected and calculated for the first and second flood seasons respectively, based on the data of 25 meteorological stations from 1960-2020. A TOPSIS method combined with CRITIC-entropy weight composition model was applied to evaluate the hazard in the first and the second flood seasons.

Results 1) The overall hazard extreme precipitation in the first flood season was higher than that of in the second flood season. The hazard in the first rainy season increased spatially from the eastern coast to the northwestern mountains and it decreased from the southeast coast to the northwestern mountains in the second flood season. 2) There was a large difference in the hazard of extreme precipitation between the first and the second flood seasons. It was divided into 3 types according to the difference in the hazard and resulted in 14 sites as the type of the significant hazard in the first flood season, 6 sites as the type of the equivalent hazard in both seasons and 5 sites as the type of the significant hazard in the second flood season. The hazard of extreme precipitation in the first and the second flood seasons was evaluated respectively, which provided more reasonably spatial-temporal variation in Fujian province.

Conclusions Because of the difference in weather systems affecting precipitation in southeast coastal areas of China, the hazard of extreme precipitation in the first and the second flood seasons was spatial-temporally different. Evaluating the hazard of extreme precipitation separately can reasonably reveal the danger of extreme precipitation and its spatial variation, and may provide scientific and technological support for regional flood control, water and soil conservation, etc.