Fifty-year rainfall change and its effect on droughts and floods in Wenzhou, China

Based on daily, monthly, seasonal and annual precipitation, monthly duration and flood-affected area data from 1954 to 2007, we examined the absolute and relative change trends of rainfall and their effects on hazard intensity in Wenzhou city, China. The long-term trend of precipitation was studied by linear regression, moving average, cumulative anomaly and Z index methods, respectively. Our results show that there was no significant downward trend of annual precipitation. In contrast, there was significant decrease in autumn, July, mid-January and early June and significant increase in early January and late May, especially in late June. During 1954 and 2007, although significant fluctuation existed in the absolute value of precipitation, the relative changes of wet and dry were not significant compared with the average. The 10-year decrease in precipitation was 23.37 mm in autumn, 14.85 mm in July, 0.33 mm in mid-January and 6.87 mm in early June; while the 10-year increase was 0.35 mm in early January, 3.05 mm in late May and 8.57 mm in late June, respectively. Moreover, we found that 1964, 1966, 1977 and 1995 were the transition periods when the rainfall Z index was at peaks, the flood intensity was high, and the drought intensity was relatively low. On the other hand, 1958, 1968, 1971, 1980, 1989 and 1992 were the periods when the rainfall Z index was at valleys, the flood intensity was low, and the drought intensity was relatively high. Taken together, we demonstrated the obvious effects of precipitation changes on flood and drought intensities.