Study of rainfall characteristics and their changes on the Hengchun Peninsula.

Rainfall characteristics including types, regimes, frequency behaviors, long-term changes, and design storm distributions were analyzed for the area of the Hengchun Peninsula in this study. The mechanism of rainfall in the study area can mainly be classified as frontal rain, thunderstorms, and typhoon rain. Each of these mechanisms dominates the type and characteristics of rainfall. Rainfall is extremely unevenly distributed in the annual cycle. About 91.03% of the rainfall is concentrated in the period from May to October (wet season), and the rest (8.97%) is in the period from November to April (dry season). There was a total of 121.7 days on average which had at least 0.1 mm d^(-1) rainfall in the yearly cycle of the study area. The percentage of rainy days in the wet season of the total rainy days in a year was 71.41%, and that of the dry season was 28.59%. This phenomenon indicates that rainfall in the wet season is relatively strong when it occurs, and that in the dry season is usually in smaller amounts. Annual and wet season rainfall totals have significantly increased in the last 4 decades. Annual rainfall has increased about 14.6 mm decades^(-1) and that in the wet season has increased about 12.8 mm decades^(-1). However, the rainfall amount in the dry season has slightly decreased at a rate of -2.5 mm decades^(-1). This phenomenon indicates that there is an increasing trend of rainfall, but the increase is mainly in the wet season. Long-term changes of rainfall-days show a steadily decreasing tendency. Rainfall-days were obviously reduced after 1990. Annual rainfall-days showed the largest declining rate, while rainy days in the dry season showed the least decline. The number of days with a rainfall amount exceeding 30 mm showed an increasing tendency in both the wet and dry seasons. In addition, the Extreme Value Type I distribution was used in this study to estimate magnitudes of 2-, 5-, 10-, 25-, 50-, and 100-year return intervals for 10-min, 1-h, 6-h and 24-h rainfall events. Design storm distributions for 3, 6, and 12 hours were also been analyzed. Results of the design storm analysis combined with those of the frequency analysis can provide a reference for designing hydraulic structures and water control works on the Hengchun Peninsula.