Abstract
Hydrological fluctuations tend to greatly vary, and rainfall characteristics have recently been significantly affected by influences of global warming worldwide. Rainfall changes and the occurrence of extreme events have received more attention because of their great impacts on ecosystem operations, agricultural activities, and design of hydraulic structures. Rainfall types, regimes, and long-term changes, and thresholds of extreme events were analyzed for the Lienhuachih area in this study using rainfall records from April 1928 to December 2016. Rainfall is unevenly distributed in the annual cycle with about 85.8% of amount and 68.8% of rainy days concentrated from April to September. Therefore, the wet season in the study area was defined as being from April to September rather than from May to October as in most other parts of Taiwan, owing to heavier spring rains. The Lienhuachih area has a mean annual rainfall of about 2409.0 mm and rainfalldays of about 144.6 d. An analysis of long-term rainfall changes showed a decreasing trend of about 28.0, 29.2, and 17.6 mm decade-1, for the annual total, and wet and dry seasons, respectively. Rainfall-days also showed a steadily decreasing trend of about 4.6, 2.3, and 2.4 d decade-1 for the annual total, and wet and dry seasons, respectively. Thresholds for extreme rainfall and drought were roughly 600 mm d-1 and 65 consecutive days of no rainfall, respectively, as estimated by quantities from the Extreme Value Type I frequency analysis of 50-yr return period. The decreasing trend for both rainfall amounts and rainfall-days and the more-frequent occurrence of extremely torrential rainfall indicate that rainfall will increasingly become temporally concentrated in the study area. This phenomenon may bring about greater difficulties for water resources management.