On the Flood Peak Distributions over China

Abstract

Time series of annual maximum instantaneous peak discharge from 1120 stations with record lengths of at least 50 years are used to examine flood peak distributions across China. Abrupt change rather than slowly varying trend is the dominant mode of the violation of stationary assumption for annual flood peaks over China. The dominance of decreasing trends in annual flood peak series indicates a weakening tendency of flood hazard over China in recent decades. Delayed (advanced) occurrence of annual flood peaks in southern (northern) China point to a tendency for seasonal clustering of floods across the entire country. We model the upper tails of flood peaks based on the Generalized Extreme Value (GEV) distributions for the stationary series, and evaluate the scale-dependent properties of flood peaks. The relations of GEV parameters and drainage area show spatial contrasts between northern and southern China. Weak dependence of the GEV shape parameter on drainage area highlights the critical role of space-time rainfall organizations in dictating the upper tails of flood peaks. Landfalling tropical cyclones play an important role in characterizing the upper-tail properties of flood peak distributions especially in northern China and southeastern coast, while the upper tails of flood peaks are dominated by extreme monsoon rainfall in southern China. Severe flood hazards associated with landfalling tropical cyclones are characterized with tropical cyclones experiencing extratropical transition, and persistent moisture transport/interactions with regional topography as demonstrated by Typhoon Nina (1975).