关键词: 负轮长, 随机模拟, 干旱指标, 年径流量序列

Abstract: A drought is recognized as natural phenomenon which is characterized by the fact that demanded water quantity surpasses natural supply. However, on its definitions and quantitative indices there is no general agreement. In order to examine the drought properties of the main rivers in china, negative run lengths (NRL) is chosen as an index indicating the quantitative degree of a drought. In this paper NRL is defined by persistent duration of annual runoff related to negative deviation from mean annual runoff. The choice of NRL is due to its clear concept, convenient computation and ability in comparison. Based on a large number of observed data on the main rivers in China, the statistical properties of NRL such as distribution, mean value, and maximum value have been exmined in this paper. The distribution type of NRL is explored and identified by using a station year method. The final results obtained in this paper can be summarized as follows: 1. Pearson Type Ⅲ distribution may be used to describe the statistical properties of NRL on the main rivers in China; 2. Mean NRL has a close relation to first order autoregressive coefficient of annual runoff; 3. Maximum NRL within a period can be taken as a measure of drought severity. Its occurrence probability may be reasonably estimated by using a stochastic generation approach. Two case studies are given for two hydologic stations: Harbin and Shanxian.

Key words: Negative run lengths, stochastic generation, drought indices, annual runoff series