关键词: 气象干旱, 联合发生概率, 游程理论, 标准化降水指数, Copula函数

Abstract:

The middle and lower reaches of the Yangtze River serves as an important grain production base in China. In the context of global climate change, the drought events will directly affect the agriculture production and people's properties and lives in this region and also the national food security. In this study, the meteorological drought events have been extracted using run theory based on the standardized precipitation index (SPI), which is defined using the gridded dataset of monthly precipitation with a spatial resolution of 0.5°×0.5° in China from 1961 to 2015. The drought events are characterized by three variables including duration, severity and peak. Exponential and Gamma functions are then selected to describe the marginal distribution of drought duration and severity, respectively. The Clayton Copula is used to construct the joint distribution of drought duration and severity. Based on the above functions, conventional univariate frequency analysis and copula-based bivariate frequency analysis are used to characterize the spatial patterns of the drought events in the study area at various time scales over the past 55 years. While univariate analysis is focused on return levels of selected return periods (5-, 10-, 20-, and 30-year) for the SPI of 3-, 6-, and 12-month time scales, the bivariate analysis is focused on the joint occurrence probabilities P₁ and P₂ of drought duration and severity, where P₁ is the probability of "drought duration and severity" exceeding their respective thresholds at the same time and P₂ is the probability of "drought duration or severity" exceeding their respective thresholds at the same time. Here, the thresholds denote the duration and severity values corresponding to selected 5-, 10-, 20- and 30-year return periods. The results show that: (1) There are some differences between the spatial patterns of drought characteristics for the 3-month, 6-month, 12- month time scales in the middle and lower reaches of the Yangtze River, and the most serious droughts mainly occur in the lower reaches of the river. (2) Both drought duration and severity are positively correlated at various time scales, that is, the areas with higher drought severity are also associated with longer drought duration and vice versa. (3) Poyang Lake watershed and areas north of the lower Yangtze River are associated with larger values of the joint occurrence probability P₁, that is, when the drought duration and severity exceed their corresponding 5, 10, 20 and 30-year return values at the same time, while smaller values of the joint occurrence probability P₁ are noted for Southern Shaanxi, Southern Henan, a major portion of Hubei and central-north of Hunan, suggesting a low risk of droughts in these areas. The results of this study can provide theoretical basis for scientific managements of water resource and the risk assessments of drought disaster in the middle and lower reaches of the Yangtze River.

Key words: meteorological drought, joint occurrence probability, run theory, standardized precipitation index, Copula function