基于Copula函数的湘江流域气象干旱向水文干旱传播特性

    Characteristics of the transmission from meteorological drought to hydrological drought in the Xiangjiang River Basin of China using Copula function

    • 摘要: 水文干旱的发生往往与气象干旱的发生密切相关,因此揭示两者间的响应关系对干旱预警及水资源管理具有极其重要意义。该研究以湘江流域为研究对象,采用标准化降水指数(standard precipitation index,SPI)和标准化径流指数(standard runoff index,SRI)分别表征气象干旱和水文干旱,采用皮尔逊相关系数确定干旱响应时间,并结合游程理论识别、融合和剔除干旱事件,分析湘江流域上、中、下游不同季节干旱的趋势性及周期性,最后建立基于Copula函数耦合贝叶斯网络模型的气象-水文干旱特征响应概率曲线。结果表明:湘江流域气象干旱到水文干旱的传播时间为2个月,阈值组合0.5、0、−0.5相较于组合0、−0.3、−0.5识别得到的干旱事件更优;下游发生水文干旱的情况最为严重,中游气象干旱最为严重,整体上水文干旱发生历时和烈度的最大值均大于气象干旱;流域整体有湿润化趋势,但是在夏季和秋季,存在干旱化的趋势;干湿变化存在几个较为明显的震荡周期,分别为3~5、6~10和18~21 a;对于联合分布函数,除了上游和下游气象-水文干旱烈度的最优联合分布为Gumbel Copula函数,其余干旱特征变量的最优联合分布均为Frank Copula函数;水文干旱历时、烈度的响应概率随气象干旱特征变量的增加而增加,但当气象干旱历时超出特定阈值时,水文干旱历时对其响应概率将趋于稳定,即当气象干旱历时超过特定阈值后,将不会进一步延长水文干旱的发生时长。该研究对深入了解湘江流域的干旱机理以及指导该地区防旱抗旱工作均具有重大意义。

       

      Abstract: Hydrological droughts are closely related to the meteorological ones in the arid and semi-arid areas. It is of great significance to reveal the response relationship between the meteorological and hydrological droughts in drought warning and water resource management. In this study, the changing patterns of drought were determined in the Xiangjiang River Basin of western China using Copula functions. Daily precipitation data was collected from 40 meteorological stations, while the monthly runoff data was from three hydrological stations (Laobutou, Hengyang, and Xiangtan stations). The standardized precipitation index (SPI) and standardized runoff index (SRI) were adopted to describe meteorological and hydrological droughts. The Pearson correlation coefficient was used to determine the response time to drought. The run theory was used to identify, merge, and remove the drought events. The Mann-Kendall trend test and wavelet analysis were employed to reveal the trend and periodicity of drought in different seasons in the upper, middle, and lower reaches. Furthermore, seven widely used distribution functions were selected to fit the duration and severity sequences of meteorological and hydrological drought that were obtained from SPI and SRI. The optimal parameter estimation was obtained in each marginal distribution using the maximum likelihood. There were strong links between meteorological and hydrological droughts using three Archimedean Copula functions (Frank, Gumbel, and Clayton Copula) by coupling the response probability curve in a Bayesian network model. Finally, the model performance was evaluated using the Akaike information criterion (AIC) and ordinary least squares (OLS). The main results demonstrate: 1) The run theory 0.5, 0, −0.5 was achieved to better identify the drought events, with a matching rate of over 90% between meteorological and hydrological droughts. There was a two-month propagation time from the meteorological to hydrological drought in the whole watershed. The SPI and SRI effectively detected the drought events and better matched them with the historical drought statistics from the different periods (in 1963, 1986, and 2003). 2) The frequency of meteorological drought was higher than that of hydrological drought. The duration and severity of hydrological drought were greater than those of meteorological one, where the downstream area experienced the most severe hydrological drought, but the midstream is the most severely affected by meteorological drought. The difference was attributed to the higher evapotranspiration and high level of socio-economic development in the downstream. 3) There was a tendency toward wetness of the historical data in the basin, which was consistent with the remote sensing monitoring of terrestrial water storage anomaly. However, there was a distinct seasonal variation in the inter-annual wetness of the basin. The upper and middle reaches showed a drier trend in the autumn, while the middle reaches were in a trend toward dryness in the summer. Additionally, there were several outstanding oscillation periods of dry and wet, namely 3-5, 6-10, and 18-21 a. 4) The optimal joint distribution of meteorological-hydrological drought severity in the upstream and downstream area was the Gumbel Copula function, while the Frank Copula function was the optimal joint distribution for the rest. 5) The response probability of hydrological drought duration and severity increased with the increase of characteristic variables in the meteorological drought. There was a stable response probability of hydrological drought duration when the duration of meteorological drought exceeded a specific threshold. This study is of great significance in understanding the mechanism of drought in the Xiangjiang River Basin as well as guiding the drought prevention and fighting work in the region.

       

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