Risk of meteorological drought in Lancang River Basin as analyzed by Copula

Drought risk assessment, quantitative analysis of drought in particular, is useful for drought prevention. Monthly precipitation and evapotranspiration sequences from 9 meteorological stations in the Lancang River Basin of Yunnan Province were used to construct standardized precipitation evapotranspiration index (SPEI). According to runoff theory, drought characteristic variables were separated, Copula function was used to establish joint distribution function to analyze drought occurrence probability and return period. With identical duration and intensity of drought events, frequency of occurrence was found higher in the northern part of Lancang River Basin than the southern part. Trend of contour change in joint return period was found to vary from region to region. Actual return period was found to be between joint return period and co-occurrence period. Values of joint return period and co-occurrence return period were within the range of predicted actual return period. Range of corresponding drought was determined by return period. Probability of mild drought in the basin was found to be 5.50% - 11.05%, moderate drought 17.43% - 39.31%, severe drought 12.89% - 17.83%, extreme drought 7.20% - 14.00%. These data would provide some useful reference for drought risk response in the basin.