Quantitative assessment of compound flood disaster in the Xijiang River Basin, considering univariable and multivariable with intra-correlation

Changing climatic conditions have escalated the risk of compound disaster, and there remains a knowledge gap of quantitative research considering univariable and multivariable with intra-correlation at river basin scale. An integrated research framework is proposed in this study to quantitatively analyze and assess the risk of future compound flood in the Xijiang River Basin based on external driving factors and internal variables. Under this framework, a multi-model ensemble of 10 preferred CMIP6 GCMs is carried out based on statistical downscaling and Bayesian weighted average method, and the multi-scale variation characteristics of precipitation and runoff during 2020 ~ 2099 are analyzed based on the ensemble data. Combined with univariate and multivariate trend analysis considering intra-correlation, the multi-class Copula functions are utilized to estimate the joint probability and return period of compound flood. The results show that: (1) The precipitation and runoff increase by 8.25%, 14.5%, and 34.05%, 55.18%, respectively, comparing to the baseline period under SSP2-4.5 and SSP5-8.5, with both displaying increasing trends at rates of 1.03%/10a, 2.66%/10a, and 2.74%/10a, 4.62%/10a on the interdecadal scale under the two scenarios, respectively. (2) The internal variables of the compound flood represented by the annual maximum peak flow (AMPF) and the annual consecutive maximum 7-day flood volume (AM7dFV) show significant increasing trends under the two scenarios. However, the annual maximum precipitation (AMPre) of the external driving factor does not show a significant trend, while the annual total precipitation (ATPre) of the external driving factor increases significantly under both scenarios. It is noteworthy that both the internal variables and the external driving factors of compound flood show significant increases in the multivariate analysis. (3) The joint variable of compound flood demonstrates a substantially increasing trend under both scenarios, along with an increase in the magnitude of the once-in-a-century flood. If the intra-correlation between multivariate is discounted, the degree of disaster would be underestimated.