Refined analysis of flood-regional composition under changing environment in the middle reach of Hanjiang River

To investigate the flood-regional composition under changing environmental conditions in the middle reach of Hanjiang River (MHR), a data-driven hydrological simulation model and its related quantitative methods were developed. The flood-regional composition of Huangzhuang(HZ) in the MHR was quantitatively analyzed, and the influence of environmental changes on river flood routing was discussed. The primary research findings are as follows: (1) A hydrological simulation model based on support vector regression machine (SVRM) is constructed to simulate the daily average flow process of HZ from 1965 to 2021. The Nash-Sutcliffe Efficiency (NSE) coefficient achieved values above 0.95, and the overall relative error (RE) was within +/- 1 %, indicating excellent simulation performance. (2) A quantitative analysis method has been proposed to identify the composition of flood areas. The results indicate that the upper reach of Hanjiang River (UHR) is the primary contributor to floods in the MHR, accounting for 60.62 % to 78.05 % of the total. The Tangbai River (TR) contributed between 14.1 % and 27.4 %, whereas the Nan River had a smaller contribution of only 6.83 % to 8.85 %. (3) Trend analysis indicates that the proportion of floods originating from the UHR increases in the summer flood season and decreases in the autumn flood season, while those changes of TR and Nanhe River (NR) are coincidental, especially in the autumn flood season, the proportion of floods in the TR increases significantly. The impact of floods from the UHR and TR cannot be ignored when implementing flood control measures. (4) A comprehensive analysis method has been proposed to quantify the integrated impacts of environmental changes. The results show that environmental changes had a relatively minor impact on flood routing and its flood-regional composition. However, they did affect the flood propagation process, resulting in earlier occurrences in peak flow, increased in peak discharge, and rapid rise and fall of floodwaters for floods exceeding 12,000 m(3)/s. These research findings provide strong foundational support for designing flood-regional, as well as flood control and disaster reduction systems in the MHR.