Impact of Changjiang-Hanjiang Water Diversion Project on the runoff process in the lower reach of Hanjiang River

Inter-basin water transfer is a commonly adopted measure for improving hydrological and ecological conditions within watersheds. However, systematically quantifying the actual improvement effects of water diversion projects on watershed runoff conditions remains a weak link in current hydraulic engineering evaluation. This study conducts complex modeling of the Changjiang-Hanjiang Water Diversion Project (CHWD), the Middle Route of South-to-North Water Diversion Project (MR-SNWD), the Dongjing River diversion, and the cascade reservoirs in the middle and lower reaches of the Hanjiang River with Soil and Water Assessment Tool (SWAT). Based on this modeling framework, the daily runoff process is simulated in the lower Hanjiang River under a scenario with no CHWD, and the impact of the CHWD on the runoff process is investigated in the lower Hanjiang River. The results indicate that: (1) the water supplementation from the CHWD is insufficient to balance the reduction impact on downstream Hanjiang River runoff caused by water withdrawal from the MR-SNWD; (2) the water supplementation from the CHWD is primarily utilized to supply the Dongjing River diversion, with limited effect on increasing runoff in the Hanjiang River section downstream of Dongjing River; (3) the water supplementation from the CHWD contributes to enhancing the stability of the runoff process in the lower Hanjiang River. This study yields a significant insight that gravity-flow water conveyance is suboptimal for inter-basin water transfer in flat terrain regions. These findings offer valuable implications for the planning and design of such projects globally.