Algal bloom prediction influenced by the Water Transfer Project in the Middle-lower Hanjiang River

River algal blooms have become a complex and challenging environmental issue due to the impacts of human activities and inter-basin water transfer project. As the water source of the Middle Route Project (MRP) of South-to-North Water Diversion Project (SNWDP), the Hanjiang River (HR) has experienced frequent algal blooms and this caused potential risk to the drinking water safety of residents along the river. Here, an integrated river algal bloom model was developed to simulate river algal blooms and analyze the effects of MRP of SNWDP on algal bloom occurrence patterns in the middle-lower HR. The integrated river algal bloom model considered multiple physical-chemical-biological processes, which include the hydrological cycle of basin, the interactive mechanisms of river hydrodynamics and the improved algal dynamics process by adding a hydrology factor. The results show that the relative error between simulated and observed algae density at the Qin Duankou (QDK) and Zongguan (ZG) sections are 17.1% and 18.5% during the calibration period and are 18.3% and 19.7% during the validation period, indicating that the proposed integrated algal bloom model met the basic requirements for simulating river algal blooms. Additionally, our analysis indicates that the occurrence of algal bloom events in the middle-lower HR will increase by about two-fold under the water transfer scenario of 9.5 billion m3 year–1 and 2.5-fold under the scenario of 13 billion m3 year–1 at the QDK section. This study potentially provides scientific tool for local governments to carry out optimal scheduling of water conservancy projects for the protection of water ecological environment.