Abstract: In recent years, water quality problems have occurred frequently in the middle and lower Hanjiang River and its environmental discharge is yet unclear, which threatens the safety of water use in the regions along it. To assess its water quality and environmental flows, this paper describes a single-factor method to evaluate their spatial and temporal distribution characteristics based on data observation. First, we collect field data for five water quality indicators－pH, dissolved oxygen, permanganate index, ammonia nitrogen, and total phosphorus－which were observed in 2015-2019 at seven control sections in the middle and lower reaches: Huangjiagang, Xiangyang, Huangzhuang, Shayang, Qianjiang, Xiantao, and Hanchuan. Then, we develop a one-dimensional hydrodynamic and water quality model and a corresponding method for calculating the environmental discharge that meets the requirement of water quality at each control section, and calculate the environmental discharges of the river using different water sources inputs. The results show two failure cases－ρ (NH3-N) in the high flow period, and ρ (TP) in the low flow period, which are unable to meet the class Ⅱ standard at the section downstream of Huangzhuang. To guarantee the environmental discharge, the operation of Water Transfer Project from Yangtze River to Hanjiang River should be combined with the discharge from the Danjiangkou reservior. In the high and low flow periods, when a flow of 500 m3/s or 350 m3/s is diverted from Yangtze, the Danjiangkou reservoir should discharge at 214 m3/s or 315 m3/s respectively to meet the demand by the river environmental discharge. This study has explored the schemes and verified their improvement on water quality security and rational allocation of the reservoir water resources.

Key words: middle-lower reaches of Hanjiang River, water quality assessment, hydrodynamic and water quality model, environmental discharge