Effect of inter-basin water transfer on water quality in an urban lake: A combined water quality index algorithm and biophysical modelling approach

To improve water quality and alleviate eutrophication in a polluted urban lake, water diversion, a common method of basin management, is planned. In this study, we used the physiology-based water quality model, the General Ecosystem Module (GEM) in FVCOM, to investigate the potential influence of transferred water on the water quality index (WQI) and comprehensive eutrophication state index (TLI) of the lake. A fuzzy synthetic evaluation algorithm was applied to quantify the value of the WQI objectively. Our results indicate that total nitrogen and total phosphorus (TN and TP) are the most dominant pollutants influencing water quality, accounting for 63.6% of the weighting. In addition, overflow of municipal sewage after rainfall contributed to the decrease of the WQI (class IV in spring and class V in summer). This slow-flow shallow water body was categorised as a light eutrophic lake according to the TLI calculation. As an important factor that influences pollutant retention and water exchange, residence time (τe) was also considered. A multi-objective optimisation method was used as an effective method to guide practical management, considering the hydrodynamic conditions (τe), water quality (WQI) and economic cost (M). First, the WQI, τe and economic cost of different parts of the lake were investigated under different inflow discharge scenarios using our combined approach. Then, the Wolves Colony Search Algorithm was used to minimise the dimensionless objective function, which is comprised of the three factors above. The calculated environmental water demand of Lake Gantang (GTL) and Lake Nanmen (NML) was 0.0456 and 0.0417 million m3 d−1, respectively. Under optimal water diversion conditions, this lake changes to a mesotrophic lake of class III with good water body exchange capacity. Overall, our combined model can be a guide for the local government and decision-makers and used to evaluate the optimal amount of transferred water at a minimum cost for inter-basin management.