Nutrient's response to water transfer in an urban river-network

Through water transfer, the hydrodynamic conditions of the river network can be improved, and the biochemical degradation of pollutants can be promoted, but at the same time, the endogenous release may be intensified. It is therefore possible to subdivide the response of nutrient content to water transfer into three components: (a) Direct Impact (DI) of dilution through water diversion, (b) Indirect Impact (II) by sediment release, and (c) Self Impact (SI) of nutrient degradation and attenuation of nutrients. By combining field investigation, laboratory experiment, and numerical simulation, the contribution weight of DI, II, and SI to the change of nutrient concentration was quantitatively analyzed. The results show that: (1) in the upper reaches of the river network, II plays a leading role in that the weight is more than 75%, and increasing the amount of water diversion increases the nutrient content; (2) SI plays a leading role in the tributary of the central part of the river network, the weight is more than 60%, and when the auxiliary pumping station is opened, the weight of II is less than 1%; (3) in the southeast of the river network, the nutrient release rate is sensitive to the change of hydrodynamics, and the weight of II is more than 90%. The results of this study can provide a reference for the formulation of a comprehensive hydrodynamic water quality management control scheme in the urban river network.