Linking hydrological, hydraulic and water quality models for river water environmental capacity assessment

The water environment of a river network can self-clean to a certain extent; however, when the wastewater discharge load exceeds a certain threshold, the balance of nature is disrupted, leading to water pollution. This emphasises the urgent need to evaluate river water environmental capacity (RWEC) as a necessary parameter for sustainable development. However, to quantify the RWEC, it is important to estimate the hydrological and hydrodynamic factors in the basin, leading to the linking of these models. The present investigation aims to propose an integrated framework, named RWEC, consisting of hydrological and hydrodynamic models, a database system, and GIS to evaluate the water environmental carrying capacity of the selected river network. The rain - runoff (RR), hydrodynamic (HD), ecological (ECO), and RWEC models were used. Groups of data, including meteorology, hydrology, and the environment, combined with topographic data and waste sources, were applied. Groups of models and data were integrated into a seven-step framework to calculate the RWEC. The case study is a basin in Binh Duong Province, Vietnam and four pollutants were selected: NH₄⁺, BOD₅, NO₃^-, PO₄^3-. The flow and water quality factors in the river basin in the study area were measured based on hydraulic models, and the water quality was calibrated. The role of hydrological, hydraulic, and water quality models in the RWEC calculation was clarified. According to the baseline and forecast scenarios, the calculation of the RWEC for the scenarios was performed. In the baseline scenario, RWEC_NH4+ is in the range (-283, -22) kg/day, RWEC_BOD5 ranges from (143, 3126) kg/day, RWEC_NO3- is in the range (-778, 2166) kg/day, and RWEC_PO43- is in the range (-31, 46) kg/day. The dependence of RWEC on environmental factors, self-cleaning factors, and the difference between the baseline and forecast scenarios were clarified.