Numerical Simulation of Water Environment in the Mountainous River of the Upper Heihe River Based on MIKE21
To systematically investigate the spatiotemporal distribution of hydrodynamics and water quality under cascaded hydropower development in the upper Heihe River, a MIKE21-based water environment model was constructed for the mountainous reach. The model simulated the dynamic changes of total phosphorus (TP), total nitrogen (TN), and ammonia nitrogen (NH3-N) from January to August 2023. Calibration and validation against field data showed good performance: the hydrodynamic model achieved a coefficient of determination (R2) of 0.89 and a mean relative error (MRE) of 11.3%; the water quality model achieved an average R2 of 0.86 and an average MRE of 14.21%. Hydrodynamic simulations revealed average flow velocities of 1.78, 0.72, and 0.36 m·s−1 during wet, normal, and dry periods, respectively. Natural river sections exhibited high velocities up to 4.3 m·s−1, while reservoir sections had near-stagnant flow due to hydraulic structures. Water quality simulations indicated that TN and NH3-N concentrations were higher in dry and normal periods, whereas TP was higher in the wet period. Spatially, concentrations in reservoir sections exceeded those in natural sections: natural sections had TP, TN, and NH3-N concentrations of 0.07–0.10, 0.25–0.50, and 0.025–0.250 mg·L−1, respectively, while reservoir sections had 0.12–0.17, 0.60–0.80, and 0.10–0.45 mg·L−1. These findings provide scientific references for water environment management in the Heihe River and similar inland river basins.