Key Takeaways & Executive Findings
- •• • The coupled MIKE21 hydrodynamic-water quality model achieved R2 = 0.89 (hydrodynamic) and average R2 = 0.86 (water quality), with MRE = 11.3% and 14.21%, respectively, demonstrating high predictive accuracy for mountainous river systems. • • Average flow velocities during wet, normal, and dry periods were 1.78, 0.72, and 0.36 m·s−1, respectively; natural sections reached up to 4.3 m·s−1, while reservoir sections approached stagnation, highlighting the hydraulic impact of cascaded dams. • • Spatially, reservoir sections exhibited significantly higher nutrient concentrations (TP: 0.12–0.17 mg·L−1; TN: 0.60–0.80 mg·L−1; NH3-N: 0.10–0.45 mg·L−1) compared to natural sections (TP: 0.07–0.10; TN: 0.25–0.50; NH3-N: 0.025–0.250 mg·L−1), indicating nutrient retention in reservoirs. • • Temporally, TN and NH3-N peaked during dry and normal periods (0.5 and 0.1 mg·L−1, respectively), while TP peaked during wet and normal periods (~0.1 mg·L−1), underscoring the need for seasonally adaptive pollution control strategies.
Abstract
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.
1. Introduction
Cascaded hydropower development in mountainous inland rivers fundamentally alters hydrodynamic conditions, reducing flow velocities and increasing water residence times, which in turn modifies nutrient transport and transformation. Traditional point-sampling methods fail to capture the continuous spatiotemporal dynamics of water quality, particularly in data-sparse mountainous reaches. Existing water quality models, such as SWAT and QUAL, often lack the resolution or hydraulic structure representation needed for simulating the complex interactions between reservoirs and natural river sections.
This study addresses these limitations by applying MIKE21, which incorporates hydraulic structure modules for dams and gates, to the upper Heihe River—a typical high-cold inland river with steep gradients and significant seasonal flow variability. The model integrates hydrodynamic and water quality simulations to resolve the spatial heterogeneity between reservoir and natural sections, providing a robust tool for assessing nutrient dynamics under cascaded development. The findings offer critical insights for water environment management and ecological operation in similar sensitive inland river basins.
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WANG Yu, WEI Xiaozhen, LUO Tianfeng, WANG Hao, TIAN Miao, SUN Chao, ZHANG Xiaolong, REN Weilong, HU Feiyan (2026). Numerical Simulation of Water Environment in the Mountainous River of the Upper Heihe River Based on MIKE21. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511055
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Frequently Asked Questions
What are the key limitations of the MIKE21 model when applied to steep mountainous rivers with ice cover, and how were they addressed?
The model was calibrated using field data from January to August, which includes ice-cover periods. However, the model does not explicitly simulate ice dynamics, which could affect hydrodynamic accuracy during winter. The study achieved R2 = 0.89 for hydrodynamics, suggesting acceptable performance, but further refinement may be needed for ice-affected periods.
How does the model handle the representation of hydraulic structures (dams) and their impact on flow and water quality?
MIKE21 includes a hydraulic structure module that simulates dams, gates, and weirs. This allowed the model to reproduce near-stagnant conditions in reservoir sections, with velocities approaching zero, and to capture the resulting nutrient accumulation (e.g., TP up to 0.17 mg·L−1 in reservoirs vs. 0.10 mg·L−1 in natural sections).
What are the implications of the observed spatial nutrient gradients for monitoring network design?
The significant differences between reservoir and natural sections (e.g., TN 0.60–0.80 vs. 0.25–0.50 mg·L−1) indicate that monitoring stations must be placed in both zones to capture representative conditions. Relying solely on natural sections would underestimate nutrient loads in reservoirs, potentially leading to ineffective management.
How transferable is this modeling approach to other high-cold inland rivers with limited data?
The approach is transferable if sufficient calibration data (water levels and nutrient concentrations) are available. The model's performance (MRE < 20%) suggests it can be adapted, but site-specific parameterization is essential. The study provides a framework for parameter estimation and validation that can be replicated.
What are the practical implications of the seasonal nutrient patterns for reservoir operation and pollution control?
Higher TN and NH3-N during dry periods (0.5 and 0.1 mg·L−1) suggest that reservoir releases during low-flow periods could exacerbate downstream nutrient loads. Operational strategies, such as timed releases to enhance dilution during dry periods, could mitigate this. The model can be used to test such scenarios.
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