Performance Evaluation of the QUAL2Kw Model for Water Quality Simulation in Mountain Rivers with Step Pool Morphology

Nesari, Mahdi; Majdzadeh Tabatabai, Mohammad Reza; Ghoreishi Najafabadi, Seyed Hossein

doi:10.48306/juem.2026.588676.1164

Performance Evaluation of the QUAL2Kw Model for Water Quality Simulation in Mountain Rivers with Step Pool Morphology

Authors

Mahdi Nesari ¹

Mohammad Reza Majdzadeh Tabatabai ²

Seyed Hossein Ghoreishi Najafabadi ²

¹ PhD Candidate of Civil, Water and Environmental Engineering Department, Shahid Beheshti University, Tehran, Iran

² Civil, Water and Environmental Engineering Department, Shahid Beheshti University, Tehran, Iran

10.48306/juem.2026.588676.1164

Abstract

Mountain rivers with step pool morphology represent complex hydraulic systems in terms of water quality dynamics; however, the performance of conventional water quality models such as QUAL2Kw, which are designed based on the assumption of uniform flow, has rarely been evaluated in such environments. This study assessed the performance of the QUAL2Kw model in simulating water quality in these rivers. A 66 m reach with step pool structure was selected on the Dohezar River in Mazandaran Province, Iran. Water quality sampling for temperature, dissolved oxygen, nitrate, and inorganic phosphorus was conducted at four monitoring stations (upstream, step crest, step toe, and downstream) over an eight month period under both low flow and high flow conditions. The model was calibrated using the first four months of data and verified with the remaining four months. Results showed under low flow conditions, the QUAL2Kw model performed satisfactorily for all parameters, with NSE exceeding 0.84. However, nitrate and inorganic phosphorus simulations exhibited lower errors compared to temperature and dissolved oxygen, attributed to increased residence time and reduced turbulence, which allowed sufficient time for kinetic reactions. Under high flow conditions, the model overestimated all four quality parameters, particularly at the step pool structure. This error is attributed to reduced residence time and the dominance of physical processes over kinetic reactions. Based on the findings, the model can serve as an efficient tool for water quality simulation in mountain rivers during low flow periods, and its results are valuable for the environmental management of these ecosystems.

Keywords

QUAL2Kw model

Water quality simulation

Step pool morphology

Mountain river

Nutrient modeling

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