System Dynamics Modeling for Predicting Nitrogen Load in Euphrates River Within Iraq

ABSTRACT Mechanistic understanding of the nitrogen cycle in natural freshwater systems, such as the Euphrates River, is critical for sustaining ecosystem services and environmental protection. A model framework of the biogeochemical processes of nitrogen dynamics has the potential to analyze system performance and could highlight the most effective solutions for water quality pollution. The dynamics of mineral nitrogen, including ammoniacal N (NH 4 + +NH 3 ), oxidized N (NO 2 − +NO 3 − ) and total mineral N (TIN) in the Euphrates River within Karbala city in Iraq, are investigated between April 2018 and September 2019. In this study, a system dynamics model is developed to examine the dynamics of mineral N and explore new insights for mitigation strategies. Findings obtained from this research indicate the ability of the system model to reproduce field observations reasonably. The root mean squared error (RMSE) value for the modeled and observed total ammoniacal N is 0.619 g N m −3 , which reflects a best fitted description for the steady state concentration. A higher RMSE value observed for total oxidized N (3.591 g N m −3 ) could be attributed to the higher nitrifying capacity of the system than model predictions. Field observations and model predictions confirm that the NO 3 − is the predominant N form in the Euphrates River (2.64 ± 1.54 g m −3 ), suggesting the important role of metabolic pathways in N cycling in the river system. These findings are also supported by the changes in the hydraulic water budget and physicochemical responses. Research outcomes highlight the modeling framework as an important approach to improving knowledge of N dynamics, system analysis, and helping decision‐making to explore best management practices in the Euphrates River.