AbstractEutrophication in lakes and reservoirs occurs all over the world and has attracted wide attention in recent years. MIKE21, a numerical model focusing on eutrophication in lakes and reservoirs, was used to construct a two-dimensional eutrophication model. Douhe Reservoir in North China was used for the study. A hypothetical case study was applied in order to simulate a situation in which variables like air temperature, transferred water volume, or exogenous pollutant load might reach extreme values. A “Comprehensive nutrition state index” was used to evaluate the reservoir eutrophication level in different scenarios. The causes and trend of eutrophication were found, which could be a reference for eutrophication prevention and control. The main conclusions are that: (1) the warm-water emission from the power plant is the main reason for the rise in water temperature in Douhe Reservoir, which has a not negligible effect on eutrophication; (2) changes in outside temperature and reservoir storage capacity have just a small impact on a eutrophication trend; an increase in exogenous load can rapidly escalate eutrophication in Douhe Reservoir; (3) higher water temperature and higher TN/TP ratio lead to a lower Chlorophyll a concentration in water bodies of a nearby warm-water discharging spot than in the other two monitored areas.