<p>Evaluating the eutrophication risk of receiving lakes is essential for developing a scientifically based water replenishment plan. However, the substantial influence of water replenishment on both the quantity and quality of lake water complicates eutrophication assessments. In this study, we aimed to propose a novel approach to evaluate eutrophication risk by accounting for the combined effects of water quantity and water quality, specifically total phosphorus (TP) concentration. Applying nonparametric kernel estimation to the copula model enables direct estimation of the marginal distribution from the original data, thereby improving the accuracy of the marginal probabilities. Additionally, a Gaussian copula model, selected from five candidate copula models, was employed to establish a joint probability distribution function for water replenishment volume and TP concentration. The eutrophication risk of the lake was assessed through 10 replenishment scenarios involving six water volumes and four time periods. The results were as follows: (1) water replenishment significantly mitigated the eutrophication risk in Caohai Lake; (2) an increase in water replenishment volume correlated with a decreased probability of exceeding the TP concentration standard, achieving a probability of 0.44 when ecological water needs were met; and (3) the probability (0.49) of exceeding the water quality standard was considerably lower in scenarios with dry season replenishment. These findings provide valuable insights for assessing eutrophication risk and managing water replenishment projects in plateau lakes.</p>