A framework for determining the maximum allowable external load that will meet a guarantee probability of achieving water quality targets.

Quantifying the maximum external pollutant loading is extremely important for environmental management and ecological restoration. However, huge uncertainty exists in the process of determining accurate external pollutant loads discharging into surface water bodies (e.g., rivers, reservoirs, and bays). In this paper, a comprehensive framework is proposed for determining the maximum allowable external load by combining a dynamic nutrient-balance model with the guarantee probability of achieving a specific water quality target. As an important drinking water source for Beijing, the Miyun Reservoir was chosen as a case study because it is experiencing increasing eutrophication. The main results are as follows. ① The nutrient-balance model has shown a good fit to field observations both in calibration and validation periods using the modified Generalized Likelihood Uncertainty Estimation (GLUE). ② Feasible concentration targets were determined for total phosphorus (TP), total nitrogen (TN), and chlorophyll-a as 0.01 mg/L, 0.76 mg/L, and 4.91 μg/L, respectively. ③ The allowable external load of TP is estimated as 45.10-54.14 t, 23.76-29.58 t, and 8.30-12.78 t for guarantee probabilities of TP control target (e.g., 0.01 mg/L) of 25, 50, and 70%, respectively. While the external TN flux should be reduced by 200.21-480.73 t, 429.33-764.45 t, and 642.40-1069.59 t to meet the TN control target (e.g., 0.76 mg/L) at 25, 50, and 70% guarantee probabilities, respectively, The wide range of allowable external nutrient loading reflects the 95% confidence intervals of the load reduction analysis and indicates the importance of model simulation uncertainty and interpretation of the water quality objective. This paper provides a scientifically sound approach to water quality maintenance for the Miyun Reservoir and other surface water bodies.