Simulation-based cost-risk analysis of phosphorus reduction alternatives: application to a mountainous watershed

This paper proposes a model-based cost-risk analysis (CRA) approach for reducing phosphorus (P) load in the Amir Kabir Dam watershed, an important mountainous region in Iran, where high P concentrations are the key driver of eutrophication. The proposed CRA examines the tradeoff between 'economic efficiency' and 'P reduction effectiveness' within an uncertainty-aware framework that helps decision-makers meet the goal of P reduction with a restricted budget at various levels of risk. The SWAT model is employed for watershed-scale simulation of the flow, sediment, and total phosphorus (TP) load based on the current situation of the watershed. The validated model was then used to simulate different scenarios, including baseline and five P mitigation strategies. The output uncertainty arising from the model parameters is subsequently assessed through Monte Carlo simulations (MCSs) and then employed to estimate a risk index and draw a risk-cost plot. Besides, the cost-reliability ratio (CRR) is utilized to reframe the problem of optimal P mitigation and represent it as a discrete minimization problem. The simulation results indicate that the model could accurately predict measured flow, sediment, and total phosphorus (TP) due to the acceptable values of NSE, R-2, P-factor, and R-factor. CRA results illustrated that P reduction in defined mitigation strategies could meet the reduction target at risk values ranging from 5 to 64%. However, according to CRR, cost-effectiveness decreases by reaching lower risks. In the Amir Kabir Dam watershed case study, one percent risk reduction costs, on average, $0.08 M at higher risks to $0.65 M at lower risks.