Evaluating Watershed Pollutant Trading Schemes under Uncertainty: a Computational Approach

We present and illustrate an approach to evaluate the options for watershed pollutant trading based on the development and application of a comprehensive simulation model. In particular, attention is focused on phosphorus as a nutrient/pollutant, and our illustrative case focuses on a portion of Chattahoochee watershed, between the outflow from Lake Lanier at the Buford Dam and the inflow to Lake West Point, i.e., passing southwards through metropolitan Atlanta. The model – in fact, an assembly of three sub-models – simulates the full dynamic behavior of not merely nonpoint sources of pollutants but also point-source discharges, and the consequent propagation of variations in stream flow, sediment flux, and phosphorus flux through the principal tributaries and main stem of the Chattahoochee River. In order to account for uncertainties, which are significant in current representations of transient variations in stream water quality, the entire model is embedded in a framework of Monte Carlo simulation. We use this general framework and the specific case study to show how various candidate options for pollutant trading can be evaluated on a consistent basis for their reliability in the presence of uncertainty. We consider this kind of “in silico” evaluation especially important at a time when there is a strong aspiration to see pollutant trading schemes implemented, but little direct evidence of their successes/failures in actual practice.