Risk analysis of total maximum daily loads in an uncertain environment using EUTROMOD

Abstract

A two-phased Monte Carlo procedure is presented for estimating the probability distribution of annual phosphorus load to a lake and the response of the lake to the load. A watershed-level nutrient loading and lake response model, EUTROMOD, and a geographic information system (GIS) were used. The uncertainty in loading and lake response due to natural variability and parameter uncertainty were propagated separately throughout the analysis. The methodology was applied to Wister Lake in Oklahoma with the lake and its trophic state as the endpoint for total maximum daily load (TMDL) analysis. The watershed contributing to Wister Lake covers approximately 260,000 ha and contains a variety of point and nonpoint sources of pollution contributing to the degradation of the lake. Model results compared well with measured water quality data. EUTROMOD simulations indicated that the lake is eutrophic under current land use and management conditions. Nonpoint source (NPS) pollution was estimated to contribute nearly 90 percent of the annual phosphorus load with the remainder attributed to point sources. The majority of this NPS load was attributed to agriculture. Alternative management evaluations indicated that an average reduction of agricultural NPS loads by 25 and 35 percent would be needed to meet our water quality goal with and without point source controls, respectively. Due to uncertainties inherent in the evaluation procedure, the required reductions had large confidence intervals which must be taken into consideration in the decision making process.