Evaluation of Uncertainty in Estimated Flow and Phosphorus Loads by FHANTM

Abstract

The South Florida Water Management District uses a computer model, the Field Hydrologic and Nutrient Transport Model (FHANTM), to estimate flow (runoff and subsurface lateral flow) and phosphorus (P) loads leaving agricultural fields in the Lake Okeechobee watershed. Uncertain knowledge of FHANTM parameter values leads to uncertainty in model estimates. This study used First Order Analysis (FOA) and Monte Carlo Simulation (MCS) to quantify uncertainty in model outputs. A sensitivity analysis was conducted to identify parameters that have the greatest effect on model output. Six parameters significantly affected model output and were used in the uncertainty analysis. In the case of surface runoff, FACTOR (monthly potential evapotranspiration factor) accounted for 64% of the uncertainty in terms of variance. For P concentrations in surface runoff, PADD (the mass of phosphorus added by animals each day) accounted for 79% of the uncertainty in terms of variance. Thus, improving knowledge of these parameters has a significant impact on reducing the uncertainty in the predicted runoff and phosphorus loads.