Watershed Loading Model Development for the Indian River Lagoon TMDL: Spatial Watershed Iterative Loading Model (SWIL 1.0-3.0) and Outfall Prioritization Tool

FDEP identified the Indian River Lagoon (IRL) and Banana River Lagoon (BRL) as impaired due to nutrient over-enrichment. The TMDL was established on the basis of a relationship between nutrient loading and segment-specific target seagrass depth limits. The Advancing Restoration in the Indian River Lagoon working group (IRL stakeholders) funded a study to analyze the health of the IRL ecosystem and the accuracy of SJRWMD’s watershed loading model (PLSM).

As a critical part of this study, Applied Ecology, Inc. led the development of an advanced pollutant loading model utilizing current databases for rainfall, land uses, BMP treatment, and EMCs. A customized GIS model, called SWIL (Spatial Watershed Iterative Loading), created a spatially varied, continuous monthly simulation of runoff over a 16-year period that provided a more spatially and temporally accurate representation of pollutant loadings and fresh water volumes in the IRL. Five updates to the model have been completed including those that addressed FDEP comments and improved monthly calibration and overall performance. The model output has been used for re-analysis of seagrass impairments in the IRL, defining a new IRL TMDL, and is currently under consideration by the FDEP for future BMAP applications, including allocations.