Coupled Hydrologic–Hydraulic–Hydrodynamic Modeling Framework for Coastal watersheds/Estuaries and MFL Evaluation

Shimelis Setegn, SFWMD/FIU Boynton Beach Florida, United States. ssetegn@gmail.com

Freshwater inflow is a primary driver of salinity dynamics, water quality, and ecosystem health in coastal and estuarine environments. In highly managed systems such as the St. Lucie, Caloosahatchee, and Loxahatchee estuaries in South Florida, understanding the magnitude, timing, and sources of freshwater inflow, including contributions from ungauged tidal basins, remains a critical challenge for water resource management, ecological restoration, and regulatory decision-making (Setegn et al., 2026).

This study presents the development and application of an integrated hydrologic, hydraulic, and hydrodynamic modeling framework based on the WaSh and estuary model. The WaSh model is a coupled watershed water quality model. The modeling system integrated watershed-scale hydrology, groundwater flow, surface-water routing, and water quality processes with downstream hydrodynamic simulations of estuarine circulation and salinity transport. This integrated approach enables a physically consistent representation of the continuum from upland watershed processes to coastal receiving waters (Setegn et al., 2026). The WaSh model incorporates key management features, such as reservoirs, stormwater treatment areas, irrigation demands, and land-use dynamics, making it well-suited to the highly regulated environments typical of South Florida. A key contribution of this work is the application of integrated hydraulic–hydrodynamic modeling as a decision-support tool for developing and re-evaluating Minimum Flows and Levels (MFL) criteria under changing environmental conditions. The framework is used to assess how alterations in freshwater inflow, driven by climate variability, land-use change, and projected sea-level rise, affect estuarine salinity regimes and ecological thresholds (Obeysekera et al., 2011; IPCC, 2021). By explicitly linking watershed processes to estuarine response, the model provides a robust basis for evaluating MFL criteria and identifying adaptive management strategies. The model is implemented using long-term hydro-meteorological datasets, including rainfall, temperature, evapotranspiration, land use, soils, and detailed hydrographic networks. Results demonstrate that the integrated system successfully reproduces observed freshwater inflows and salinity patterns, with satisfactory performance metrics (e.g., NSE ≈ 0.7), and improves the quantification of inflows from ungauged regions (Setegn et al., 2026). This work highlights the importance of fully coupled, process-based modeling approaches for representing interactions among the surface, subsurface, and coastal environments. It supports improved decision-making for estuarine management in the face of climate change and sea level rise.

Keywords: Coastal hydrology, hydrodynamic modeling, MFL, sea level rise, climate change, estuarine salinity, WaSh model, coupled systems

2026 CSDMS meeting-043

Coupled Hydrologic–Hydraulic–Hydrodynamic Modeling Framework for Coastal watersheds/Estuaries and MFL Evaluation