The Importance of Process Representation for Simulating Coupled Surface-Groundwater Flow in Karst Watersheds: A Comparison of SWAT, SWAT-MODFLOW and DisCo

The highly permeable matrix and conduit-dominated flow of karst systems make them vulnerable to contamination due to rapid infiltration and groundwater transport. Critical issues in karst regions with intensive agriculture include water overallocation and high nutrient loads, motivating the need for management strategies to reduce further impairment. But, developing strategies for aquifer and aquatic ecosystem protection is challenging due to the need for hydrological models that capture the spatiotemporal variability in nutrient loading and water use and adequately represent the complex flow dynamics common in karst aquifers. Models that can simulate land and water management strategies and capture the complexity in karst systems are limited. Surface-runoff models such as the Soil and Water Assessment Tool (SWAT) are useful for simulating changing land management practices and quantifying pollutant loads, but they apply simplistic groundwater routines. Notably, SWAT can now be coupled to the USGS groundwater flow model MODFLOW to overcome these limitations, but simulating integrated surface and groundwater flow systems may still be limited when coupling two distinct models. Fully coupled surface and groundwater models such as Discrete Continuum (DisCo) can simulate 3-D surface and groundwater flow using Richards equation for variably saturated subsurface flow and capture turbulent flow in discrete conduits. While fully coupled models may better represent the flow physics in karst terrain, their inability to simulate management strategies remains a major limitation for guiding watershed management. In this work, we compare the ability of SWAT, SWAT-MODFLOW and DisCo to reproduce surface and groundwater interactions and hydrodynamics in a karst watershed in northern Florida. Model development is part of the USDA-NIFA funded Floridan Aquifer Collaborative Engagement for Sustainability (FACETS) project, which aims to understand land use changes needed to achieve agricultural water security while meeting environmental regulations. Preliminary results show all models perform well, but DisCo captures baseflow and storage in karst systems more effectively. We use these results to discuss the limitations and benefits of each model for karst watershed management in the context of the FACETS project