Quantifying the Effects of Land Use and Management on Receiving Water Quantity, Quality, and Ecosystem Health in a Karst Watershed

The Upper Floridan Aquifer (UFA), one of the most productive aquifers in the world, supports productive agricultural and silvicultural industries, and provides drinking water to more than 10 million people. A significant portion of the UFA is unconfined and thus is rapidly recharged by rainfall and easily polluted by a variety of land uses. As a result, the aquifer and the springs and rivers it feeds face significant threats to water quality, water quantity and ecosystem health. In Florida, minimum flows and levels (MFLs) and numeric nutrient criteria (NNC) for nitrate-nitrogen (NO3-N) have been set to protect the ecological integrity springs and rivers fed by the UFA, but these standards are not currently being met throughout much of the state.This study focuses on the Santa Fe River Basin, where it is estimated that a 11.3 % reduction in water withdrawals and 35% reduction in NO3-N leaching to the UFA are needed to meet the mandated MFL and NNC. Land use in the basin is dominated by production forestry and agriculture (primarily corn, peanut, hay, pasture). The objective of this study was to quantify NO3-N loads and net groundwater recharge across the range of production practices currently being implemented for these land uses and to understand changes needed to achieve environmental standards. Results showed that, for current practices, corn-peanut rotations yield average net recharge of -170 mm/year and average NO3-N leaching of 200 kg/ha/yr. For grazed pastures, hay, and production forestry, average net recharge was 504, 476, and 316 mm/yr, and average NO3-N leaching was 110, 3.4, 4, kg/ha, respectively. We found that required reductions in irrigation pumping and NO3-N leaching can be achieved by conversion from more intensive (corn, peanut, pasture) to less intensive (hay, forest) land uses, or by strict adoption of advanced irrigation and nutrient management strategies such as soil moisture sensor triggered irrigation, reduced rates and improved timing of fertilizer application, reduced stocking densities, and rotational grazing. Current work is investigating the regional economic impact of required changes and assessing both the public’s willingness to pay and producers’ willingness to accept incentives to bring about these changes

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