Documenting the Development and use of coupled Surface and Groundwater Models to Determine the Fate of Nutrients in a Karst Aquifer

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Coupled surface water-groundwater (SW-GW) models are increasingly being used to help guide the management of karst watersheds, where quick infiltration and high spatial variability can lead to rapid and complex transport of dissolved pollutants. In the Santa Fe River basin in northern Florida, nutrient loading to the karstic Floridan Aquifer System (FAS) from agriculture and other anthropogenic sources has caused increasing nitrate concentrations associated with harmful algal blooms and aquatic ecosystem degradation. Increasing agricultural and municipal water use also threatens the region’s natural ecosystems. Best Management Practices (BMPs) have been developed to manage water and fertilizer use and subsequent nutrient loading while striving to maintain agricultural yields. However, given the complexities of GW-SW interactions in the watershed, assessing the efficacy of different BMPs on FAS water quality across multiple spatial and temporal scales is challenging. To address this challenge, we are using an integrated SW-GW modeling approach, applied at nested spatial scales, to capture the effects of climate, land-use, and land management (via BMPs) on water quantity and quality. We use the Soil and Water Assessment Tool (SWAT) to capture spatiotemporal variability of land uses and farming practices and to simulate BMPs via changes to fertilizer application rates, irrigation schedules, and crop rotations. We then couple SWAT to the groundwater model MODFLOW to capture the geological heterogeneity of the FAS and represent SW-GW interactions that support critical habitats (e.g. springs). Here, we detail our efforts to develop the model at scales ranging from farm-scale calibration of specific crops and BMPs to a fully coupled watershed-scale model. Preliminary results of the coupled model highlight the relative roles of climate/weather variability vs. land use and BMP application in helping meet regulatory water quantity and quality targets.

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Presented at the AGU Fall Meeting on December 13, 2018 in Washington, D.C.