Towards improving agricultural and environmental sustainability in the lower Apalachicola-Chattahoochee-Flint River Basin by understanding the agricultural and climate change impacts on the surface- and groundwater resources
Type of DegreePhD Dissertation
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Surface- and groundwater resources in the lower Apalachicola-Chattahoochee-Flint (ACF) River Basin play an important role in the economic and ecological vitality of the region. Agriculture, which contributes more than $2 billion annually to the region’s economy, is heavily dependent on the underlying Upper Floridan Aquifer (UFA) for irrigation. As the aquifer is close to the land surface and in direct connection with many surficial streams, it also plays a critical role in sustaining streamflow and maintaining the habitat of a wide range of flora and fauna including the endangered mussels species in the region. Intensive groundwater withdrawal from UFA for irrigation, which is further projected to increase in the future, has, however, led to decreases in streamflow and groundwater levels. This has led to detrimental impacts in the habitat of the aquatic species and also threatened the sustainability of agriculture in the region. The issue is further compounded by the emerging and potential impacts of climate change in the watershed hydrologic components in the region. As a result, the overarching goal of this dissertation was to help improve the agricultural and environmental sustainability of the region by evaluating the benefits of the change in agricultural management practices for the major row crops (cotton and peanut) along with understanding the regional impacts of the projected increase in irrigation and changing climate in the surface- and groundwater resources of the region. A field-scale SWAT model can help evaluate the agricultural management practices in detail and also help to increase the confidence of the stakeholders in the evaluation results. As a result, a detailed literature review of the different methods SWAT can be set up as a field-scale model was first performed to identify the most suitable method for setting up the model for this study. The review study identified five different ways with which a SWAT model could be set up for field-scale evaluation. Evaluating the range of agricultural management practices using a field-scale SWAT model for cotton and peanut production for irrigation water use, crop yield, and nitrate loss identified the management scenario with soil moisture sensor-based irrigation, cover crop, and strip tillage as having the highest potential for reducing irrigation water use and nitrate loss while maintaining high agricultural productivity. Although the management scenarios that are most adopted in the region, which included checkbook irrigation and no cover crops, had similar crop yields, irrigation water use and nutrient loss were considerably high. Evaluating the regional impacts of the projected increase in irrigation using the MODular groundwater FLOW (MODFLOW) model showed that groundwater levels would decrease by as much as 2.38 m when compared to levels observed in a significant drought year of 2011. Reduction in groundwater levels was highest in the region of the lower ACF River Basin where the aquifer was comparatively thin. It was also observed that groundwater discharge from the UFA to the surficial streams would decrease by as much as 33% indicating that the projected increase in irrigation may not be sustainable, especially during prolonged drought conditions. Assessing the impacts of streamflow and evapotranspiration (ET) calibration on groundwater recharge simulation by SWAT to determine if there is a calibration approach that results in improved groundwater recharge simulation showed that calibration of streamflow followed by ET provided the best estimates for groundwater recharge. The study also identified streamflow as the most important variable to be calibrated for groundwater recharge simulation while calibration of ET alone had a negligible impact in groundwater recharge simulation. A comparison of SWAT simulated groundwater recharge to estimates derived from RORA also showed that SWAT can accurately simulate groundwater recharge in the lower ACF River Basin. Information derived from this study was critical in setting up and calibrating the SWAT-MODFLOW for evaluating the impacts of climate change. Evaluation of the regional impacts of climate change in the surface- and groundwater resources of the lower ACF River Basin using the calibrated SWAT-MODFLOW model showed an increase in streamflow in most months throughout the region. There were, however, certain months, mostly at the beginning of the year, during which streamflow decreased in the Spring and Ichawaynochaway watersheds under future climate. Comparison of flow duration curves (FDCs) between the baseline and future climate identified a considerable increase in streamflow in extreme events indicating the possibility of flooding events in the future. Although the study indicated to an increase in surface- and groundwater (SW-GW) exchange (groundwater discharge to streams) in the main-stem of the Flint River, groundwater discharge to streams reduced in the ephemeral streams and the Spring and Ichawaynochaway watersheds signaling to the vulnerability of these watersheds to the change in the climate. Overall, results from the field-scale evaluation of the agricultural management practices of the major crops can help reduce irrigation water while maintaining high agricultural productivity, which can be important, especially during prolonged drought conditions. Regional evaluation of the projected increase in irrigation as well as climate change helped us understand the challenges that lay ahead for the agricultural and ecological sustainability of the region, an understanding of which can help us plan and make better management decisions for the future.