The salty saga: a coastal landscape's response to saltwater intrusion

Abstract

Coastal farmlands in the eastern United States face escalating threats from soil salinization, driven by saltwater intrusion (SWI) resulting from rising sea-levels and more frequent coastal storms among other factors. These changes diminish agricultural productivity and trigger significant land cover transformations, including the conversion of farmland into salt patches or marshes, the expansion of ghost forests, and the migration of coastal marshes. Despite the growing impacts of salinization, large-scale monitoring remains challenging due to the complex spatial and temporal dynamics of salt-affected landscapes. In this dissertation, I use remote sensing, machine learning, and multi-source and multi-decadal ancillary data to (i) assess long-term trends in land use transitions, (ii) quantify salinity-driven land cover changes, and (iii) identify key environmental drivers of salinization. In Chapter 2, I examine long-term land use and land cover (LULC) changes in response to salinity variations from 2000 to 2016. Analysis of Global Soil Salinity Maps shows non-saline areas expanded (from 15,081 km2 to 16,219 km2) while extremely saline regions declined (from 3,644 km2 to 2,811 km2) in Delmarva, indicating reduced soil salinity over time. Land cover classification using Landsat imagery reveals significant losses of farmland (4.9%) and marsh (5.5%) area, highlighting the persistent threat of SWI to the coastal ecosystem. However, the coarser spatial resolution (30m) used in Chapter 2 limited the ability to fully capture the fine-scale and heterogeneous impacts of SWI on the landscape. Therefore, in Chapter 3, I present a spectral unmixing approach to map and quantify the fractional abundance of salt patches and marshes across the Delmarva Peninsula, which includes 14 coastal counties in Delaware, Maryland, and Virginia. This analysis reveals a three-fold increase in salt patch area between 2011 and 2022, growing from 4.7 km2 to 14.9 km2. In Chapter 4, I apply the methodology from Chapter 3 to create annual salt patch datasets for Delaware from 2019 to 2023, documenting a sharp rise in salt-affected farmland areas. I also investigate topographical and environmental factors driving salinization, finding that elevation plays a limited role, as both high- and low-elevation farmlands are vulnerable to salinization. Tidal amplitude, reference evapotranspiration, and proximity to water bodies emerge as key drivers for farmland salinization, with their relative importance varying annually. Integrating findings from Chapter 4 with recent studies suggests rapid expansion of salt patches in Delmarva between 2011-2023, growing from 4.7 km2 to 26.7 km2, indicating accelerated and complex landscape changes in recent years. Taken together, this work enhances our understanding of the spatial and temporal patterns of salinization in coastal agricultural landscapes. By integrating remote sensing and data-driven modeling approaches, this research provides valuable insights for environmental managers, policymakers, and agricultural stakeholders seeking to mitigate the impacts of saltwater intrusion and develop adaptive land management strategies and enhance coastal resilience.