Evaluating estuarine ecosystem gradation and vulnerability through SWOT observations.

Deltas and estuaries represent some of the most dynamic and complex transition zones on Earth, serving as vital ecological hubs that are increasingly vulnerable to the dual pressures of accelerated sea-level rise and anthropogenic modification. The NASA-CNES Surface Water and Ocean Topography (SWOT) mission provides a transformative capability to observe these environments by providing high spatial-resolution, wide-swath measurements of Water Surface Elevation (WSE). Unlike traditional nadir altimetry, SWOT’s interferometric SAR technology allows for the capture of two-dimensional water level gradients across scales of estuarine reaches. This study explores the utility of SWOT time-series measurements in resolving complex tidal components within intricate coastal geometries and assesses how these physical measurements can be translated into critical parameters for evaluating the health and long-term vulnerability of coastal ecosystems. In this presentation, we focus on hydroperiod and salinity.

By performing harmonic analysis on SWOT-derived WSE time-series, we demonstrate the ability to effectively resolve previously unknown but major tidal constituents in estuarine channels. These tidal components allow for the precise derivation of the hydroperiod—the frequency, duration, and depth of tidal inundation. Hydroperiod is the primary environmental driver of vegetation zonation, nutrient cycling, and carbon sequestration potential in mangroves and saltmarshes. Consequently, accurate SWOT-based mapping of inundation patterns offers a new lens through which to view coastal resilience and the potential for "blue carbon" sequestration. Another driver of ecosystem gradation and vulnerability is salinity, which distribution is a non-linear product of the interaction between tidal forcing, freshwater discharge, and complex bathymetry, making it far more difficult to resolve than surface height alone. Indeed, it requires numerical modeling, which presents significant technical hurdles.

We tested the implementation of numerical hydrodynamic models in several distinct geographical settings to evaluate the limits of SWOT-informed simulations. These sites included the high-discharge, tide-dominated Guayas Estuary in Ecuador, the marine-dominated and ecologically sensitive Langebaan Lagoon in South Africa, and the complex, Knysna Estuary, also in South Africa. Our results indicate that while SWOT provides unprecedented boundary conditions for water levels, the ability to simulate salinity and transport remains heavily constrained by a persistent lack of high-resolution bathymetry and other in situ measurements. To improve these simulations, there is an urgent need for better bathymetric data derived from bathymetric lidar for shallow sub-tidal zones and in-situ sonar transects for deeper primary channels. Secondly, the common lack of in situ data along the salinity gradient inhibit robust assessment of the methods. This work highlights the necessity of a synergetic approach, combining SWOT’s wide-swath observations with targeted bathymetric mapping and in situ data assimilation to provide the predictive accuracy required for effective coastal management and ecosystem conservation in a rapidly changing climate.