Investigating hydrodynamics within the Guayas Estuary and coastal Ecuador
The Guayas estuary, near Guayaquil, Ecuador, connects to the Gulf of Guayaquil (Figure 1). The system is a tide-dominated with a semi-diurnal tide and tidal range between 1-5m (increasing from the ocean to the Guayas River). The Guayas River drains into the estuary, with a mean annual discharge of 2000 m3/s and peak flows from January to March. Vegetation around the estuary is dominated by mangrove forests, though many forested areas have been converted to aquaculture for shrimp farming.
Salinity gradients control the structure of mangrove forests. To our knowledge, before the launch of the SWOT satellite the hydrodynamic interactions between the fresh and salt water had yet to be studies and no data were available on the water level along the Guayas estuary. Thus, new in situ measurements of water surface elevation were needed to calibrate and validate hydrodynamic models. SWOT provided long time-series of water surface elevation measurements at the mouth of estuary during the cal/val phase and currently, during the science phase, across the entire estuary. Data are now allowing to clarify to which degree SWOT are able to resolve processes within the Guayas estuaries and its multiple tributaries which are smaller channels, sometimes without wind (potentially dark water), and bordered by mangrove trees reaching 40 meters in height.
Our larger goal was to understand how the hydrology controls the structure of mangrove forests and contribute to carbon export to oceans. To achieve this goal, we sought to calibrate and validate a hydrodynamic model using in situ and SWOT data. There is still a need for in situ measurements of water surface elevation near and along the Guayas Estuary and its tributaries. The in situ measurements provided fast sampling—every 15 minutes— over a period of about 1 year. SWOT provided daily measurements during its cal/val phase that can be used to spatially extrapolate in situ-measurements across the entire region, thus improving modeling of overflow into mangrove forests. The in situ data have also served to calibrate and validate SWOT data in terms of its accuracy but also in terms of the impacts of layovers (due to mangrove tree shadow effects) and dark water (due to smooth water surface conditions resulting from lack of wind).
To do so, 11 pressure transducers and 3 barometers were deployed across the Guayas estuary and along the north shore of the Gulf going to Salinas to capture 2D variations in water level at 15’ interval for a period of about 1 year. The gauges have been installed by March 2023.
Before the start of this campaign, there was no current monitoring of the area, however, the in situ gauges served three purposes: 1-calibrate and validate SWOT data; 2-provide water level measurements with fast sampling intervals of 15’ and; 3- validate our tide and river discharge retrieval algorithms. In order to calibrate numerical hydrodynamic models in estuarine regions— strongly impact by tides—the 15’ sampling is required. We designed a non-stationary harmonic analysis that can be used with long SWOT time-series (~3 years) to resolve tidal and discharge contribution. The in situ data and calibrated numerical model have served to validate the SWOT retrievals.
Estimating fluxes of water across the land-ocean-sea-continuum (LOAC) is critical to evaluate the export of carbon and nutrient to the ocean, the deposition of sediment along the shore and to assess the vulnerability of wetlands and ecosystems in the face of sea level rise, increased storm activity and freshwater diversion projects. It is also critical to improve hydrodynamic models in these regions to support navigation, hazard contingency plans and rescue efforts. While gauging of estuaries and channels is commonly used to manage coastal wetlands, availability of these in situ measurements is in decline globally. But even the most extensive gauge networks cannot capture overflows to and within the wetlands. Our experiment demonstrated the potential of SWOT as an alternative to expensive gauges networks to calibrate hydrodynamic models and retrieve seasonal freshwater discharge. These calibrate models and discharge estimates in estuaries and river deltas can be used to improve our understanding of the water and carbon cycles, thereby providing a powerful tool to monitor salinity gradients in these regions and their contribution to ocean carbon.
The Guayas Estuary mission was funded by NASA ROSES.
Principal investigators: Marc Simard (Jet Propulsion Laboratory, California Institute of Technology), Mireya Pozo Cajas (Universidad de Guayaquil), Paola Calle Delgado (Escuela Superior Politécnica del Litoral, ESPOL)
Contact point for the study site: Marc Simard (marc.simard@jpl.nasa.gov)