Scientific High-Level Visiting Fellowships (SSHN) for Improved estimation of Indian wetland dynamics

Purpose and Objectives of the Trip: 

The main objective of this short trip is to strengthen the collaboration and develop concrete research plans for improved estimation of Indian wetland dynamics. The primary aim is to improve the estimation of tropical wetlands using the NASA-CNES Surface Water and Ocean Topography (SWOT) satellite mission and their response to changes in climatic parameters.

The sub-objectives are:

• Discussion on algorithm development for an improved high-resolution dynamic wetland extent estimation using remote sensing techniques for India.
• An accurate estimation of paddy crop coverage will help to determine how much wetland methane emission is anthropogenic. Paddy is one of the thirty most water-thirsty crops, which leads to the overexploitation of groundwater and is a significant source of methane emission.
• The team will discuss adapting the existing flood model using the SWOT data. One of Dr Singh’s PhD students is working on flood modeling.
• Additionally, the team will also explore the Inter-tidal bathymetry variation tool developed by Dr Frappart’s team for Indian coastlines.
• Visit the sites equipped with GNSS-R sensors to calibrate SWOT data to develop a similar method at Amrita University.
• Finally, the impact of climatic forcing on wetland dynamics of the tropics will be discussed.

These meetings will help to understand the large-scale response of surface hydrology to various changes in its driver. In tropical scenarios, this will help better understand the effect of changes in temperature and rainfall, wetland methane emission, and its feedback.

This will again help policymakers and scientists devise scalable mitigation measures.

Expected outcomes of the trip

Detailed discussions of wetland dynamics, coastal variability, and the possibilities SWOT offers are planned.

Some concrete expected outcomes are:

1. A framework for an adapted algorithm for Indian natural wetland dynamics estimation using SWOT satellite mission data.
2. Algorithm for estimating artificial wetland dynamics, focusing on paddy crop and future climatic variations.
3. Joint supervision of PhD students
4. Joint publications

The current collaboration between Indian and French team

Dr Frappart and Dr Singh work together in wetland dynamics using the NASA-CNES Surface Water and Ocean Topography (SWOT) satellite altimeter data. SWOT is a new type of satellite (wide-swath radar altimetry) and a top-priority mission for both NASA and CNES.

Dr Frappart, an expert and core science team member of SWOT on lakes and wetlands topics, has trained Dr. Singh’s

PhD students in altimetry data processing and analysis. He is also one of the doctoral committee members for Dr. Singh’s PhD Student (Ms Anagha V S). Dr. Frappart and the whole SWOT science

team will benefit from collaboration with  Dr. Alka on monitoring India’s wetlands. Dr. Alka’s two PhD students are working on wetland dynamics-related topics:

1. Ms Anagha is working on coastal ecohydrological dynamics. Dr. Frappart is her doctoral committee member. She previously applied for the Raman Charpak fellowship with Dr. Frederic Frappart. However, her application was not successful. Nevertheless, Dr. Frappart continued supporting her online. It is anticipated that this collaboration will result in a journal article.
2. Mr. Rajarajan is working on Methane wetland modeling. He has worked with Dr Philippe Peylin from ORCHIDEE (Organising Carbon and Hydrology In Dynamic Ecosystems), the land surface model of the Institut Pierre Simon Laplace (IPSL) for six months (July – December 2024) on biogeochemical modelling. The main objective of his dissertation is to reduce uncertainty in tropical wetland methane emissions. For this, accurate wetland extent estimation is crucial.

India is the biggest paddy producer, with multiple small natural wetlands along its coastline. The launch of the latest SWOT mission has made precise estimation of small wetland dynamics possible, and it is crucial not only for the hydrological budget but also for crop yield, methane estimation, and ecosystem resilience.

Description of Research Project 

Wetlands are very dynamic environments pressured by anthropogenic activities and climate change.

Wetlands in the tropics are understudied and different from their better-studied northern counterparts. Advanced remote sensing techniques and hydrological models derive dynamic wetland extent information. Satellite altimetry has limited coverage in the tropics compared to higher latitudes, but it also needs a large surface water area to accurately estimate changes in water level. Eventually, thousands of small Indian wetlands failed to be accounted for in the water-carbon budget. CNES and NASA launched a revolutionary satellite, the Surface Water and Ocean Topography (SWOT) mission, in December 2022. Using advanced radar technology, SWOT provides unprecedented global

observation of surface water bodies. The wide-swath altimetry mission SWOT has provided for the first time direct estimates of Surface Water Storage variations with an unprecedented spatial resolution (~100 m) (Salameh et al., 2021). In addition to high-resolution Ka-band, SWOT mission also carries a nadir altimeter (in Ku band, same as the one of traditional satellite altimetry like Jason-

3) that will provide information on the water surface elevation between the two swaths of SWOT.

Additionally, CNES has developed a SWOT hydrology simulator, which mimics the error characteristics of the Ka-band interferometric SAR over the surface of wetlands and rivers. Henceforth, the simulated observations supplement current SWOT observations to produce historical dynamics.

SWOT interferometry measures heights over water surfaces only, which prevents the use of this method over intertidal areas. Most tropical wetlands need more gauging stations to understand the water level variability and the extent of that variability. In addition to better estimating coastal wetlands using its capability to observe water bodies wider than 250 m × 250 m areas, the project is oriented to identify paddy fields. India is the biggest paddy cultivator, which taxes the water budget heavily and is also a significant source of methane.

Methane (CH4) is the second most potent anthropogenic greenhouse gas, with a warming potential of 80 times that of Carbon dioxide in 20 years (Koffi et al., 2020). The recent sharp rise in methane concentrations may derail attempts to keep the Earth’s temperature below the 1.5 °C target enshrined in the Paris Agreement (Saunois et al. 2020). The ultimate cause of the rise is unknown

because various processes of methane sources are considerably more varied and more uncertain. Tropical wetlands are a likely source of emissions that are not currently accounted for. Wetlands are the largest natural source of methane, roughly accounting for one-third of total emissions, and are the most important source of uncertainty in the methane budget (Hurtt et al., 2022; Ma et al., 2021; Saunois et al., 2020).

The lack of clear attribution to wetlands for the rise in methane concentration stems from uncertainty in wetland spatial extent and high spatial and temporal variability of methane emissions at various scales.

Integrating the model with advanced climate model data will allow the scientists to answer questions like how a wetland responds to a two °C rise or how much the methane/flood/coastline will be reduced or increased if the wetland extent is reduced by 20%, etc. During the proposed trip, the discussion will be focused on novel algorithm development to map the wetlands under the canopy efficiently using SWOT.