Modelling atmospheric CO2 and CH4 mixing ratios over mixed natural-agricultural wetlands in the Ebre River Delta

Terrestrial ecosystems play a crucial role in mitigating climate change by reducing greenhouse gas (GHG) emissions and sequestering significant amounts of atmospheric carbon dioxide (CO₂). Wetlands, particularly coastal wetlands, are highly efficient carbon sinks but can also be large sources of methane (CH₄). Natural and agricultural wetlands, such as rice paddies, contribute to 37 % of global CH₄ emissions. Monitoring wetland-atmosphere carbon exchange is essential to evaluate the effectiveness of natural climate solutions (NCS), such as wetlands restoration and sustainable agricultural practices, in reducing GHG emissions and increasing soil carbon storage. Traditional methods for quantifying GHG emissions from wetlands include chamber flux measurements and eddy-covariance flux towers. These techniques provide valuable insights into carbon dynamics at the plot and ecosystem scale levels but fail to capture carbon fluxes at a regional scale, where policy decisions are often made. Recently, atmospheric composition observations have been used at regional scales and over urban areas to constrain the spatial and temporal distribution of GHG fluxes derived from land surface models. Applying similar methodologies to wetland regions, provided sufficient atmospheric observations are available, could enhance understanding of atmospheric carbon dynamics in these areas. The Ebre River Delta, a mixed natural-agricultural wetland system of international importance in terms of sustaining economic activities and biodiversity, offers a unique opportunity to investigate carbon sequestration and GHG emissions. This potential is enhanced by the availability of atmospheric GHG observations from in situ site tower and vehicle transects conducted across the regions.

Here, we integrate advanced modelling techniques and observational data to refine our understanding of GHG fluxes in the Ebre Delta. Biogenic GHG emissions over the Delta are estimated using a high-resolution Vegetation Photosynthesis and Respiration Model (VPRM) adapted for wetland ecosystems for CO₂, and the Kaplan model embedded in the Weather Research and Forecasting (WRF) Greenhouse Gas (WRF-GHG) model to estimate CH₄ emissions.  A sensitivity analysis is performed to compare VPRM CO₂ emissions from different model configurations, entailing a default and a wetland-adapted model versions, and two sources of input satellite-vegetation indices, MODIS and Sentinel-2, with contrasting  spatial resolutions. Then, modelled atmospheric CO₂ and CH₄ mixing ratios with WRF-GHG during growing season are compared with in situ observations from the site tower and vehicle transects to assess their accuracy. The framework developed in this study will provide the basis for investigating sequestration and emission hotspots over a mosaic of wetland land-uses and evaluate the region's potential for climate change mitigation and adaptation.