Sentinel-1 SAR backscatter and coherence patterns during dry and wet periods at a large emergent wetland in the Pannonian lowlands

Emergent wetlands occupy the transitional zone between permanent wetlands and upland environments and provide a range of ecosystem services. These services strongly depend on inundation patterns and vegetation dynamics, which are often difficult to monitor using in-situ approaches due to limited accessibility, dense vegetation, and the need to minimise disturbance, particularly in protected areas. Remote sensing offers an effective means to overcome these constraints by providing consistent information over large areas in a timely and cost-efficient manner. Synthetic aperture radar (SAR), in particular, is well suited due to its sensitivity to water occurrence beneath vegetation canopies. In emergent wetlands, double-bounce scattering of the radar waves between standing water and vertical vegetation components typically results in elevated backscatter and coherence compared to other land-surface types. However, this response is influenced by several factors, including radar wavelength, vegetation structure, and water level. We analyse time series of Sentinel-1 backscatter intensity and coherence with the goal of characterising inter and intra-annual variations in surface water extent between 2015 and 2025. We interpreted the SAR-derived metrics using a comprehensive reference dataset including water level, high-resolution imagery from unmanned aerial vehicles, meteorological data and vegetation indices. The study was conducted at a long-term ecosystem research site at the shallow, subsaline Lake Neusiedl, located in the Pannonian lowlands of Eastern Austria. The lake is a Ramsar site of international importance due to its ecological significance, particularly for breeding and migratory birds. More than half its surface is covered by one of the largest continuous reed belts in Europe, dominated by Phragmites australis. During spring, the reed belt shows a clear double-bounce signature whereas in summer, high vegetation and declining water levels lead to a decrease in backscatter and coherence. During a prolonged drought period, which lasted from 2019 to 2022, water extent at Lake Neusiedl decreased significantly followed by a marked recovery starting in 2023. Our results showcase both potential and limitations of water extent retrieval in emergent wetlands based on C-band SAR data and hold important lessons for future wetland monitoring using data acquired at longer wavelengths by SAR missions, such as NISAR and ROSE-L. In addition, the observed coherence patterns offer initial indications of the potential for retrieving wetland water level changes using SAR interferometry.