# Assessment of Carbon Flux and Soil Moisture in Wetlands Applying Sentinel-1 Data
## 1 Introduction  
Wetlands are very sensitive ecosystems, functioning as a habitat for many unique plants and animals. Wetlands play a significant role in many ecological functions including: carbon balance, regulation of the hydrological cycle, flood control, and the improvement and maintenance of water quality [1]. These areas remove atmospheric CO\({}_{2}\) through sequestration and subsequent peat accumulation, which can help to mitigate climate change effects. Those climate variations may alsoprovide a significant influence on wetlands' role in the carbon cycle by shifting from a sink to a sources mode of CO2 [2]. The world's peatlands cover only 5%-8% of the Earth's land surface but store more organic carbon than any other terrestrial ecosystem i.e., an average of 118 g-C-m-2-year-1 [3]. Many wetlands have been degraded by extraction for fuel, drainage, burning, overgrazing, urbanization, and atmospheric pollution. Degraded wetlands contribute 6% of global anthropogenic CO2 [4], which could have an impact on climate change. Returning these ecosystems to a pre-anthropogenic-disturbance state by restoration is very important and can be treated as a buffer between the agricultural land and the surface water system (nutrient retention). The Net Ecosystem Exchange (NEE) of CO2 between the terrestrial ecosystem and the atmosphere reflects the balance between gross CO2 assimilation (gross primary production--GPP) and ecosystem respiration (RESP). In it, photosynthesis processes are CO2 sinks, and respiration processes are CO2 sources. The NEE level is commonly measured by ground-based techniques such as Eddy Covariance [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16] and chamber [17; 18; 19] methods. Remote sensing methods offer possibilities to evaluate NEE spatially, most often globally [20; 21; 22; 23]. Wetlands are regarded as one of the difficult areas to monitor through conventional methods due to their relative inaccessibility and sensitivity to weather conditions. In-situ measurements are expensive and capable of providing data to a single, small area, and are difficult to use for mapping spatial variations. Microwaves can penetrate through vegetation and into soil, and thus can be widely used for the assessment of biomass and soil moisture [24; 25; 26; 27; 28; 29; 30] which are considered to have an impact on CO2 flux. The relationship between microwave signal and soil moisture is complex as it depends on radar frequency, incidence angle, polarization, surface roughness and vegetation cover. For C band data, many studies showed that it is possible to estimate soil moisture with reasonably high accuracy [25; 26; 27; 28; 30; 31]. Many of them concerned bare soils and agricultural areas [25; 27; 31]. [PERSON] et al [32] studied the application of RADARSAT-2 and LANDSAT-7/8 images to investigate the potential for the combined use of Sentinel-1 and LANDSAT-8 and Sentinel-2 data for soil moisture and Leaf Area Index (LAI) retrieval over irrigated grasslands. The results showed that HH (Horizontal Transmit/Horizontal Receive--like polarization) polarization is the most relevant to soil moisture estimates. [PERSON] et al. [33] stated, that the sensitivity of microwave signals to soil moisture increases with a smaller incidence angle which allows better penetration of the wave. Also, the polarization of VV (Vertical Transmit/Vertical Receive--like polarization) has a higher penetration depth than HH. [PERSON] et al. [34] proposed the conventional empirical linear relationship approach between backscattering coefficient (\(\sigma^{\circ}\)) calculated from ENVISAT-ASAR (ESA's Environmental Satellite-Advanced Synthetic Aperture Radar) data registered in HH polarization with a dual-angle configuration and surface soil moisture of a small watershed. However, it can also be applied to vertical (VV) polarisation. [PERSON] et al.