The current study presents a methodology for water mapping from Sentinel-1 (S1) data and a flood extent analysis of the three largest floodplains in Estonia. The automatic processing scheme of S1 data was set up for the mapping of open-water flooding (OWF) and flooding under vegetation (FUV). The extremely mild winter of 2019/2020 resulted in several large floods at floodplains that were detected from S1 imagery with a maximal OWF extent up to 5000 ha and maximal FUV extent up to 4500 ha. A significant correlation (r² > 0.6) between the OWF extent and the closest gauge data was obtained for inland riverbank floodplains. The outcome enabled us to define the water level at which the water exceeds the shoreline and flooding starts. However, for a coastal river delta floodplain, a lower correlation (r² < 0.34) with gauge data was obtained, and the excess of river coastline could not be related to a certain water level. At inland riverbank floodplains, the extent of FUV was three times larger compared to that of OWF. The correlation between the water level and FUV was <0.51, indicating that the river water level at these test sites can be used as a proxy for forest floods. Relating conventional gauge data to S1 time series data contributes to flood risk mitigation.

Vertical variability of inherent optical properties (IOPs) affect the water quality retrievals from remote sensing data. Here, we studied the vertical variability of IOPs and simulated apparent optical properties (AOPs) in the Gulf of Finland (Baltic Sea) under three characteristic (non)stratification conditions. In the case of mixed water column, the vertical variability of optically significant constituents (OSC) and IOPs was relatively small. While in case of stratified water column the IOPs of surface layer were three times higher compared to the IOPs below the thermocline and the IOPs were strongly correlated with the physical parameters (temperature, salinity). Measurements of IOPs in stratified water column showed that the ratio of scattering (b(440)) to absorption (a(440)) changed under the thermocline (b(440)/a(440) < 1) i.e., absorption became the dominant component of attenuation under thermocline while the opposite is true for the upper layer. Simulated (from IOPs) spectral irradiance reflectance (R(λ)) and spectral diffuse attenuation coefficient (K_d(λ)) from deeper layers (below thermocline) have significantly smaller magnitude and smoother shape. This becomes relevant during upwelling events—a common process in the coastal Baltic Sea. We quantified the effect of upwelling on surface water properties using simulated AOPs. The simulated AOPs (from IOPs measurements) showed a decrease of the signal up to 68.8% and an increase of optical depth (z₉₀(λ)) from 2.3 to 4.3 m in the green part of the spectrum in case upwelled water mass reaches the surface. In the coastal waters a vertical decrease of K_d(λ) in the PAR region (400–700 nm) by 6.8% (surface to 20 m depth) was observed, while vertical decrease of chlorophyll-a (Chl-a) and total suspended matter (TSM) was 31.7 and 42.1%, respectively. The ratio R(490)/R(560)≥0.77 indicates also the upwelled water mass. The study showed that upwelling is a process that, in addition to biological activity, horizontal transport of OSC, and temperature changes, alters the optical signal of surface water measured by a remote sensor. Knowledge about the vertical variability of IOPs and AOPs relation to upwelling can help the parametrisation of remote sensing algorithms for retrieving water quality estimates in the coastal regions.

In the process of spatial planning in Estonia the local municipalities are required to define the recurrent flooding zone along the inland waters that locate at their territory. Estonia is well known for its large floodplains that are annually covered by water. However, information about the spatial flood extent is scarce. A methodology for mapping the flooded area from Sentinel-1 and -2 imagery was developed and applied on data covering high water seasons in 2016–2019. Statistical information about flooded areas along the inland waters were compared with other available data sources related to wetlands i.e. map of wetlands in Estonian Topographic Database. Results showed that additional information about flood extent and duration retrieved from Sentinel-1 and Sentinel-2 the data can contribute to defining the recurrent flooding zone along the inland waters in process on spatial planning.

Shipborne measurements of sea surface layer temperature in the western Gulf of Finland in July 2002 were compared with four different MODIS sea surface temperature (SST) products. The root mean square difference of the satellite and shipborne temperature series was calculated. According to the calculations the MODIS Aqua SST products were closer to the shipborne temperature measurements than the MODIS Terra SST products. A criterion of 0.4 °C temperature differences was found to detect upwelling regions from MODIS SST products. Comparison of images showed that surface accumulation of cyanobacteria causes a local increase of the sea surface temperature on the SST images.

A field study of optical and physical properties in Muuga Bay, Gulf of Finland, was performed on 30 May 2007. In one of the studied stations traces of ballast water were visible on the water surface. An Advanced Synthetic Aperture Radar (ASAR) image was received and analysed from the same area. The vertical profiles of the absorption and attenuation coefficients together with the temperature and salinity profiles were measured in 13 stations. The concentrations of oil products, chlorophyll a, coloured dissolved organic matter, and suspended particulate matter were determined from water samples. The spatial distribution of temperature, salinity, and optically active substances indicated four distinct areas: the southern coastal area with saline, cold, and chlorophyll a poor upwelling water; the western coast of the bay with warmer and chlorophyll a richer surface water; open water dominated by higher concentrations of coloured dissolved organic matter; and Muuga harbour where the water had a high concentration of suspended particulate matter and chlorophyll a, which caused stronger light attenuation and a thinner euphotic layer compared to the other parts of the bay. Regression analysis showed that the absorption coefficient at 676 nm correlated well with the chlorophyll a concentration and the scattering coefficient at 555 nm correlated with the suspended matter concentration in Muuga Bay. The correlation coefficient between ASAR data and oil products was 0.71 although the concentration of oil products was relatively low (0.01–1.72 ppm).