# Exploring the Spectral Variability of Estonian Lakes Using Spaceborne Imaging Spectroscopy
## 3 Results and Discussion
Figure 4: (**a**) PRISMA image acquired on 19 September 2020 over Lake Lammijarv, (**b**) OWT classification map, (**c**) spectral signatures extracted from the four ROIs displayed in (**a**) and following the same colors. These results refer to Application n.2. Figure 3: From left to right: spectral comparison between PRISMA data corrected with POLYMER and S3 data in Lake Peipsi (19 September 2020, 8 June 2021, 21 May 2022); spectral comparison between EnMAP L2A (21 July 2024) data and S2 (22 July 2024) data in Lake Vörtsjärv. Blue and green curves represent multispectral spaceborne data and hyperspectral spaceborne data (mean + st.dev. ), respectively. These results refer to Applications n.1 and n.6. Figure 5 shows a comparison between two PRISMA images acquired on 24 June 2020 and 9 May 2021, showing the area where the Velikaya River enters Lake Pihkva (Application n.3). The ROIs extracted from the images are located at the same position. Ancillary datasets included ERA5-derived precipitation and wind data. Notably, cumulative rainfall during the week prior to the first acquisition was 8.8 mm, increasing significantly to 62.1 mm in the week preceding the second acquisition. As for wind conditions, on the first date the average speed was 2.3 ms\({}^{-1}\) and the maximum speed recorded was 3.9 ms\({}^{-1}\), while on the second date the average speed was 6.1 ms\({}^{-1}\) with a maximum of 8.8 ms\({}^{-1}\). The combined influence of elevated precipitation and strong wind conditions contributed to an increased sediment load transported by the river into the lake [87]. Figure 6 shows Application n.4, in which the effect of the inflow from the Emajogi River--which connects Lake Vortsjarv to Lake Peipsi--was evaluated, as captured in the EnMAP image acquired on 16 May 2024. To this end, a 5 km transect was established for the extraction of spectral signatures, extending from within the river into the inner lake, with sampling points spaced at 1 km intervals. As the measurement points progress from the river toward the lake, where the influence of the inflow gradually diminishes, corresponding changes in the spectral signatures were observed. Initially, the spectra exhibited markedly higher concentrations of CDOM and TSM, which decreased in magnitude along the transect and reached their lowest levels at the final ROI (distance from the shore \(\approx\) 4 km), where the river's influence was minimally detectable. Notably, the spectral signature observed within the ROI located inside the river is similar to the characteristic spectral signature of Lake Vortsjarv, as illustrated in the example provided in Figure 2. This similarity is attributable to the hydrological connection, wherein water entering Lake Peipsi originates from Lake Vortsjarv via the river [88]. This result suggests that the combined spatial resolution and spectral properties of satellite sensors such as EnMAP can support analyses regarding the influence of river discharge into lakes. Therefore, understanding of the hydrological processes in lakes can progress, reaching the maturity of what has been achieved for many years by monitoring river plumes entering sea waters [89; 90; 91]. Figure 5: From left to right: PRISMA images acquired on (**a**) 24 June 2020 and (**b**) 9 May 2021 over Lake Pihkva, (**c**) spectral signatures extracted from the two ROIs located at the same position on different dates. These results refer to Application n.3. Figure 7 shows Application n.5, in which the concentration of PC was estimated (using the MDN model) for a PRISMA image acquired on 16 August 2022 over Lake Peipsi. The average water temperature for the previous month has increased by 3 \({}^{\circ}\)C over the last three years, which may be associated with the elevated concentrations of PC [92], the presence of which was identifiable from the PRISMA spectral signatures (absorption peak around 620 nm and reflectance peak around 650 nm), while remaining undetectable when using the S2 sensor, given its lower spectral resolution.