## 5 Discussion  
According to the results, there is a strong agreement (R \(>\) 0.83) between the \(R_{rs}\) data derived from PRISMA and Sentinel-2. Nevertheless, the PRISMA-based \(R_{rs}\) at short-wavelength bands (442 nm and 492 nm) are overestimated with respect to those of Sentinel-2 (Figures 4 and 5, Table 3). More agreement statistics including MAD and bias are reported in Table 3. The bias at blue bands (442 nm and 492 nm) is up to 50% (overestimation of PRISMA with respect to Sentinel-2). These short-wavelength bands are those having the most impact from the atmosphere. Thus, the cross-sensor bias in \(R_{rs}\) (442) and \(R_{rs}\) (492) data can be attributed to the differences in the atmospheric correction. The other \(R_{rs}\) bands show good agreement as the biases are close to unity.  
The agreement statistics indicate a high consistency among the TSM retrievals from hyperspectral PRISMA and multispectral Sentinel-2 images. The agreement is also relatively strong for Chl-a and CDOM retrievals (Table 4). However, PRISMA vs. synthesized Sentinel-2 retrievals of Chl-a and CDOM are stronger (R \(\approx\) 0.8) than when comparing with those derived from real Sentinel-2 images (R \(\approx\) 0.7). Note that shorter wavelengths carry more information about CDOM and Chl-a than TSM particularly in turbid waters [80]. The short wavelengths are more affected by the atmosphere and thus the higher mismatches between PRISMA and Sentinel-2 derived Chl-a and CDOM can be attributed to the differences in the atmospheric corrections. The PRISMA vs. synthesized Sentinel-2 agreements are stronger than PRISMA vs. Sentinel-2 for Chl-a and CDOM. This can be attributed, to some extent, to the one-day difference between PRISMA and Sentinel-2 overpasses as the distribution and magnitude of the constituents can have some variations. The overpasses of PRISMA and Sentinel-2 are simultaneous for 23 April 2020. For this date also, PRISMA vs. Sentinel-2 agreements of Chl-a and CDOM are not as strong as those for PRISMA vs. synthesized Sentinel-2. Given the acquisitions being simultaneous (23 April 2020), the synthesized Sentinel-2 vs. Sentinel-2 differences can be related to other factors that affect all the cross-sensor comparisons: differences in the atmospheric correction, radiometric sensitivity, and SNR of PRISMA and Sentinel-2 sensors. It is worth noting that the magnitudes of the retrieved constituents are in accordance with the reported characteristics of the lake and the range of parameters (Section 2). This is an important point that the PRISMA derived water quality parameters are within the expected range for Trasimeno, which shows the potential of PRISMA for this application.  
As is evident, \(g_{dd}\) is significantly higher for PRISMA-based imagery than Sentinel-2. The high value of \(g_{dd}\) suggests that the atmospheric path radiance is underestimated in PRISMA level 2D products. This is in line with the results derived from the cross-sensor agreement analyses among the \(R_{rs}\) data. As mentioned above, the agreement analyses indicated that the \(R_{rs}\) at short-wavelength bands (442 nm and 492 nm) of PRISMA are overestimated compared to Sentinel-2. The inversion residual based on hyperspectral PRISMA data is slightly smaller than when using multispectral Sentinel-2.