Sentinel-2 Forel–Ule Index as a Proxy for Ecological Status in Reservoirs: A Case Study in Southern Portugal

Water color is an important optical proxy for trophic status and water quality, but its integration into regulatory assessment frameworks is still limited. This study assesses the potential of the Forel–Ule Index (FUI) derived from Sentinel-2 as a proxy indicator to support the assessment of the ecological status of reservoirs under the European Union’s Water Framework Directive (WFD). Seventeen reservoirs located in semi-arid Mediterranean climate agricultural basins in southern Portugal (Sorraia, Sado, and Guadiana) were analyed, combining 4316 FUI observations (2017–2024) with in situ water quality data and official WFD ecological status classifications. The results showed that the values on the FUI scale (which ranges from 1 to 21) fell, for the most part, between 12 and 18 and with marked spatial and seasonal contrasts, particularly between more transparent reservoirs and persistently turbid ones, probably eutrophicated reservoirs. Principal component analysis showed that the first component (PC1, 39.5% of variance) represents a trophic gradient dominated by turbidity, chemical oxygen demand and chlorophyll-a, and is positively, albeit moderately, correlated with FUI (Spearman’s ρ = 0.439, p < 0.001), while the second component, dominated by nitrogen, showed no significant association. To make the Water Framework Directive (WFD) regulations compatible with the structure of the available dataset, ecological status was dichotomized into “Satisfactory” and “Deterioration”. Binary logistic regression showed that increasing FUI values were significantly associated with a lower probability of classification as “Satisfactory” (β = −0.682, p = 0.0137; odds ratio = 0.51, 95% CI: 0.29–0.87). The model performance was moderate (balanced accuracy = 0.682; AUC = 0.754), with better identification of “Deterioration” conditions than “Satisfactory” conditions. The Mann–Whitney U test confirmed that mean FUI values differed significantly between the two ecological status groups (U = 65, p = 0.0166), with lower values associated with reservoirs meeting the “Good” threshold. Overall, FUI proved to be a low-cost and temporally flexible screening and early-warning tool, particularly useful for identifying departures from favorable ecological conditions. However, the index is not a direct substitute for the official ecological classification and is best applied in combination with physicochemical and biological metrics when assessing changes in water quality over a shorter period of time.