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

Water colour 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 analysed, combining 4,316 FUI observations (2017–2024) with in situ water quality data and official WGD ecological status classifications. FUI values covered virtually the entire scale (1–21), with most observations 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. The ordinal logistic regression relating the FUI to the ecological status classes of the WFD captured the expected quality gradient, with FUI values between 10–13 reliably identifying the status ‘Good’ (probability > 0.70), but with greater uncertainty for the intermediate range (14–16) and a tendency to underestimate “Poor/Bad” conditions when the FUI > 16. Overall, the FUI proves to be a low-cost, high-frequency screening and early warning tool that is effective in detecting good conditions and state transitions. However, it should be complemented by physical-chemical and biological metrics when a fine distinction between WFD classes is required.