Fitness-for-Purpose of Sentinel-2 Chlorophyll-a and Cyanobacteria Proxies in a Cerrado Reservoir: Implications for Hydropower Monitoring

Hydropower reservoirs in the Brazilian Cerrado combine dendritic morphology, regulated water levels, and seasonal stratification, and their forebays and tributary arms are rarely covered by fixed monitoring stations. We used the João Leite Reservoir, a dendritic Cerrado impoundment of the scale of regional small hydropower plants, as a retrospective case to test whether Sentinel-2 MSI imagery could support chlorophyll-a and cyanobacterial monitoring and to define the evidence a hydropower operator needs before adopting a satellite layer. The record comprised 64 surface observations at five stations over 14 monthly periods but only 11 unique Sentinel-2A acquisitions. We retrieved Level-2A products, extracted median reflectance from 60 m × 60 m footprints, and evaluated six prespecified indices under leave-one-scene-out and leave-one-station-out validation. In 29 matchups from seven scenes, the NDCI leave-one-scene-out R2 was −0.122 for chlorophyll-a and −0.240 for cyanobacterial abundance; RMSE was 1.80 μg L−1 and 1.74×105 cells mL−1. Scene-level atmospheric residuals of the Level-2A product exceeded the red-edge signal expected across the observed 0.7–6.5 μg L−1 range, and the MSI lacked the 620 nm phycocyanin band, so no prespecified index achieved positive out-of-scene skill. Removing the shoreline station or applying the Brazilian regulatory cyanobacterial triggers, which were exceeded in every strict matchup, did not change the result. The record is therefore unfit for quantitative mapping, and event-screening performance remains unestablished. The transferable contribution is a five-gate evidence protocol, a four-state fitness framework, and a sensor-by-variable observability assessment for hydropower reservoir monitoring.