Evaluation of the Landsat 8 Provisional Aquatic Reflectance Product for Inland Water Quality Monitoring

The USGS Landsat 8 Provisional Aquatic Reflectance (PAR) product offers a standardized basis for inland water monitoring, but its reliability is constrained by spectral uncertainty and valid-data availability. This study evaluated PAR using GLORIA in situ observations, Lake Lanier field measurements, and cross-sensor comparisons, with ACOLITE and OC-SMART as alternative atmospheric correction methods. In the full GLORIA comparison, PAR showed median absolute percentage differences (MAPDs) of 115.9% and 75.2% at 443 and 483 nm, respectively, compared with 40.0% at 561 nm and 66.5% at 655 nm. Moreover, 310 of 579 PAR matchups contained negative reflectance in at least one band, substantially limiting usable observations. Water quality retrievals evaluated using common matchups yielded PAR MAPD values of 71.6% for chlorophyll-a, 71.0% for total suspended solids, and 117.7% for colored dissolved organic matter absorption, indicating substantial retrieval uncertainty even after sample availability was controlled. Over Lake Lanier, narrow water branches exhibited substantial PAR data loss and less reliable retrievals, highlighting limitations associated with shoreline proximity and mixed pixels. Differences in valid-data coverage also constrained direct comparisons among methods at this site. Cross-sensor comparisons over Lake Victoria and Lake Michigan, nevertheless, showed that PAR captured broad spatial patterns, while time-series comparisons suggested potential for resolving seasonal variability, particularly in chlorophyll-a. Overall, PAR may support monitoring of larger inland waters when sufficient valid observations are available. Applications in small or narrow waters, and retrievals sensitive to blue-band errors, require particular caution. Broader operational and long-term use requires further validation across sites and years.