UAV Hyperspectral Characterization of Spatial Color Heterogeneity in Alpine Karst Lakes

Alpine karst lakes in Jiuzhaigou exhibit spatial variations in optical properties associated with differences in water depth, underwater substrates, and travertine distribution. Conventional satellite observations may not provide sufficient spatial resolution to resolve the small-scale optical variations present in these shallow and heterogeneous lakes. In this study, UAV-based hyperspectral imagery was used to quantify spatial optical variations among three representative cascade lakes located in the “Y-shaped” valley network of Jiuzhaigou, China. Hyperspectral reflectance data were transformed into the CIE L*a*b* color space to quantify perceptual color variability, while the Euclidean color difference metric (ΔE) was used to assess relative spatial heterogeneity. In addition, seven empirical optical proxies were developed to describe spatial variations in apparent optical properties without attempting absolute retrieval of water constituents. A tiered unsupervised K-means clustering approach was applied to resolve optical heterogeneity across multiple spatial scales. Global-scale clustering separated the major optical differences among lakes, whereas lake-specific clustering revealed distinct intra-lake optical zones, including areas with stronger benthic influence, transitional optical conditions, and zones dominated by water-column signals. Non-parametric Kruskal–Wallis H tests showed significant differences among optical zones, indicating that the selected proxies can distinguish spatial optical variability. The results show that UAV hyperspectral observations combined with color metrics and empirical optical proxies can effectively describe fine-scale spatial optical variations in alpine karst lakes. Rather than retrieving absolute biogeochemical or bathymetric parameters, this approach provides a reference description of relative optical variability that may assist future repeated observations and spatial monitoring of protected aquatic ecosystems.