Statistical Relationship Assessment and Temporal Variability Analysis of Water Turbidity Using Satellite Time-Series Data in the Kok River, Chiang Rai Province
Keywords:
Turbidity, NDSSI Index, Time-Series Data, Google Earth EngineAbstract
This study aims to evaluate the statistical relationship between water turbidity and surface reflectance derived from satellite imagery and to analyze the temporal variability of turbidity in the Kok River using time-series Sentinel-2 satellite data acquired between 1 January 2023 to 31 March 2025, processed on the Google Earth Engine (GEE) platform. The rank correlation analysis between field turbidity (NTU) and NDSSI showed a strong inverse relationship, with Spearman’s rank correlation and Kendall’s rank correlation coefficients of –0.83 and –0.64, respectively, indicating an association between NDSSI values and the ordinal variation of turbidity in the Kok River. Temporal analysis revealed a seasonal pattern in turbidity, with low to moderate levels during the dry season and a rapid increase during the rainy season. The maximum turbidity observed in the dataset was 6,150 NTU, while the minimum NDSSI value detected was –0.52, occurring at different times. Anomaly analysis indicated that turbidity tended to increase during the rainy season. Turbidity values exceeding 720 NTU were classified as positive anomalies, whereas NDSSI values lower than –0.13 were classified as negative anomalies. However, anomaly detection for NDSSI was limited to periods with available satellite imagery, as some months during the main monsoon season did not meet the image selection criteria. Overall, the results suggest that time-series NDSSI derived from satellite imagery may be useful for examining turbidity variation trends and identifying anomalous turbidity conditions in the Kok River, Chiang Rai Province.
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