Assessment of Spatio-Temporal Variability of Turbidity using the Normalized Difference Turbidity Index (NDTI) in Jepara Coastal Waters (Sentinel-2)

Abstract This study investigates the spatio-temporal variability of turbidity in the coastal waters of Jepara Regency from January to August 2025 using the Normalized Difference Turbidity Index (NDTI) and Normalized Difference Water Index (NDWI) derived from Sentinel-2 imagery via Google Earth Engine (GEE). Statistical analysis reveals a strong positive correlation ( r = 0.63) between monthly precipitation and median NDTI values, confirming that rainfall-driven terrestrial runoff is the primary driver of coastal turbidity. Turbidity peaked during the Northwest Monsoon in January (median NDTI = −0.084), coinciding with maximum precipitation (638.71 mm). While turbidity generally declined toward the inter-monsoon period (reaching a minimum in June/July), a distinct “August Anomaly” was identified where NDTI increased to −0.165 despite low rainfall. Spatial analysis integrated with bathymetric data suggests that this dry-season turbidity is driven by wind-induced sediment resuspension in shallow nearshore zones (0–10 m depth). An independent T-test ( p = 0.066) further indicates a clear seasonal shift in water quality. These findings demonstrate that turbidity dynamics in Jepara are governed by the interplay of monsoonal forcing and shallow-water geomorphology. This study highlights the effectiveness of GEE-based multi-temporal monitoring for managing coastal water quality in cloud-prone tropical regions.