Spatial Analysis of Rainfall Distribution in Lebak Regency, Indonesia, Using the Isohyet Method: Implications for Flood Mitigation

Lebak Regency is characterized by mountains, hills, lowlands, coastal areas, and river networks, making rainfall distribution an important component of hydrometeorological disaster assessment. This study analyzes the spatial distribution of rainfall in Lebak Regency in 2025 using the isohyet method and discusses its use as supporting information for flood mitigation. Rainfall observations from five locations—Rangkasbitung, Malingping, Bayah, Sajira, and Leuwidamar—were spatially processed in ArcGIS. Regional rainfall was estimated by area-weighting the rainfall represented by successive isohyet zones. Observed monthly mean monthly rainfall ranged from 160 to 219 mm/month. Bayah recorded the highest mean monthly rainfall (219 mm/month), followed by Sajira (215 mm/month), Rangkasbitung (180 mm/month), Leuwidamar (162 mm/month), and Malingping (160 mm/month). The isohyet calculation used seven zones with a total mapped area of 3,076 area units and a weighted rainfall sum of 555,623.5, producing a mean areal monthly rainfall of 180.63 mm/month. The mapped pattern indicates spatial rainfall variability, with relatively high observed values in Bayah and Sajira. These results can support rainfall monitoring and preliminary prioritization for flood preparedness. However, the resulting map should not be interpreted as a flood-risk map because flood risk also depends on drainage capacity, river conditions, land use, topography, watershed characteristics, exposure, and vulnerability.