Monthly Water Budget and Stress Index for Water Security Assessment Due to Seasonal Rainfall Variability in Indonesia

Indonesia, an equatorial country with a tropical climate, experienced distinct dry and rainy seasons influenced by monsoonal, equatorial, and local rainfall patterns. These patterns significantly affected the spatial and temporal distribution of water availability. The concentration of population and agricultural activity, particularly on Java Island, intensified the challenges of meeting water demand. To address these challenges, this study refined water security assessments by integrating global remote sensing data with government and locally sourced datasets. This integrative approach enabled more accurate, monthly-scale analyses at the watershed level, captured seasonal variability, and improved the reliability of water budget and water stress index (WSI) estimations. The WSI was used to quantify the ratio between water demand and available resources, highlighting periods and areas of water stress. The results showed that Java Island, especially the Bengawan Solo and Brantas watersheds, exhibited the highest water demand for irrigation and household use. These watersheds were identified as the most critical among 16,955 nationwide, with significant water deficits due to high population density and irrigation intensity. The monthly WSI analysis revealed recurring water stress during the dry season, particularly in Java, Bali, West Nusa Tenggara, and East Nusa Tenggara, where lower precipitation coincided with peak water demand. In contrast, regions such as Borneo and Sulawesi experienced less pronounced seasonal variation in water stress. This study highlighted the importance of combining multi-source data to better capture regional and seasonal water dynamics. Understanding these patterns was essential for developing targeted water resource management strategies, particularly in regions with high vulnerability to water stress.