The study effectively applied the Otsu algorithm to monitor surface water changes in Dau Tieng Reservoir using Sentinel-1 radar imagery during the period 2015–2025. This method allows for the automatic determination of threshold values, enabling clear separation between water bodies and land surfaces in satellite images. The results show that the water surface area fluctuated significantly over time, with distinct phases such as sharp shrinkage (2015–2016), expansion (2016–2017), rapid increase (2021–2022), decline (2023–2024), and a trend toward stabilization (2024–2025). In addition, the integration of Google Earth Engine for powerful satellite image processing and Google Colab for spatial data visualization has created a flexible, automated, and highly reproducible cloud computing environment, contributing to enhanced water resource monitoring in the context of climate change.