Climate change, combined with the accelerating pace of urban development, has significantly amplified flood hazards in semi-arid regions over recent decades. This study aims to evaluate and delineate flood risk by examining the influence of climate variability and environmental dynamics in a semi-arid setting during the period 1990–2024. A mixed descriptive–analytical methodology was applied using GIS and the Google Earth Engine cloud platform. Precipitation indicators—including annual total rainfall, the number of heavy precipitation days (R10mm), and maximum one-day rainfall (Rx1day)—were analyzed to identify long-term trends in extreme hydrological events. Subsequently, flood risk in Tehran was assessed through the integration of land cover indices (NDVI, NDWI, and NDBI) and key topographic variables such as elevation, slope gradient, and slope aspect. The results indicate an upward trend in annual precipitation and an increase in the frequency of heavy rainfall events, whereas maximum one-day rainfall exhibited only minimal variation. Land cover and terrain analyses further reveal substantial urban expansion and morphological changes throughout the study period. Flood risk zoning demonstrates that although most of Tehran lies within low-risk categories, moderate to high-risk areas are primarily concentrated in densely urbanized districts extending from the central city toward its southern regions.