Abstract— The Greater Zab River basin is a vital source of water for agriculture, industry, and domestic use in the Turkey and Kurdistan region of Iraq. The aim of this research is to determine the amount of water that flows on the surface in the Greater Zab basin using Geographical Information System (GIS), Remote Sensing (RS) techniques and analyse suitability of the study area for rainwater harvesting. This study used the Soil Conservation Service Curve Number method to estimate surface runoff in the basin. The study employed the Analytic Hierarchy Process (AHP) technique to determine the weight of each criterion. The research revealed that in the Greater Zab basin, the yearly depth of surface runoff has been around 60.1 mm during the last four decades. Additionally, the typical amount of rainfall recorded was 634.5 ± 117 mm, indicating a significant potential for water harvesting in the region. According to the findings, 4.3% of the study area was deemed highly suitable or suitable for water harvesting, while 48% had moderate suitability. The study's findings have important implications for sustainable water resource management in the Greater Zab basin. The study's findings on appropriate locations for collecting rainwater can provide valuable insight for future water resource management initiatives in the area. Overall, the study underscores the importance of proactive water resource management strategies to ensure the long-term sustainability of the Greater Zab River basin.