New investigation techniques for detecting hydro-meteorological data trends are required due to significant changes in precipitation and streamflow affecting the planning and operation of water resources. In this study, the trends analysis is carried out for the observation of climatic and hydrologic parameters such as temperature, humidity, precipitation and streamflow in the Euphrates Basin located in the Southeastern Anatolia Project (referred to as GAP from the Turkish acronym for Güneydoǧu Anadolu Projesi) region, which is the most important integrated water resources project in Turkey. The relationships between different meteorological and hydrological parameters in the basin are evaluated using the overlay mapping technique, popular in various engineering disciplines. The significant trends are indicated on the thematic maps and the visible changes are observed in different parts of the basin. Decreasing trends are detected in minimum streamflow, while the increasing trends in the annual mean and maximum temperature and in the annual mean and maximum humidity parameters are observed for many stations. The study concludes that the overlay mapping technique can be used effectively to observe the climatic changes and trends in hydrometeorological variables. The results are expected to help water resources managers and policy makers in decision-making for better planning in water resources planning and management.