[en] The monitoring of stable isotopes (δ¹⁸O and δ²H) in water can provide a sensitive indicator of water loss by evaporation. We obtained water samples from surface water and groundwater from both the young and old alluvial plains in the central part of the Nile Valley of Egypt. Groundwater is the only source for irrigation in the old alluvial plains while both surface water (River Nile and irrigation canals) and groundwater are used in the young alluvial plain. Results showed different isotopic compositions between each group of samples and hydrologic connections between shallow groundwater and surface water in the young alluvial plain. The δ¹⁸O and δ²H relationship of the samples collected from the desert areas of the old alluvial plains below agricultural lands define an evaporation line with a slope of 4.5 and low deuterium excess of <− 14‰. These values can be attributed to return flow of irrigation water that has been subjected to evaporative processes, further amplified by intense agricultural practices. Average evaporative losses were estimated to be between 31% and 36%. - Highlights: • Stable isotopes were used to study groundwater/surface water interaction. • The study area in Egypt which is in arid climate zone. • The study area is under intense year-long agricultural practices. • The study estimated the evaporative loss and it is implication on water management. • This is the first study of its kind from the study area.