Objective: This study aimed to investigate the isotopic variations of precipitation, determine the local meteoric water line, and evaluate the probable moisture source in the Talar watershed, Mazandaran Province.

Methods: In February 2025, precipitation samples were collected from five stations, Behnamir, Kiakola, Qaemshahr, Zirab, and Sangdeh, at elevations ranging from −15 to 1300 m above sea level, and their δ¹⁸O and δ²H isotope values were measured. Isotopic variations with elevation and environmental conditions were also examined, and deuterium excess (d-excess) values were calculated.

Results: The local meteoric water line of the Talar watershed was determined as δ²H = 5.58δ¹⁸O − 13.39. Isotopic values showed a non-uniform spatial pattern, with Sangdeh at 1300 m exhibiting the greatest isotopic depletion. The δ¹⁸O elevation gradient was estimated at approximately −0.27‰ per 100 m. The d-excess values suggest the probable influence of moisture source conditions and sub-cloud evaporation. Comparison of regional meteoric water lines also indicates the Black Sea as the primary vapor source for precipitation in the region.

Conclusion: The findings indicate that precipitation in the Talar watershed is influenced by environmental factors and moisture source, and the Black Sea is identified as the main source of water vapor in the region.