Water samples were taken monthly during the study period for precipitation, which was collected on each rainy day. All samples were analyzed for stable oxygen and hydrogen isotope compositions. The samples are collected in a 60 ml, Polyethylene bottle directly from the source and clearly labelled with all details. It is made sure the bottles are tightly capped. During sampling, storage and transportation to the laboratory, care is taken to avoid evaporation of the sample.The stable isotopes were analyzed using Isotope Ratio Mass Spectrometer (Finnigan Delta Plus XP Continuous Flow Isotope Ratio Mass Spectrometer) at the Isotope Hydrology division of Central Water Resources Development and Management (CWRDM), Kozhikode. The oxygen isotope (δ¹⁸O) was analysed using CO₂–H₂O equilibration method. The hydrogen (δD) analyses were performed by the H₂–H₂O equilibration technique using platinum catalyst supported on a hydrophobic material.