Forest ecosystem plays a major role in controlling moisture dynamics over the continents, particularly in the humid tropics. The montane forest ecosystem in South India supports a characteristic warm tropical climate which affects the weather pattern and the monsoon system. This study focuses on better understanding of the influence of dual monsoonal rainfall on the surface water and groundwater in southwest India, and the role of the Western Ghats mountain belt in governing the water isotope characteristics (i.e., isotopic elevation, rainfall amount and continental effects) in the humid tropics of South India. This is achieved through a spatial study of stable isotope ratios of surface and subsurface water (oxygen, δ 18 O and hydrogen, δ 2 H), collected from different tropical river basins located between Kozhikode (Kerala, 10°30′ N) and Udupi (Karnataka, 13°30′ N), in the wettest places and highest peaks of the Western Ghats between 2011 and 2014. The results on stable isotope ratios of ground water, river water and springs show that the water from the tropical mountain belt of the Western Ghats exhibit a low elevation effect with an isotopic lapse rate of 0.09‰/100 m for δ 18 O up to 2050 m asl. Beyond 2050 m asl, a considerable effect of elevation with an isotopic lapse rate of 2.5‰/100 m for δ 18 O is observed. The water samples from the Nilgiri ranges (1950-2300 m asl) exhibited higher isotopic lapse rate of 1.5‰/100 m for δ 18 O unlike that of the mountains (Ezhimala, Agumbe and Chembra) close to the eastern Arabian Sea. The difference in isotopic lapse rate is mainly dependent on (i) the dominating seasonality of oceanic source moisture over the subcontinent leading to higher depletion of heavy isotopes for more inland groundwater during the winter monsoon on the eastern slopes of the Western Ghats (ii) the degree of terrestrial moisture feedback mechanisms along windward slopes of the Ghats belt (i.e., Arabian Sea coast of India) leading to relative enrichment of heavy isotopes in groundwater fed by the highly recycled vapour sources rather than depletion due to amount, elevation or continental effect, and iii) deep cooling of orographically uplifted air moisture at high elevations of inland tropical mountains. The observed isotopic elevation effect on the groundwater and surface water is unique and constrained by specific time scale and mountain ranges within the Western Ghats belt. This study provides new understanding on factors controlling hydrological budget along higher parts of the Western Ghats mountain belt in South India experiencing humid tropical climate.