Evidence of elevation effect on stable isotopes of water along highlands of a humid tropical mountain belt (Western Ghats, India) experiencing monsoonal climate

•Synoptic scale observation of water isotopes over high elevations of Western Ghats.•Study reports elevation effect on stable isotope ratio of water in the Western Ghats.•Reports role of humid tropical mountains in controlling seasonal monsoon moisture.•Traces moisture source and mixing processes along highlands of the Western Ghats.•Provides insight into processes controlling regional hydrology in the Western Ghats.•Unravels factors affecting local microclimate to seasonal climate in South India. 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 south-west 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, δ18O and hydrogen, δ2H), 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 δ18O 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 δ18O is observed. The water samples from the Nilgiri ranges (1950–2300 m asl) exhibited higher isotopic lapse rate of 1.5‰/100 m for δ18O 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