Isotopic fingerprinting of dual monsoon moisture sources, evapotranspiration process and microclimate manifestation over the tropical rainforest region, western part of the Western Ghats, India

•Stable isotope ratios of rainwater with space and time over Southwest India.•Evidence of microclimate impact on water sources in tropical rainforest of India.•Isotopic amount effect over the humid tropical coast of India is demonstrated.•Isotopic reversal of continental and elevation effects is evident in small humid basins.•Fingerprinted critical water reserve in humid tropics through isotope systematics.