Water circulation and governing factors in humid tropical river basins in the central Western Ghats, Karnataka, India
Authors: Tripti M. , Lambs L. , Gurumurthy G. P. , Moussa I. , Balakrishna K. , Chadaga M. D. 
Published: December 2015

Rationale

The small river basins in the narrow stretch of the Arabian Sea coast of southwest India experience high annual rainfall (800–8000 mm), with a higher proportion (85 %) during the summer monsoon period between June and September. This is due to a unique orographic barrier provided by the Western Ghats mountain belt (600–2600 m) for the summer monsoon brought by the southwesterly winds. This study is the first of a kind focusing on the water cycle with an intensive stable isotopes approach (samples of river water, groundwater, rainwater; seasonal and spatial sampling) in this part of the Western Ghats in Karnataka and also in the highest rainfall‐receiving region (with places like Agumbe receiving 7000–8000 mm annual rainfall) in South India. In addition, the region lacks sustainable water budgeting due to high demographic pressure and a dry pre‐monsoon season as the monsoon is mainly unimodal in this part of India, particularly close to the coast.

Methods

The stable isotopic compositions of groundwater, river water and rainwater in two tropical river basins situated approximately 60 km apart, namely the Swarna near Udupi and the Nethravati near Mangalore, were studied from 2010 to 2013. The δ18O and δ2H values of the water samples were measured by isotope ratio mass spectrometry, and the d‐excess values calculated to better understand the dominant source of the water and the influence of evaporation/recycling processes.

Results

The water in the smaller area basin (Swarna basin) does not show seasonal variability in the δ18O values for groundwater and river water, having a similar mean value of −3.1 ‰. The d‐excess value remains higher in both wet and dry seasons suggesting strong water vapor recycling along the foothills of the Western Ghats. In contrast, the larger tropical basin (Nethravati basin) displays specific seasonal isotopic variability. The observation of higher d‐excess values in winter with lower δ18O values suggests an influence of northeast winter monsoon water in the larger basin.

Conclusions

The narrow coastal strip to the west of the Western Ghats displays unique water characteristics in both tropical river basins investigated. For the smaller and hilly Swarna basin, the dense vegetation (wet canopies) could largely re‐evaporate the (intercepted) rain, leading to no marked seasonal or altitude effect on the water isotope values within the basin. The larger Nethravati basin, which stretches farther into the foothills of the Western Ghats, receives winter monsoon water, and thus exhibits a clear seasonal variability in rainfall moisture sources. The degree of water vapor recycling in these wet tropical basins dominates the isotopic composition in this narrow coastal stretch of South India. An insight into the soil water contribution to the river water and groundwater, even in the rainfall‐dependent tropical basins of South India, is provided in this study. Copyright © 2015 John Wiley & Sons, Ltd.