Moisture recycling disturbing elevation effect on isotopes in
north-eastern Himalayan River catchment during dry season
Abstract
Himalayan rivers are prone for drying subject to continuous drop in the
glacial meltwater contribution and groundwater level. The present study
is conducted in the region of West Kameng District of Arunachal Pradesh,
India, covering catchment of 2 tributaries of Kameng river, viz., Tenga
& Dirang-Bichom. We used stable isotopic method to trace the origin of
water feeding the river as it flows from the headwater to the region of
flood plain. Our observation allowed defining the Local Water line (LWL)
in this region for the dry period, with the relation δD = (8.1 ± 0.3)×δ
18O + (11.6 ± 2.5‰). This equation is derived from
analysis of river water sample collected during March 2021. This LWL is
identical to that reconstructed using the monthly (from India, April –
October 2007 ) precipitation isotope data from a station at Mawlong,
Meghalaya, [δD = (8.1 ± 0.1) ×δ 18O + (11.8 ± 0.9)
‰]. The d-excess values from the two set of data are similar at 11.2±
1.8 ‰ and 11.3± 2.7‰, respectively, implying that river water is mainly
derived from rainwater. Such coincidence of observation is interpreted
as a common source of water for river and groundwater. Further, we
compared present observation with other studies on the surface water
composition in other Himalayan River systems and documented a consistent
elevation pattern for stable isotopes. Our observation on spatial
variability showed maximum altitude effect in the North-Western Himalaya
and drop in isotope ratios with height with pronounced participation of
recycled moisture in the Eastern Himalaya with presence of terrestrial
biosphere.