Stable isotopes of river water and groundwater along altitudinal gradients in the High Himalayas and the Eastern Nyainqentanghla Mountains

Abstract

Study Region This study considers river water and groundwater in seeps and springs collected from the non-monsoon season in the valleys of the Dudh Koshi River in eastern Nepal and the Niyang River of eastern Tibet, both in the Himalaya Mountains. Study Focus Data from this study comprise water samples that provide a single season snapshot of δ18O and δD values that give additional information into the sources of moisture and the altitude lapse rates for the southern flank of the High Himalaya of Nepal and the Eastern Nyainqentanghla Mountains of the Tibetan Plateau. New Hydrological Insights The local water line for Nepal samples, δD=(7.8±0.3) · δ18O + (4.0‰±4.6‰), was moderately lower in slope than for Tibetan Plateau samples, δD=(8.7±0.1) · δ18O + (24.3‰±2.0‰); evaporation has a greater influence on the Nepal samples—consistent with warmer temperatures in Nepal versus Tibet within the same altitude range. Mean d-excess values for Tibet samples (13.1‰±2.0‰) implies that recycled continental moisture has more influence than marine moisture observed for the Nepal samples (7.4‰±4.4‰). Altitude lapse rates of δ18O and δD for Nepal samples (-2.8‰‰km−1 and −24.0‰km−1) do not significantly differ from Tibet samples (−3.1‰‰km−1 and −27.0‰km−1) and regional measurements; the lapse rates are reduced above 4500m and are not influenced by exceptionally high elevations in the Dudh Koshi River watershed.