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

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 4500 m and are not influenced by exceptionally high elevations in the Dudh Koshi River watershed.