Stable isotopes of precipitation in Nepal Himalaya highlight the topographic influence on moisture transport

Abstract

The Nepal Himalaya is a transition zone for pollutants transport from the South Asia onto the Tibetan Plateau, with the moist convection identified as dominating the chemical composition that would be transported onto the Plateau. Yet little is known regarding the atmospheric water vapor transport over this region, and how the summer monsoon flows would interact with local topography, with traces detectable from ground sample and observations. The stable oxygen and hydrogen isotopes in precipitation (δ18Op and δDp) are important tracers for understanding various hydrological processes. This study reports, for the first time, stable isotope data in daily precipitation at five stations along a north-south transect on the southern slope of Himalaya, with altitudes ranging from 102 m to 5050 m above sea level (a.s.l). The altitude effect of δ18Op across the region of interest is apparent with an altitudinal lapse rate as 0.17‰/100 m. Seasonal trends in δ18Op and δDp show low values during June–September and higher values during October–May at all of the study sites, with the noticeable decrease of δ18Op during summer indicative of the presence of the Indian monsoon over the eastern Nepal Himalaya. The average d-excess values in the summer precipitation increase with altitudes til 3800 m a.s.l before decreasing afterward to 5050 m a.s.l. This corroborates with previous meteorological discovery in the region, and probably confirms the replacement of prevailing westerly by the southerly monsoon flows during summer monsoon season.