The main controls of the precipitation stable isotopes at Kathmandu, Nepal

Precipitation stable isotopes (²H and ¹⁸O) are adequately understood on their climate controls in the Tibetan Plateau, especially the north of Himalayas via about 30 years’ studies. However, knowledge of controls on precipitation stable isotopes in Nepal (the south of Himalayas), is still far from sufficient. This study described the intra-seasonal and annual variations of precipitation stable isotopes at Kathmandu, Nepal from 10 May 2016 to 21 September 2018 and analysed the possible controls on precipitation stable isotopes. The enriched δD and δ¹⁸O values were identified during non-monsoon season and depleted values were found during monsoon season, showing remarkable intra-seasonal characteristics of monsoon influence. The local meteoric water line suggested a strong influence of evaporation during rainfall in non-monsoon season and significant impact of non-equilibrium processes on precipitation during monsoon season. Temperature–δ¹⁸O exhibited negative correlation for overall samples and showed no significant correlation in seasonal scales, which was attributed to the influence of monsoon moisture. The positive correlation was observed between δ¹⁸O and outgoing longwave radiation (OLR) in monsoon season, suggesting the significant impact of convective activity on temporal variations of precipitation stable isotopes. During April, and May of 2016 and 2017, variation of precipitation stable isotopes are probably related with the mixing of multi-moisture combined with the westerlies transport. Our study suggested that the moisture transport processes are the main controls of precipitation stable isotopes at Kathmandu.