Investigation of temporal variations in the stable d18 O and d D isotopes from Kathmandu's precipitation events shows that the relatively enriched d18 O and d D values in the winter(the dry season, dominated by the westerlies) were positively correlated with temperature,indicating a temperature effect controlling the changes of d18 O and d D. However, the d18 O and d D values were depleted in the summer(the wet season, dominated by the Indian monsoon), which were negatively correlated with precipitation amount, indicating an amount effect. In addition, the comparison of stable isotopes in precipitation from Kathmandu and Mawlong(near the Bay of Bengal)shows that the overall trends of d18 O and d D values at Kathmandu generally approximate those at Mawlong.However, there remain many differences between the details of the isotopic changes at Kathmandu versus those at Mawlong. Compared with those at Mawlong, the further rainout effect and the more intense lift effect of the oceanic moisture by the high mountains resulted in the moredepleted d18 O and d D values in summer precipitation at Kathmandu. A deuterium excess and the local meteoric water lines reveal that evaporation at Kathmandu exceeds that at Mawlong. The data also show that the Indian monsoon activities at Mawlong are more intense than those at Kathmandu.
The lapse rate of water isotopes is used in the study of the hydrologic cycle as well as in the estimation of uplift of the Tibetan Plateau.The greater elevation contrast in the Southern Himalayas allows for a detailed discussion about this lapse rate.We analyze variations of 18δO in precipitation and river water between 1320 m and 6700 m elevations in the Southern Himalayas,and calculate the specific lapse rate of water 18δO.The results show that the multi-year average lapse rate in precipitation over this region is 0.15‰/100 m.The one-year average lapse rate is 0.17‰/100 m from three sites along the Southern Himalayas.The two results agree,but are much lower than the global average of 0.28‰/100 m.This work also shows that there is a difference in precipitation 18δO lapse rate between the monsoon and non-monsoon seasons.The calculated precipitation lapse rate is much lower than that in surface water.
