Moisture Sources and Pathways Determine Stable Isotope Signature of Himalayan Waters in Nepal

The Himalayan mountain range produces one of the steepest and largest rainfall gradients on Earth, with >3 m/yr rainfall difference over a similar to 100 km distance. The Indian Summer Monsoon (ISM) contributes more than 80% to the annual precipitation budget of the central Himalayas. The remaining 20% falls mainly during pre-ISM season. Understanding the seasonal cycle and the transfer pathways of moisture from precipitation to the rivers is crucial for constraining water availability in a warming climate. However, the partitioning of moisture into the different storage systems such as snow, glacier, and groundwater and their relative contribution to river discharge throughout the year remains under-constrained. Here, we present novel field data from the Kali Gandaki, a trans-Himalayan river, and use 4-year time series of river and rain water stable isotope composition (d(18)O and d(2)H values) as well as river discharge, satellite Global Precipitation Measurement amounts, and moisture source trajectories to constrain hydrological variability. We find that rainfall before the onset of the ISM is isotopically distinct and that ISM rain and groundwater have similar isotopic values. Our study lays the groundwork for using isotopic measurements to track changes in precipitation sources during the pre-ISM to ISM transition in this key region of orographic precipitation. Specifically, we highlight the role of pre-ISM precipitation, derived from the Gangetic plain, to define the seasonal river isotopic variability across the central Himalayas. Lastly, isotopic values across the catchment document the importance of a large well-mixed groundwater reservoir supplying river discharge, especially during the non-ISM season. Plain Language Summary Himalayan rivers are the essential water source for downstream communities. However, it is still not well understood where precipitation moisture comes from and how it is transferred to rivers. In particular, the role of intermediate water storage in groundwater, snow, and glacier is not well understood. To resolve these issues, we collected river water, groundwater, snow, glacier, and rain samples across Central Himalayas in Nepal and analyzed their isotopic composition. We determine characteristic signatures of the different moisture sources to trace water transfer in and out of the study area. Combining field observations with satellite observations and numerical models, we show that rainfall before the beginning of the Indian Summer Monsoon undergoes several evaporation-precipitation cycles and is derived from the Gangetic foreland, while monsoon precipitation is derived from the Indian and Arabian Sea. During our 4-year observation period, pre-Monsoon precipitation resulted in the largest variation in river water isotopes. We also identify groundwater as the principal source of river discharge during the dry season. This work highlights the importance of detailed spatiotemporal isotopic analysis to determine precipitation moisture sources, how water sources change throughout the year, and how they contribute to river flow in the central Himalayas.

Automated summary

The Himalayan mountain range produces large rainfall gradients and the Indian Summer Monsoon contributes to most of the precipitation in the central Himalayas. However, the partitioning of moisture into different storage systems and their contribution to river discharge remains unclear. In this study, field data and stable isotope composition of river and rain water were used to understand hydrological variability. The study found that pre-ISM rainfall is distinct from monsoon rain and groundwater. The research highlights the importance of pre-ISM precipitation and groundwater as sources of river discharge in the central Himalayas.