A New Estimate of Meltwater Contribution to Domestic Water Resources in the Khumbu Valley, Nepal

Retreating glaciers and snowpack loss threaten high-altitude communities that rely upon seasonal melt for domestic water resources. But the extent to which such communities are vulnerable is not yet understood, largely because melt contribution to water supplies is rarely quantified at the catchment scale. The Khumbu Valley, Nepal is a highly glaciated catchment with elevations ranging from 2000 m to 8848 m above sea level, and more than 80% of annual precipitation falls during the summer monsoon from June to September. In 2018, the first estimates were made of meltwater contributions to local domestic water resources in the dry, pre-monsoon season. Sampling was conducted in April and May of 2016 and 2017 along the major trekking route between Lukla and Everest Base Camp. Samples were taken from the rivers, tributaries, springs, and taps used by the communities for their domestic water supply. In addition to these, meltwater samples were collected directly from the base of the Khumbu Glacier in 2016, and several rain samples were collected in 2017. Estimates were calculated from δ¹⁸O isotopic data using a two-component mixing model with the Khumbu glacial melt and pre-monsoon rain as endmembers. Here, melt contribution estimates are revised using a calculated annual average isotopic composition of precipitation in place of the collected rain endmember. Considering the predominance of monsoonal precipitation in the Khumbu Valley, an annual value may enable more accurate calculations. According to these revised estimates, melt contributes up to 79% of domestic water resources during the pre-monsoon season with an average of 30%. These new findings suggest the communities of the Khumbu Valley are vulnerable to the loss of a third or more of their dry-season domestic water supply as glaciers retreat and snowpack declines. Continued monitoring will yield more accurate melt contribution estimates and support the sustainability and resilience of high-altitude communities in the face of climate change.