𝐍𝐂𝐇𝐌 𝐑𝐞𝐩𝐨𝐫𝐭 𝐖𝐚𝐫𝐧𝐬 𝐨𝐟 𝐆𝐥𝐚𝐜𝐢𝐞𝐫 𝐌𝐚𝐬𝐬 𝐋𝐨𝐬𝐬 𝐢𝐧 𝐁𝐡𝐮𝐭𝐚𝐧’𝐬 𝐌𝐚𝐣𝐨𝐫 𝐑𝐢𝐯𝐞𝐫 𝐁𝐚𝐬𝐢𝐧𝐬

…𝒄𝒐𝒎𝒎𝒖𝒏𝒊𝒕𝒊𝒆𝒔 𝒂𝒏𝒅 𝒉𝒚𝒅𝒓𝒐𝒑𝒐𝒘𝒆𝒓 𝒂𝒕 𝒓𝒊𝒔𝒌 𝒂𝒔 𝒈𝒍𝒂𝒄𝒊𝒆𝒓𝒔 𝒔𝒉𝒐𝒘 𝒏𝒆𝒈𝒂𝒕𝒊𝒗𝒆 𝒎𝒂𝒔𝒔 𝒃𝒂𝒍𝒂𝒏𝒄𝒆𝒔

By Kezang Choden

The Annual Report of the National Center for Hydrology and Meteorology (NCHM) for the fiscal year 2024-2025 highlights growing concerns over Bhutan’s glaciers, warning of negative mass balances in several river basins over the coming decades. According to the report, the mass balance of a glacier is the sum of all processes that add or remove ice. A glacier is considered stable when its mass balance is nearly zero, meaning it is in equilibrium with the climate and neither retreats nor advances. A negative mass balance indicates that melting exceeds accumulation, leading to glacier shrinkage. Conversely, a positive balance means accumulation outweighs melting, allowing the glacier to expand.

By 2040, glaciers in the Wang Chhu basin are projected to experience the most negative mass balance, while several glaciers in the Manas Chhu and Punatsang Chhu basins are also expected to undergo significant losses. The outlook becomes more alarming in the mid-term (2041-2060) and long-term (2081-2100), with most glaciers in the Manas Chhu and Punatsang Chhu basins likely to show negative balances, and only a few along the northern border projected to maintain positive balances. The report highlights the vulnerability of Bhutan’s glaciers to climate change, warning of consequences for water resources, ecosystems, and communities that rely on glacier-fed rivers.

Bhutan’s glaciers are vital for sustaining life and livelihoods across the region. They provide water for drinking, agriculture and hydropower, serving as a backbone for both communities and the economy. However, climate change and rising temperatures are altering glacial runoff, with serious consequences for people and infrastructure downstream. Glacial mass balance, the difference between snow accumulation and ice loss, determines how much runoff is generated in glacier-fed areas. Many of the country’s rivers depend heavily on this process for year-round water supply. Scientific projections indicate that the future of this balance is increasingly uncertain.

Under the best-case pathway, known as Representative Concentration Pathway (RCP) 2.6, strict measures to reduce carbon emissions are assumed to take effect by 2020, allowing the glacial mass balance to stabilize or improve slightly. By contrast, under moderate RCP 4.5 and high-emission RCP 8.5 scenarios, the glacial mass balance is projected to decline steadily through the end of this century. In the short term, this may increase runoff as glaciers release more melt water, but accelerated retreat could sharply reduce water flow in the long term, creating risks for agriculture, energy generation and drinking water supplies.

The NCHM will dispatch a technical team to Lunana this September to test the siphon method as part of efforts to manage risks posed by melting glaciers and changing water flow patterns. Karma Dupchu, Director of NCHM, said, “By testing the siphon method, we are trying to find effective solutions to mitigate potential disasters. It is about safeguarding our people, infrastructure, and future.” He explained that a negative mass balance indicates glaciers are losing more ice than they are gaining, a trend that directly affects rivers, streams, and glacial lakes. Communities downstream are increasingly vulnerable to flash floods, water scarcity, and changes in river volume, while hydropower generation faces growing challenges.

The siphon method is regarded as a practical approach to lower water levels in glacial lakes and reduce the risk of glacial lake outburst floods (GLOFs). With climate change accelerating glacier retreat, the NCHM has been closely monitoring glaciers and glacial lakes across the highlands.
A 2020 study conducted by Sherubtse College in collaboration with the NCHM identified the Paa Chhu basin as a sub-basin of the Wang Chhu basin, containing 31 glaciers over an area of 28.39 square kilometre (km2). The Jichudrakey glacier is the largest, covering 9.24 square kilometre (km2). This data has been verified through the Bhutan Glacier Inventory maintained by NCHM, which continues to monitor and record changes.

Rivers in Bhutan rely heavily on melt waters, making them sensitive to climate change. Variations in glacier melt, shrinking snow packs, and rainfall shifts are expected to alter natural river flows, affecting communities, agriculture, and hydropower at lower elevations. To better understand these contributions, tracer-based mixing models were applied in the Paa Chhu basin. Three-component models using Delta 18O and Sulphate (SO4²-) as tracers showed glacier ice melt contributes about 33.3 percent of river flow, while groundwater accounts for 44.9 percent. Glacier melt increased from May through July, dipped in August, and rose again in October. Groundwater became increasingly important with distance from glacier headwaters.

Results from a four-component End-Member Mixing Analysis (EMMA) revealed glacier melt as the most dominant contributor, averaging 49.6 percent across all sampling seasons. Rainfall contributed 30 percent, moraine outflow 19.9 percent, and snow 14.5 percent. EMMA and three-component models confirmed that July recorded the highest average contribution from glacier ice melt. Limited snow and rainfall samples may have affected precision, but the study provides critical insights for managing Bhutan’s water resources.

Kaka, Gup of Lunana Gewog under Gasa Dzongkhag, expressed concern over climate change in the highlands and acknowledged NCHM’s ongoing mitigation efforts. “About sixteen years ago, snowfall used to arrive on time, blanketing the mountains and feeding the glaciers,” he said. “But now the snow melts very quickly, and the mountains that were once covered in white remain barren for much of the year.” Changes in seasonal snowfall and glacial retreat are among the most visible effects of climate change in Lunana, one of the country’s most vulnerable gewogs.
The Gup emphasized the dependence of highland communities on glaciers and snow-fed rivers for water, agriculture, and livestock. Unpredictable snowfall and faster melting trends are placing mounting pressure on traditional ways of life. He noted that NCHM’s work, including installing early warning systems and testing the siphoning technique to lower water levels in glacial lakes, is a welcome step in ensuring community safety. “The risks are growing, but the efforts to mitigate them bring hope to the people of Lunana,” he said.