Yangyel Lhaden

The National Centre for Hydrology and Meteorology (NCHM) is conducting a study to assess the contributions of different water sources to the Mochhu. This research aims to enhance understanding of the hydrological processes that govern river flow within these basins.

This study is the third of its kind, following similar research on the Chamkharchhu and Pachhu.

For this study, water samples are collected from all contributory sources of the river such as ground water, tributaries, snow, glaciers, in all seasons.

Mochhu is the western branch of Punatsangchhu, consisting of two tributaries, one originating from Masang Gang region in the east and the other from Gangchen Tag region in the west. This sub basin consists of 66 glacial lakes covering an area of 4.25 square kilometres.

Mochhu is also one of the three major sub basins for Punatsangchhu.

Chief of Cryosphere Services Division, Karma, said that understanding these processes was crucial for assessing the vulnerability of water supplies for downstream populations, particularly in the context of a changing climate.

A recent publication of the International Centre for Integrated Mountain Development (ICIMOD), Snow Update Report 2024, indicates a 20 percent decline in snow cover in the Hindu Kush Himalaya (HKH) between November 2023 and April 2024. This is the lowest recorded in the past two decades and affects the water security of millions in the HKH region and billions downstream. Bhutan is within the HKH region.

Glacier ice meltwater and snow play a critical role in Bhutan’s river flow, essential for hydropower generation.

Glaciers in Asia are often referred to as “water towers” because they store vast amounts of ice and gradually release meltwater that feeds major river systems.

In Bhutan, previous studies have also shown that glaciers are a significant source of water for rivers.

A study on the Chamkharchhu, conducted by lecturers from Sherubtse College between 2014 and 2017, found that glacier ice meltwater contributes 23 percent and snowmelt contributes 39 percent to the Chamkharchhu during the pre-monsoon season.

“The Pachhu survey also reports similar results, indicating that glaciers and snow dominate the water source in rivers, which is significant,” Karma said. “Imagine the condition of rivers under the same climatic conditions when glaciers are the majority contributors to these rivers.”

In the Pachhu, glacier ice melt and groundwater were dominant contributors to river flow in all sampling seasons, with an average contribution of 49.6 percent.

Karma said that even snow pattern was changing with erratic frequency although there was no data to back it up. “From my personal experience it has never snowed in July in Lunana since 2009.”

Executive glaciologist, Phuntsho Tshering, said that the state of our glaciers was unhealthy. “Most of the glaciers have surpassed the equilibrium line altitude (ELA), which divides the glacier into two parts. One side needs continuous snow accumulation to replenish the other side. Without adequate snowfall, the ELA has exceeded, leading to glacier melting.”

He said that understanding the various factors affecting water sources and river flow was important to determine and understand the contribution of each river source, which was why studies like this were crucial. “We have completed sampling and are yet to send the last sample to India for assessment. It could take the entire financial year for the results to be out.”

The study is expected to inform the government about water resource management, climate adaptation strategies, and policymaking to ensure sustainable water supply and mitigate the impacts of climate change, among others.

Globally, such studies provide governments crucial data-backed facts to work with.

For example, in the United States, the management of the Colorado River exemplifies how governments use studies on river water sources: they allocate water rights, manage dams based on flow patterns, conserve ecosystems, and plan for droughts, ensuring equitable distribution and environmental sustainability across multiple states and sectors.