24 Oct 2016 | News

Monsoon Discharge Measurements of Langtang Khola and Lirung Outlet

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To catch the highest discharge of Langtang Khola and Lirung outlet, a team of glacio-hydrologists from the International Centre for Integrated Mountain Development (ICIMOD) and Kathmandu University conducted a field expedition to Langtang valley from 29 August to 6 September, 2016. The expedition was part of regular data collection field activities conducted by the Cryosphere Initiative.

Obtaining accurate discharge measurements from mountain streams is difficult since they are highly turbulent and their beds are irregular. The team recently acquired sensors to perform salt dilution based measurements of discharge, a reliable method for such studies in this context.

ICIMOD installed automatic hydrological stations which provide water level measurements every ten minutes on Langtang Khola and at Lirung Glacier outlet near Kyanjing Gompa. To compensate for the lack of resources to carry out regular discharge measurements and convert water levels into useful discharge data, experts at ICIMOD measure the most representative high, average and low flows during monsoon, fall, spring and winter. The collected discharge data are used to construct a clear relationship between the measured water levels and the actual discharge. The automatically recorded water levels can then be converted to discharge data which help researchers understand the amount of the river flow contributed from different sources such as glacier melt, snow melt and rain fall.

Rating curve for Langtang Khola, linking discharge to height measurements derived from discharge measurements obtained with the salt dilution method. Data presented here includes all the measurements obtained since may 2016 with the new salt dilution sensor.

Discharge data are used to calibrate glacio-hydrological model to better explain hydrological processes in the region. Models represent the flow in the river. Results from these studies can be used by the government to develop policies on water resource management and hydro power planning. Researchers also use such data for climate change impact research.

Discharge data during the monsoon period is important for the highest flow of the river. Monsoon and high melt rates of glaciers add to the additional flow, bringing the water level (stage) higher in rivers. Monsoon therefore is a crucial period of the year to study as the river discharge is at its highest. With more observed data on discharge and stage height, researchers can better explain the relationship between discharge and water level using rating curves.