Prevailing uncertainties are of major concern because some projections of their future have serious implications for water resources. One of the most closely studied areas is the Langtang valley, a catchment of nearly 600 km‑ on the border with China and a quarter of which is covered by glaciers. The highest point is the Langtang Lirung, at 7,234 meters a peak feared and revered by mountaineers because of the difficulty in reaching the summit.
Most of the rain falls in summer when the monsoon hits the Indian subcontinent and the Langtang region is on its northern frontier, before the Tibetan Plateau. During winter, in the areas above 4,000 meters, many meters of snow accumulate each year and most of it is melting at very fast rates.
Langtang Valley in the Himalaya
Weather stations across the Nepalese Himalaya
Studies of precipitation and melt in the valleys of the Nepalese Himalaya is a collaboration of the International Centre for Integrated Mountain Development (ICIMOD), the Nepalese Department of Hydrology and Meteorology, Kathmandu University and Utrecht University in the Netherlands.
Our work here has been going on for many years, currently funded by the Norwegian Government, European Research Commission and the Netherlands Organisation for Scientific Research. We are an international team comprising Nepali, Dutch, Belgian, Finnish, Norwegian, German and Austrian co-workers studying the local climate, glaciers, snow cover and how it all relates to water resources.
The project team maintains a number of automatic weather stations in the Langtang River catchment on the border with China that help us to judge the quality of larger datasets derived from satellites. We visit twice every year to maintain our stations and conduct field experiments and we use the data to make very detailed studies of melt and precipitation in the valley.
Our highest automatic weather station is located at 5,200 meters above sea level on the Yala Glacier. Bi-annual glacier mass balance measurements there are among the most detailed in the Himalaya making it one of the most important sites to help understand the future of glaciers in the region based on field measurements. Our AWS measures air temperature, relative humidity, wind speed and direction, snow depth, as well as all four radiation components with a Kipp & Zonen CNR4 net radiometer.
This data helps understanding the local meteorological drivers of glacier mass change. Yala Glacier overall is losing mass due to intense solar radiation and dry and windy conditions that facilitate snow and ice melting and sublimation (the phase change from solid to gas without going through the liquid state).
Maintenance on site, twice a year
Maintenance of the automatic weather station at this high elevation can be tricky; temperatures in winter fall below -20°C and wind speeds can be very high. Our team visits the station twice a year, before and after the monsoon, to read out data and check equipment functionality.
On clear days, solar radiation at this altitude is fierce and the reflection from the white snow feels like a grill even when air temperatures are just above the freezing point. When ablation of snow and ice is very strong in summer the metal poles of the weather station often need to be re-drilled into the ice by three to four meters to prevent it from melting out and falling over, and potentially damaging sensors like the CNR4 in the process.
Getting data from these stations is not technically difficult, but the local conditions are a challenge. Oxygen levels are 50% lower than here in my office in the Netherlands, and that has an effect on one’s concentration. Combine this with freezing temperatures in the morning and the beating sun in the afternoon and you have quite difficult operating conditions.
How the CNR4 comes into play
Kipp & Zonen CNR4 net radiometer
The CNR4 is an essential centre-piece of our measurements, and we have it running on three of our weather stations. It allows us to measure incoming short-wave radiation, the major driver for ice and snow melt. At the same time it also records the out-going shortwave radiation, from which we then deduce the albedo of the ice or snow cover, a variable that is essential to understand melt events.
While short-wave radiation is measured on many stations and can also be relatively easily modelled, the CNR4 provides insights into a much more challenging variable - long-wave thermal radiation, coming from the atmosphere, clouds and the surrounding terrain. The data analysis is ongoing and in future will hopefully help us to better understand the processes leading to runoff in a changing climate.
CNR4 irradiance measurements over snow and ice during the monsoon season
Future work
After installing new sensors for so many field seasons our main focus now lies on maintaining these setups and analysing the data. Future endeavours will hopefully also lead us into other areas to see whether insights gained here over the years are applicable elsewhere. Collaborations with local and international partners are key to that, an effort that takes much time in meetings but makes work on these stations even more exciting.
Find out more about ICIMOD at www.icimod.org and the Utrecht University mountain hydrology projects at www.mountainhydrology.org