Asian rivers are fed from the High Asian Mountains such as the
Hindu-Kush-Himalaya-Karokoram (HKH) range. Snow and glacier melt are
important hydrological processes there, but liable to be significantly
affected by climate change.
A major uncertainty in Asian mountain hydrology is the highly variable
spatial distribution of precipitation; our understanding of this has
been poor owing to the region’s remoteness and the lack of reliable
measurement networks. Glacio-hydrological modelling needs accurate
information on precipitation distribution, but only a very limited
number of high-altitude measuring stations exist. A long-term monitoring
system is needed to collect high-elevation (above snowline) and
middle-elevation (just below snowline) climate data in the HKH.
This report is based on work in the Langtang catchment in Nepal.
High-quality pluviometers measured total precipitation (rain and snow),
snow height and temperature; cheaper tipping buckets installed below the
snowline allowed accurate measurement of rain only; surface temperature
sensors were installed along an altitudinal gradient and used to detect
the presence of snow cover. These measurements in combination with other
meteorological and hydrological observations by project partners were
analysed and key findings were identified.
Precipitation patterns in the valley are highly variable in time and
space. During monsoon there is almost daily precipitation, as moist air
from the Bay of Bengal collides with the Himalayas. In winter the system
works quite differently: precipitation is produced by disturbances from
the west leading to low-pressure areas along the southern periphery of
the Tibetan plateau. These low-pressure areas cause cyclonic circulation
that transports warm moist air from the south, resulting in winter
precipitation. Such events are infrequent, but substantial. There is
also great spatial variation in precipitation, e.g. Kyangjin village is
twice as dry (867 mm/year) as Lama Hotel (1819 mm/year). These spatial
patterns are also seasonal: during monsoon there is a general decreasing
trend in precipitation following the valley gradients, in winter the
opposite is observed. Precipitation also increases with altitude, but
maximum precipitation during monsoon is at a higher elevation than
during winter owing to the different mechanisms.
Temperature is widely assumed to decrease with altitude by about 6.5
°C/1000 m elevation gain. This “lapse rate” is very important in
modelling as it determines the temperature at higher areas where snow
and ice are melting. It is also a very sensitive parameter, e.g., an
error of 2 °C/1000 m could result in a difference of 4 °C from the
valley floor which may greatly affect the amount of melt water modelled.
Observations showed a clear seasonal cycle and high correlation with
elevation throughout the year, as expected; however the lapse rates show
great seasonal variation. During monsoon the temperature decreases only
by 4.6 °C/1000 m, whereas in winter the lapse rate is -5.8 °C/1000 m.
The steepest lapse rate is observed in the pre-monsoon season from March
to mid-June (- 6.4 °C/1000 m). There is also strong variation in diurnal
variations throughout the valley.
The high mountains of Asia are a hugely important water resource.
Climate change is likely to affect the timing and patterns of water
availability, so accurate understanding of the water cycle in this
region is imperative. A first step is to quantify solid and liquid
precipitation. This project made importance advances and the following
conclusions were drawn:
- There are very large seasonal, diurnal and spatial differences in
precipitation, caused by complex interactions of the topography, the
monsoon in summer and westerly disturbances in winter.
- Temperature varies very strongly with elevation, season and between
day and night. Its decrease with altitude is less than the
environmental lapse rate as a result of local circulation and seasonal
humidity; use of a constant annual lapse rate is incorrect and may
result in erroneous temperature fields.
- Using local observations and incorporating spatial variation in
precipitation and temperature has a profound impact on rain–snow
portioning, snow and glacier melt and river runoff. It is essential to
incorporate this in water availability and climate change impact
- If local observations are not available, results from nearby
catchments with similar climatic conditions may be transferred, but
utmost care must be taken to verify datasets using as much local
observation as possible.
- If for budgetary, logistical or safety reasons local observations
cannot be obtained, there are proxies, e.g. mass balances of glaciers,
which may be used in a first-order assessment of high-altitude
. Field-based research in high-altitude regions of Asia is very
challenging for several reasons which should be noted:
- The logistics of field expeditions to the Himalayas are complex.
Accessibility is poor and there is a strong reliance on local people,
their knowledge of the mountains and their strength and endurance.
- High-altitude work poses the risk of Acute Mountain Sickness from lack
of oxygen. Proper medical knowledge and precautionary measures are
- The weather in the mountains is highly unpredictable: twice during a
field expedition, a cyclone from the Bay of Bengal caused heavy
snowfall, preventing access to observational sites.
- Nepal is in a tectonically very active area and the Gorkha earthquake
on 25 April 2015 hit Nepal and the Langtang catchment in particular
very hard. Many people died, villages were severely damaged and most
of the hydro-meteorological equipment put beyond repair.
- Strong partnerships are essential for this type of research; the
international cooperation between ICIMOD, ETH, Utrecht University, PMD
and Kathmandu University was successful and none of the outputs would
have been possible without this.
- Local institutions like the Department of Hydrology and Meteorology,
Kathmandu University and Tribhuvan University are obvious partners to
manage observational networks. However, the capacity of these
institutions to manage high-altitude observatories independently needs
to be further developed.
- Much of the region is unstable geopolitically, and security issues can
constrain scientific research considerably. This was experienced at
first hand when a terrorist attack in Pakistan forced the project to
- Data management is important, but it can be challenging to quality
control, homogenise and store the data in a high-quality manner.
The following recommendations are made:
- The 2015 Ghorka earthquake destroyed much of the Langtang
high-altitude observatory; it is strongly recommended to rebuild it to
act as a benchmark catchment for the entire Himalayas.
- More catchments in contrasting climate zones should be equipped with
hydro-meteorological instruments to improve the scientific basis for
water availability projections.
- The capacity of local institutions, government bodies and universities
should be improved by hands-on training in the field, with transparent
selection procedures based on intellectual capacity, physical and
mental strength and motivation.
- International collaboration should be promoted, with mechanisms to
ensure long-term commitment, maintenance of observatories and central
Immerzeel, W.W.; Bierkens, M.F.P.; Shea, J.; Shrestha, A.B.; Pellicciotti, F.; Rasul, G. Calibrating Above and Below Snow Line Precipitation as Inputs to Mountain Hydrology Models - Final Report. Utrecht University, Utrecht, Netherlands (2015) 205 pp.