Calibrating Above and Below Snow Line Precipitation as Inputs to Mountain Hydrology Models - Final Report

Abstract

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 studies.
  • 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 precipitation

. 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 essential.
  • 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 cancel fieldwork.
  • 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 data management.

Citation

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.

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