Precipitation in the Himalayas

By: Naresh Kumar, B P Yadav and Sanjay Bist
The western Himalayan region experienced an increasing trend of one-day heaviest precipitation, during both winter and monsoon through 1978 to 2007. While one day heaviest precipitation is on the rise in high-altitude stations during winter, the same is observed in low altitude stations during the monsoon.
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Extreme weather, such as heavy precipitation, extensive avalanches, cloud burst, flash floods and landslides, regularly happen in the western Himalayan region. These extreme events generally occur under the influence of mid-latitude westerly systems during the winter months and due to monsoonal systems in summer. The western Himalayan region climate is influenced by the southwest monsoon from June to September and by westerly systems from November to March.

Several scientists have studied the precipitation pattern and trends in this region so far. The variability in discharge data of four rivers in the north-western Himalayas, namely Beas, Chenab, Ravi and Satluj (Bhutiyani et al., 2008) and the variation in precipitation over the northwest Himalayas using monthly precipitation data from Shimla, Srinagar and Leh have been studied by a group of scientists (Bhutiyani et al.,2010). There was no discernible change observed in winter precipitation but a significant decreasing trend was evident in monsoon precipitation during the period under study. The rainfall variability over Uttarakhand using 80 years’ data is also studied (Basistha et al., 2009). The variations in seasonal and annual rainfall as well as rainy days over Kashmir valley have also been studied (Kumar & Jain, 2010). A recent study (Kumar et al., 2015) examined the temporal variation in winter frequency of the westerly systems which bring winter precipitation over Himachal Pradesh and documented the synoptic features that cause extreme weather in the form of heavy precipitation over the region. Another study (Kumar & Jaswal, 2016) has examined the variation in monthly, seasonal and annual precipitation over western Himalayan Region and their linkages with the North Atlantic Oscillation (NAO) and Southern Oscillation Indices (SOI). Their study had indicated a significant decreasing trend in monsoon precipitation over the region.

In the present study, temporal and spatial variability in monthly, seasonal and annual one-day heaviest precipitation over western Himalayan region, by using the daily precipitation data of 15 surface observatories of India Meteorological Department (IMD) for the period 1978-2007 has been analysed. The meteorological data is from observatories in Dehradun, Bhunter, Dharamshala, Batote, Srinagar, Kupwara, Banihal, Badarwah, Qazi Gund, Kokernag, Mukhim, Manali, Pahalgam, Mukteshwar and Gulmarg in Jammu and Kashmir, Himachal Pradesh and Uttarakhand. From the daily precipitation data, monthly, seasonal and annual one-day heaviest precipitation for each of the stations, as also the western Himalayan region was calculated.

For the entire western Himalayan region as a whole, seasonal and annual one-day heaviest precipitation was calculated by taking the mean of the data for all stations for a particular period. Annual one-day heaviest precipitation of a station was calculated by noting one-day heaviest precipitation from January to December. Seasonal one-day heaviest precipitation was calculated for winter (January-February), pre-monsoon (March-May), monsoon (June-September) and post-monsoon (October-December) seasons. For the trend analysis, the non-parametric Mann-Kendall test (Kendall, 1976; Kumar & Jaswal 2016) was used. The trend was considered significant, if the confidence level of the series was 95 per cent or more.

Analysis of the data reveals that there was no noticeable trend in annual one-day heaviest precipitation over western Himalayan region during the study period. However, season-wise analysis reveals an increasing trend of precipitation in the winter and monsoon period and a decreasing trend in pre-monsoon and post-monsoon period. The monthly analysis of data shows increasing trend in monthly one-day heaviest precipitation in July, August and September and decreasing trend in all other months. Overall, no significant change is observed in monthly, seasonal and annual one-day heaviest precipitation for the western Himalayan region as a whole.

Station-wise analysis of data shows increasing trend in annual one-day heaviest precipitation over eight stations, decreasing trend over six stations and no trend over one station. However, all stations in lower altitudes (less than 1,500 m) show an increasing trend in annual one-day heaviest precipitation. In winter, eight stations show an increasing trend, while the remaining seven stations show a decreasing trend. However, all high altitude (>2300 m) stations show an increasing trend in winter one-day heaviest precipitation.

Precipitation in the Himalayas

The pre-monsoon season has only four stations showing an increasing trend, while all the remaining stations show a decreasing trend. Batote, Badarwah and Kukernag, in particular, showed a significant decrease in pre-monsoon one-day heaviest precipitation. During the monsoon, eight stations indicate an increasing trend in monsoon one-day heaviest precipitation with a significant increase of 9.7 mm per decade over Bhunter (Fig. 1).

In the post-monsoon season, almost all stations show a decreasing trend in one-day heaviest precipitation. Monthly analysis of data shows a decreasing trend at all stations, with a significant decrease over Dehradun in November followed by March during which, all 14 stations show a decreasing trend. In September, 11 stations show an increasing trend in monthly one-day highest precipitation. Thus, in general, increasing trends are observed in May, June, July and September in lower altitude stations and in January, September and October months at higher altitude stations.

 

Endnote

Although there is no significant change noticeable in annual precipitation of the entire western Himalayan region, season-wise analysis yields noticeable trends in precipitation pattern. The seasonal variation in one-day highest precipitation can also help policy makers and planners make suitable interventions when heavy rainfall warnings are issued by the Indian Meteorological Department (IMD).

 

References:

Basistha, A., Arya, D.S., & Goel, N.K. (2009). Analysis of historical changes in rainfall in the Indian Himalayas. International Journal of Climatology, 29, pp555-572.

Bhutiyani, M.R., Kale, V.S., & Pawar, N.J. (2008). Changing streamflow patterns in the rivers of northwestern Himalaya: implications of global warming in the 20th century. Current Science, 95(5), pp703-708.

Bhutiyani, M.R., Kale, V.S., & Pawar, N.J. (2010). Climate change and the precipitation variations in the northwestern Himalaya: 1866-2006. International Journal of climatology, 30, pp535-548.

Kendall, M. (1976). Time series. London: Charles Griffin & Co.

Kumar, N., Yadav, BP., Gahlot, S., & Singh, M. (2015). Winter frequency of western disturbances and precipitation indices over Himachal Pradesh, India: 1977-2007.Atmósfera 28(1), pp67-74. doi:10.1016/S0187-6236(15)72160-0

Kumar, N., & Jaswal A.K. (2016). Historical temporal variation in precipitation over western Himalayan Region: 1857-2006. Journal of Mountain Science, 13(4), pp672-681. DOI: 10.1007/s11629-014-3194-y.

Kumar, V., & Jain, S.K. (2010). Trends in seasonal and annual rainfall and rainy days in Kashmir Valley in the last century. Quaternary International, 212, pp64–69.

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