Mountains in the Chaning World, Mountains in the Chaning World

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Rainfall Variability of Upper Bagmati Basin, Nepal
Sandhya Thapa, Khadga Bahadur Thapa

Last modified: 2017-08-24


The amount of precipitation and its distribution over Nepal is controlled by southwest monsoon system. The monsoon reaches eastern Nepal with a modal onset date of 10 June and advances westwards covering the whole country within a week. The summer months June, July, August and September receives on an average 80% of the total annual precipitation and the remaining eight months receives only 20%. Though there are various studies on the precipitation pattern over Nepal however there are limited studies on the precipitation pattern and the change in distribution pattern of daily rain rate of different threshold over the Kathmandu valley capital of Nepal. The study Comparison the variability of rainfall of 2015 with the long-term mean annual Precipitation of upper Bagmati basin. Also it helps to determine the optimum numbers of rainfall station for the study area. For the study of rainfall variability Arithmetic mean method and isohyets methods were used. The rainfall variability over upper Bagmati basin was investigated on the basis of observed rainfall data from different stations (twenty one stations)  for the period of 1985-2014.  The mean monsoon precipitation of the study area has been calculated as 1661 mm whereas 2015 received only 1389 mm. Kathmandu valley received only 84% of annual rainfall in 2015 in comparison with long-term mean annual precipitation. The study revealed a large variation in the spatial distribution of monsoon rainfall in a small geographical area, from 565 mm at Chapagaun to 1962 mm at Kakani. Highest monthly rainfall was measured in August when all the stations received rainfall in excess of 300 mm except Khumaltar, Khokana, Nagkhel, Godavari and Chapagaun. Kakani recorded 778 mm, the highest in the region followed by Jitpurphedi and Panipokhari 490 mm while Chappagoun recorded 204 mm, the lowest. The south facing slopes in the northern belt of Kathmandu valley receives the heaviest rainfall and may be considered as the windward slopes of the northern east monsoon system. The stations of valley floor are adequate whereas if additional raingauge are to be installed in future then they are to be installed on hill slopes where variability of rainfall occurs.

Keywords: precipitation pattern, raingauge, long-term mean, adequate

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