Rainfall Pattern Over Kathmandu Valley During Summer Monsoon Season and its Long - Term Change

Abstract

In this study, the spatial variability of rainfall over Kathmandu valley is investigated on the basis of observational record of rainfall from 15 stations of valley for the period 1970-2005(With more than 10 years data in each station).The spatial distribution of summer monsoon rainfall is similar to the annual distribution with the highest rainfall pocket area in the Northwestern mountain in the periphery of Kakani with more than 2300 mm of mean monsoonal rainfall, while the lowest rainfall is found in southern side of valley floor in the periphery of Khumaltar with average monsoonal rainfall less than 950 mm. Rainfall pockets over valley are also noted in the periphery of Mountain stations Nagarkot, Godavari, Thankot, Sundarijal and Sankhu with mean monsoonal rainfall greater than 1500 mm. Inter annual variability of monsoonal rainfall is found to be relatively lowin the central part of valley floor than that of the base and top of mountains. Based on daily rainfall data of 6 stations of valley for the period of 1971-2005, overall (Valley average) trend on various rainfall characteristics over Kathmandu valley during monsoon season are found as: 1) Overall (Valley average) rising trend in total monsoon rainfall but it does not have statistical significance. 2) Overall (Valley average) significant rising trend in no. of rainy days (> 1 mm /day) in Kathmandu valley. (At 5% level) (3) Overall(Valley average) significant rising trend (at 1% level) in no. of days with precipitation rate 0.1-10 mm /day but the rising trend on that threshold in valley floor is not significant. 4) Though, there is overall (Valley average) falling trend in rain of categories 60-90 mm /day in valley, it doest have statistical significance. (5Overall (Valley average) rising trend in accumulated rain with rain rate 40-60 mm /day and > 90 mm /day with low statistical significance. Study of diurnal variability of rainfall in the valley floor has also been carried out by using hourly data of Kathmandu airport for the period 1992-2002 .The result shows the two pronounced peak, one in midnight and another in evening.