Investigating the rainfall variability in the Kathmandu Valley, Nepal

Abstract:

Assessment of spatio-temporal variability in rainfall patterns is vital to assess climate-induced changes and water resources planning and management. This paper investigates spatio-temporal variability in rainfall and assesses the frequency of daily rainfall observations from seven stations in the Kathmandu Valley, Nepal, from 1971-2015. Our findings revealed that while more than half of all rainy days were class ‘A’ (light rain, daily rainfall < 10 mm), cumulative class ‘A’ rainfall contributed to less than one-fifth of the total rainfall observed. In contrast, class ‘B’ rainfall events (between 10-50 mm in a day) accounted for 37% of the rainy days but contributed to 63% of total rainfall. Class ‘B’ and class ‘C’ (between 50-100 mm day-1) events occurred more frequently at the mountainous stations (Thankot and Sankhu) compared to the Valley floor where daily rainfall events of <10 mm were more frequent. Mountain stations showed a falling trend for rainfall in all seasons, ranging from -8.4 mm/year at Sankhu to -21.8 mm/year at Thankot, whereas, a mixed pattern was found on the Valley floor. Mean annual rainfall in the Valley was 1610 mm (1880 mm in mountain areas and 1410 mm on the Valley floor). Local factors like topography and elevation play an important role in the spatial distribution of rainfall. A rising trend of 4.3 mm/year was observed at Kathmandu Airport station located in the central part of the Kathmandu Valley. These trends were more frequently larger and statistically significant in the mountains than the Valley floor. Both annual rainfall and the number of rainy days decreased in the Kathmandu Valley over the study period. The study indicated a significant reduction in rainfall after 2000. In the Kathmandu Valley, all the streams and springs are rain-fed and shallow aquifers depend on rainfall for recharge. Since springs and shallow groundwater are the primary sources of water supply, it is apparent that decreasing rainfall will have an adverse impact on domestic, industrial, and agricultural water supplies, and the livelihoods of people.