Less rain and rainy days—lessons from 45 years of rainfall data (1971–2015) in the Kathmandu Valley, Nepal

Abstract

Understanding spatiotemporal variability in rainfall patterns is crucial for evaluating water balances needed for water resources planning and management. This paper investigates spatiotemporal variability in rainfall and assesses the frequency of daily rainfall observations from seven stations in the Kathmandu Valley, Nepal, from 1971 to 2015. Daily rainfall totals were classified into five classes, namely, A (light rain, daily rainfall < 10 mm in a day), B (moderate rain, between 10 and 50 mm), C (heavy rain, between 50 and 100 mm), D (storm, between 100 and 150 mm), and E (large storm, > 150 mm). An ordinary kriging method was used for spatial interpolation using QGIS. We performed Mann–Kendall (MK) test in conjunction with Theil-Sen’s (TS) slope estimator to detect monotonic trends, their significance, and magnitude. We find that the mountain stations depict a decreasing rainfall trend for all seasons, ranging from − 8.4 mm/year at Sankhu to − 21.8 mm/year at Thankot, whereas a mixed pattern is found on the Valley floor. Since the surrounding mountains are the chief source of surface runoff across the valley, rivers, and rivulets are substantially affected by falling rainfall tendency. Both annual rainfall amount and the number of rainy days decreased in the Kathmandu Valley over the study period. We observe a significant reduction in rainfall after 2000. As springs and shallow groundwater are the primary sources of water supply in the Kathmandu valley, it is apparent that decreasing rainfall will have (and is already having) an adverse impact on domestic, industrial, and agricultural water supplies and the livelihoods of people.