Spatio-temporal rainfall variability in the Himalayan mountain catchment of the Bagmati River in Nepal

Rainfall records between 1970 and 2015 from 12 rain gauge stations are used to examine the variability of rainfall patterns in the Himalayan mountain catchment of the Bagmati River. Based on monthly and annual rainfall distributions derived from daily records, this study analyzes the spatial and temporal variation of long-term trends and departures of 5-yearly average rainfall from the long-term mean. Both monthly and annual results show considerable spatial variabilities over rather short distances (< 25 km). Long-term trend results based on the Mann-Kendall test on homogenized time series showed a significant decrease in annual rainfall at four stations and significant increase at one of the stations. The 5-yearly departures from long-term mean showed that most of the stations received higher rainfall in the 1970s while the rainfall between 2005 and 2015 was substantially lower at several stations. Monthly breakdown of long-term trends and 5-yearly departures provided additional insight in rainfall variations that are not necessarily reflected by the annual results. Some stations have significant changes in rainfall for few months even though the annual rainfall does not change significantly, while a few of the significant monthly trends were, in fact, opposite to the annual trend. Month-wise investigation of 5-yearly departures from the long-term mean also suggests that the annual results are cumulative results of varying combinations of monthly changes rather than due to any particular group of months behaving similarly. The valuable information derived from monthly analysis is expected to assist in planning and management of water resources.