Abstract
Changes in rainfall and land use/land cover (LULC) can influence river discharge from a catchment in many ways. Homogenized river discharge data from three stations and average rainfall records, interpolated from 13 stations, were examined for long-term trends and decadal variations (1970–2017) in the headwater, upper and middle catchments of the Bagmati River. LULC changes over five decades were quantified using multitemporal Landsat images. Mann–Kendall tests on annual time series showed a significant decrease in river discharge (0.61% per year) from the entire Bagmati catchment, although the decrease in rainfall was statistically insignificant. However, declines in river discharge and rainfall were both significant in upper catchment. Decadal departures from long-term means support these trend results. Over tenfold growth in urban area and a decrease in agricultural land were observed in the upper catchment, while forest cover slightly increased in the entire catchment between 1975 and 2015. Correlation analysis showed a strong association between surface runoff, estimated using the curve number method, observed river discharge and rainfall in the upper catchment, while the relationship was weaker in the headwater catchment. These results were also supported by multiple regression analysis, suggesting that human activities together with climate change have contributed to river discharge changes in the Bagmati catchment.
Highlights
Atmospheric and ocean temperatures are rising at an unprecedented rate and the rise is projected to continue during the 21st century [1]
Long-term changes in rainfall and river discharge of the Bagmati River catchment were analyzed with Mann–Kendall trend tests and decadal departures from the long-term mean using historical records between 1970 and 2017
The impacts of rainfall and LULC changes on river discharge were examined using correlation and multiple regression analysis, and their relationship to surface runoff was estimated based on the curve number method
Summary
Atmospheric and ocean temperatures are rising at an unprecedented rate and the rise is projected to continue during the 21st century [1]. This warming can alter rainfall patterns, including the frequency of extreme precipitation/drought events, evaporation and total runoff patterns, and the availability of water resources [1,2]. The Nile River showed a decreasing discharge in the 20th century [6], while high summer precipitation and rainstorms are linked to increased summer discharge and floods in the lower Yangtze River basin [7]. In the Ganges basin (of which the Bagmati catchment forms part), some studies have predicted increasing monsoon discharge [8,9], while others suggested a likely decrease of annual and monsoon discharge [10,11]
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