Flow regimes of Himalayan rivers of Nepal: nature and spatial patterns

A large-scale perspective is provided upon the spatial distribution of river flow regimes across the Nepalese Himalaya by classifying long-term average monthly runoff data for 28 river basins. The classification methodology is shown to be a useful tool for identifying underlying spatial structure in flow regime shape (timing of peak) and magnitude (low, intermediate and high) in an extreme physical environment where hydrological patterns are complex and poorly known. Low, marked August peak regimes occur across far-western Nepal but also in some eastern basins, which have a short summer monsoon and snow- and ice-melt. Low, July–August peak regimes are found in the central to eastern High Mountains and High Himalaya and the eastern Middle Mountains where the summer monsoon arrives earliest, meltwaters contribute but topography limits precipitation amount. Low–intermediate, August–September peak regimes dominate the central Middle Mountains due to an extended summer monsoon and greater groundwater contributions. Intermediate–high magnitude regimes occur along the Middle Mountains–High Mountains boundary with July–August peaks in western–central areas and marked August peaks at higher elevations in eastern–central and eastern Nepal, reflecting differences in summer monsoon penetration. The practical implications of these results for assessment of water resources and prediction of runoff from ungauged basins are highlighted.