The hydrologic regime of Himalayan catchment basins is not well-defined. The lack of a basic understanding of runoff sources and timing in the rivers of South and Central Asia creates problems in resolving questions related to specific aspects of the importance of elements of the water budget cycle, such the current concern over the impact of the retreat of Himalayan glaciers on water supplies. As a result of a general unavailability of data describing the hydrology, climate and topography of the Himalayan catchment basins, application of hydrologic concepts and models developed for mountain catchments in Europe or North America is problematic. Initial definitions of this regime could be based on existing data describing streamflow, and topography from available maps. As a first approximation, spatial variations in streamflow formation have been calculated from published data as the product of the variation of specific runoff (depth/unit area) and surface area with altitude in the catchment basins of the Nepal Himalaya. This paper presents the results for 15 gauged catchments, together with some speculation in the significance of the concept and results.
Mean annual specific runoff for the rivers flowing from the Nepal Himalaya decreases with increasing mean basin altitude, ranging from maximum values of approximately 3000 mm at 1000 m, to values between 500 ? 1000 mm at 5000m. This negative orographic gradient of runoff contrasts sharply with the positive gradient characterizing most mid-latitude mountain catchments in Europe and North America. A direct transfer of generally-accepted procedures in hydrologic modeling based on assumptions, concepts and procedures developed for the mountains of North America and Europe will require modification when applied to the catchments of the Himalaya as a result of the differences in meteorology, altitude and local relief.