UC Santa Barbara

Between June and September each year, the Indian monsoon typically delivers about 80% of the Nepalese Himalaya's annual precipitation. Topography on the windward (southern) flank of the range modulates persistent spatial variations in precipitation along the length of the range. Where topography is stepped with an initial abrupt rise as the Himalaya abut the foreland and a second rise as topography ascends toward the high peaks, two bands of high precipitation prevail, each where relief passes a threshold value. In contrast, more uniform, northward rising topography localizes a single high-rainfall band near the front of the range. A dense meteorological network that was operated in the Marsyandi catchment from 1999 to 2004 in central Nepal gives more insight on spatial variability in precipitation. Annual precipitation decreases ten-fold between the rainfall peak (of ∼4. m/yr) on the southern flank of the Himalaya and the semi-arid rain shadow on its northern flank (40-50. km farther north). Modest contrasts in rainfall between ridges versus valleys during the monsoon are replaced by strong altitude-dependent precipitation contrasts in the winter. Strikingly, above ∼4. km altitude, ∼40% of the total precipitation arrives as winter snowfall. Four years of daily discharge and suspended sediment measurements on the main-stem and on several tributaries of the Marsyandi during the monsoon document a strong north-south gradient in average erosion rates. Based on a suspended-to-bedload ratio of 2:1 (as estimated from grain-sizes in a landslide-dammed paleo-lake), erosion rates range from ∼0.1. mm/yr in the northern rain shadow to ∼2. mm/yr in the monsoon-drenched south. This strong modern spatial gradient in erosion rates mimics the precipitation gradient across the same area and broadly scales with specific discharge. In the wetter regions, nearly a meter of rain is required before significant sediment fluxes occur. After this initial meter of rain, the daily rainfall required to trigger sediment pulses (attributable to landsliding) gradually decreases during the remaining monsoon season from ∼40. mm to 10. mm. In the higher altitude rain shadow to the north, water discharge is more closely linked to temperature than to precipitation: a linkage suggesting that melting of snow and ice, rather than rainfall, modulates the runoff. The sediment flux in the rain shadow during the monsoon season displays a marked temporal hysteresis: fluxes broadly scale with discharge during the first two months of the monsoon, but decouple from discharge later in the monsoon. This behavior suggests that the sediment flux is supply limited. We interpret that much of the sediment is subglacially derived and that its transport into the river network is restricted either by limited bedrock erosion or subglacial hydrology. © 2012 Académie des sciences.