Appearance of extreme monsoonal rainfall events and their impact on erosion in the Himalaya

Monsoonal rainfall in the Himalaya dominates erosion and sediment transport through the fluvial system. In addition to the strong seasonal nature of the Indian summer monsoon, striking interannual variations in monsoonal strength characterize longer records. For example, during any given year, rain may penetrate further into the orogen, even though peak rainfall amounts almost always occur at topographic barriers in regions with high relief, regardless of overall monsoonal strength. Tropical Rainfall Measurement Mission (TRMM) product 3B42 rainfall is first used to document the spatial rainfall distribution and then the TRMM time series are used to identify temporal and seasonal rainfall variations. A simple, but robust magnitude-frequency relation for each rainfall pixel is used to show that rainfall greater or equal to the 90th percentile occurs at least twice as often in mountainous terrain as in low-elevation regions. Previous field-based observations and measurements show that this significantly higher number of extreme events leads to higher erosion volumes and greater fluvial-mass transport rates. The spatiotemporal context of these extreme events helps to predict occurrences of high sediment flux and could underpin the strategic development of preventative measures. Improved statistics for extreme events are key to mitigating the filling of hydropower reservoirs and abrasion of hydropower turbines, as well as to sustaining infrastructure and successful agriculture in the downstream section of the Himalaya.

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