Spatial variability in precipitation has received little attention in the study of connections between climate, erosion, and tectonics. However, long-term precipitation patterns show large variations over spatial scales of ~10 km and are strongly controlled by topography. We use precipitation rate estimates from Tropical Rainfall Measuring Mission (TRMM) satellite radar data to approximate annual precipitation over the Himalaya at a spatial resolution of 10 km. The resulting precipitation pattern shows gradients across the range, and from east to west along the range, and fivefold differences between major valleys and their adjacent ridges. Basin-wide average precipitation estimates correlate well with available measured mean runoff for Himalayan rivers. Estimated errors of 15%-50% in TRMM-derived annual precipitation are much smaller than the spatial variability in predicted totals across the study area. A simple model of orographic precipitation predicts a positive relationship between precipitation and two topographically derived factors: the saturation vapor pressure at the surface and this pressure times the slope. This model captures significant features of the pattern of precipitation, including the gradient across the range and the ridge-valley difference, but fails to predict the east-west gradient and the highest totals. Model results indicate that the spatial pattern of precipitation is strongly related to topography and therefore must co-evolve with the topography, and suggest that our model may be useful for investigation of the relationships among the coupled climate-erosion-tectonic system.

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Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km2, with a maximum depth of 680 m and contained an estimated 832 km3 of water; the smaller lake contained an estimated 80 km3 of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 14C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 14C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 X 106 m3 s–1. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.

[1] To investigate processes related to the interaction of topography and precipitation, a tropics-wide (±36°latitude) high resolution (0.1°) ten year (1998 -2007) rainfall climatology is presented from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) using algorithm 2A25 version 6 near-surface rain. We observe a tight coupling between precipitation and topography with distinct precipitation-topography relationships present in northwest South America and South Asia. An error model is developed by subsampling the TRMM Multi-satellite Precipitation Analysis as sampled by the PR. The error model predicts observed sampling error as a function of resolution, rain rate and sampling frequency with an r 2 of 0.82. This error model indicates that the precipitation climatology at 0.1°r esolution does resolve precipitation gradients in regions with large average daily rain totals including the Andes, Western Ghats, and Himalaya.

Observations of precipitation fall speeds and precipitation patterns suggest that precipitation phase (rain versus snow) is a signifi cant control on the relationship between precipi tation patterns and topography, due to the potential for increased downwind advection of snow relative to rain. A coupled model of orographic precipitation and surface erosion shows that for a range of climate variables, steady-state precipitation patterns vary from nearly uniform and maximizing over the highest topography, to highly spatially variable, closely coupled to topography and reaching a maximum on low slopes. Precipitation patterns are a fi rst-order control on modeled range scale and ridge-valley scale relief, channel concavity, and the position of the drainage divide. An association between cool climates, spatially uniform precipitation, and effi cient erosion of high topography is indicated. The importance of precipitation phase to the evolution of precipitation patterns and topography further demonstrates the fundamental importance of the coupled climate, erosion, and tectonic system in the evolution of mountain topography.