Changes in river channel erosion or deposition affect the geomorphic evolution, aquatic ecosystems, and river regulation strategies. Fluvial processes are determined by the flow, sediment and boundary conditions, and it has long been expected that increasing sediment supply will induce aggradation. Here, based on thorough field surveys, we show the unexpected undercutting of the piedmont rivers influenced by the 2008 Wenchuan (Ms 8.0) Earthquake. The rivers flow from the Longmen Mountain with significant topographic relief to the flat Chengdu plain. In the upstreams, sediment supply increased because of the landslides triggered by the earthquake, causing deposition in the upstream mountain reaches. However, the downstream plain reaches suffered undercutting instead of deposition, and among those rivers, Shiting River was the most seriously affected, with the largest undercutting depth exceeding 20 m. The reasons for this unexpected undercutting are proposed herein and relate to both natural and anthropogenic causes. In addition, we also demonstrate, at least for certain conditions, such as rivers flowing from large-gradient mountain regions to low-gradient plain regions, that upstream sediment pulses may induce aggradation in upstream and degradation in downstream, causing the longitudinal profile to steepen to accommodate the increasing sediment flux.
Pan-evaporation, an important indicator of atmospheric evaporative demand, is a combination of complex meteorological interactions that is critical for agricultural irrigation planning and drought monitoring (Roderick et al. 2007, Cong et al. 2009, Wang et al. 2012. Changes in pan-evaporation affect the hydrological cycle and energy balance. This is more important in energy-limited conditions when evapotranspiration approaches the level of pan-evaporation (evaporative demand) (Ertek et al. 2006, Kahler & Brutsaert 2006. Research conducted over recent decades has widely reported that pan-evaporation has decreased while air temperature has increased worldwide, which is known as the 'Evaporation Paradox' (Brutsaert & Parlange 1998, Roderick & Farquhar 2002, Ren & Guo 2006. The causes of the panevaporation decrease have been hotly discussed and associated with climate change (Milly & Dunne 2001, Moratiel et al. 2010). An acceptable interpretation is that reductions in wind speed and solar radiation conributed to the decrease in pan-evaporation (Roderick & Farquhar 2002, Rayner 2007
The Yellow River basin could be divided into three sub-regions, which makes it as the ideal target for studying regional climate change. On the basis of daily precipitation at 62 meteorological stations in the Yellow River basin, spatial distribution and temporal trends of annual and monthly mean precipitation and extremes were analyzed during 1961-2012. The Mann-Kendall trend test and linear least-square method were used to detect trends and magnitudes in annual and monthly mean precipitation and extremes. The results indicate that mean precipitation and extremes have different trends, and the three sub-regions also have distinct trends.
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