Influence of Loess Plateau check dams on catchment flood hydrology across varying stages of lifespan

<p>The over 100,000 check dams constructed across the Loess Plateau for soil and water conservation may have substantially changed the hydrological processes in the region, which, however, has not been understood yet. As a critical step towards revealing the lumped effect of check dams at the regional scale, this study explored the modified flood hydrology induced by check dams in Wangmaogou catchment, a representative small Loess Plateau watershed. A coupled hydrological and hydraulic modeling approach was applied to simulate the flooding process for different stages of deposition and topographic changes in the check dam reservoir. The results suggest a paradigm shift of the dam effect on flood attributes, which transits from a total interception in the early stage of the dam to peak reduction and flood detention, rather than a complete loss of flood control functions, when it approaches the maximum capacity of sedimentation. Under a given level of deposition, the reduction to a minor flood by a check dam was higher than that to a major flood. With the progression of siltation behind the check dam, the flood peak reduction rate, flood volume reduction rate, and flood lag time decreased accordingly. Although the check dam with a reservoir fully filled by sediment lost its ability of intercepting floods, it still exhibited a considerable ability to reduce the peaks of floods. The topographical changes contributed to the reduction of flood peak appreciably by reducing the flow velocity and retarding the flood propagation. Noticeably, this reduction augmented with the advancement of siltation and the topographic change, indicating the persistence of the hydrologic effect of check dams in a long run. As a result of hydrological changes, the reduction in flood flow velocity due to check dam suggests a substantial reduction in sediment transport and channel erosion during floods. In addition, a dam system containing multiple, cascading check dams exhibits much more significant effect in modifying both hydrologic and hydraulic properties of flood than individual dams. The current research provides a mechanistic understanding of the check dam effect on watershed hydrology under heavy rainstorms in small catchments, which sheds light on evaluating the upscaled effect of the large number of check dams on Loess Plateau regional hydrology and water resources.</p>

Hydrologic State Influence on Riverine Flood Discharge for a Small Temperate Watershed (Fall Creek, United States): Negative Feedbacks on the Effects of Climate Change

Watershed flooding is a function of meteorological and hydrologic catchment conditions. Climate change is anticipated to affect air temperature and precipitation patterns such as altered total precipitation, increased intensity, and shorter event durations in the northeastern United States. While significant work has been done to estimate future meteorological conditions, much is currently unknown about future changes to distributions of hydrologic state variables. High-resolution hydrologic simulations of Fall Creek (Tompkins County, New York), a small temperate watershed (324 km2) with seasonal snowmelt, are performed to evaluate future climate change impacts on flood hydrology. The effects of hydrologic state and environmental variables on river flood stage are isolated and the importance of groundwater elevation, unsaturated soil moisture, snowpack, and air temperature is demonstrated. It is shown that the temporal persistence of these hydrologic state variables allows for an influence on watershed flood hydrology for up to 20 days. Finally, six hypothetical climate change forcing scenarios are simulated to estimate the influence of catchment conditions on the watershed runoff response. The possibility of drier summers and wetter springs with a reduced winter snowpack in the Northeast is also simulated. These hydrologic changes influence flood discharge in the opposite direction as climate effects because of a reduced snowpack accumulation and melt time. Strong hydrologic state influence on flood discharge may be most attributable to increased air temperature and decreased precipitation. Hydrologic state variables may change both the location and shape of seasonal flood discharge distributions despite expected consistency in the shape of precipitation statistic distributions.