Changes in Flood Regime of the Upper Yangtze River

River flooding affects more people worldwide than other natural hazards. Thus, analysis of the changes in flood regime caused by global warming and increasing anthropogenic activities will help us make adaptive plans for future flood management. The nonstationary flood behavior in the upper Yangtze River was examined comprehensively in terms of trend, change point, and periodicity with co-usage of different methods. Results show that there are decreasing tendencies in the corresponding series of annual maximum flood peak flow and flood volume in four out of six control stations, except Pingshan and Wulong stations in the Jinsha River and the Wu River, respectively, and the flood peak occurrence time appears earlier mostly. The uniformity of flood process increases in four main tributaries, while it decreases in mainstream of the Yangtze River (Yichang and Pingshan stations). The rates of both rising limb and recession limb of all the typical flood process flowing through the six stations were analyzed. 77.8% of the rates of rising limb decrease, while 61.1% of the rates of recession limb increase, which is almost consistent with the variation reflected by the uniformity. The change points of most evaluation indicators happened in 1970s–1990s. The first main periodicity of evaluation indicators in Yichang is about 45 years, while that of other stations is about 20 years. Invalidity of stationarity in the flood series can be attributed to the intensified construction on major water conservancy projects, changes of underlying surface, and influences of climatic variables. The contributions of both climatic control and the Three Gorges Dam (TGD) to the variation of the annual flood peak in Yichang station were further quantitatively evaluated, which has verified that the construction of the TGD has played a positive role in peak-flood clipping.

A paradigm shift in predicting stormflow responses in an active tectonic region through a similarity analysis of pressure propagation in a hydraulic continuum

Soil layers on hillslopes acts as systems in quasi-steady states generating rainfall-stormflow responses that are controlled by pressure propagation in a hydraulic continuum established when the rainfall volume is sufficiently large. A similarity analysis for quantifying the sensitivity of the stormflow response and recession limb to topographic and soil properties in a sloping permeable domain showed that the deviation of stormflow responses in the hydraulic continuum decreases due to the macropore effect. The rapid responses seem to be naturally derived from the evolution of the soil layer with the assistance of the vegetation-root system and effective drainage systems in zero-order catchments in active tectonic regions with heavy storms. To predict stormflow responses using distributed runoff models, a paradigm shift to consider this evolution process is needed because the simple stormflow responses and complex and heterogeneous catchment properties are poorly related, but may be mainly determined by soil evolution processes.

Patterns of grain-size temporal variation of sediment transported by overland flow associated with moving storms: interpreting soil flume experiments

This study describes and interprets the evolution of grain-size distribution of sediment yields generated in an experimental soil flume subjected to downstream and upstream moving rain storms. Results of laboratory experiments show that downstream moving storms cause more soil loss than do upstream moving storms. The pattern of sediment grain-size evolution in time during a runoff event exhibits a clear dependence on the direction of storm movement. A strong relationship between overland flow discharge and mean sediment size is found. Nevertheless, the mean grain-size of sediments transported during the rising limb of the hydrograph is coarser than during the recession limb of the hydrograph. This is more marked for downstream moving storms.

Forest watershed runoff changes determined using the unit hydrograph method

Unit hydrograph is a basic method to show changes in runoff in the watershed. The investigation of runoff changes was carried out in the U Dvou louček watershed situated at the summit part of the Orlické hory Mts., East Bohemia. The waveform ordinates of recession limbs of unit hydrographs obtained using a common approach had to be approximated by the least-squares method. Final hydrographs reflected both drainage treatment and forest stand growth influencing the runoff from the watershed. Both factors increase culmination in synergy and reduce runoff on the recession limb of the hydrograph. We confirmed increased maximum runoff taking up 25–30% of the total runoff time when waterlogged sites were drained. The culmination increased by 0.2–0.8 mm/hour indicates the runoff increased by 2–8 m³/ha/hr.

On the pararneterisation of drainflow response functions

A procedure for the parameterisation of drain flow hydrographs is proposed. This involves the derivation of empirical linear response functions, which are themselves parameterised. The parameters are the time and height of the peak, and the recession characteristics. The recession limb of the hydrograph can be approximated best by the Youngs (1985) analysis, which requires two parameters. The merit of this method is illustrated by an analysis of data from a drainage experiment at North Wyke, Devon, UK; this shows that the model fits the data very well. Keywords: Drainflow hydrographs; response functions