Impacts of socio-economic factors on sediment yield in the Upper Yangtze River

In recent years, the role of human activities in changing sediment yield has become more apparent for the construction of hydraulic engineering and water conservation projections in the Upper Yangtze River, but it has not been evaluated at the macro scale. Taking Sichuan Province and Chongqing City as an example, this paper studies the relationship between socio-economic factors and sediment yield in the Upper Yangtze River based on section data in 1989 and 2007. The results show that sediment yield is significantly correlated with population density and cultivated area, in which the former appears to be more closely related to sediment yield. Moreover, in the relation of sediment yield vs. population density, a critical value of population density exists, below which the sediment yield increases with the increase of population density and over which the sediment yield increases with the decrease of population density. The phenomenon essentially reflects the influence of natural factors, such as topography, precipitation and soil property, and some human activities on sediment yield. The region with a higher population density than critical value is located in the east of the study area and is characterized by plains, hills and low mountains, whereas the opposite is located in the west and characterized by middle and high mountains. In the eastern region, more people live on the lands with a low slope where regional soil erosion is slight; therefore, sediment yield is negatively related with population density. In contrast, in the western region, the population tends to aggregate in the areas with abundant soil and water resources which usually lead to a higher intensity of natural erosion, and in turn, high-intensity agricultural practices in these areas may further strengthen local soil erosion. It is also found that population tends to move from the areas with bad environment and high sediment yield to the areas with more comfortable environment and less sediment yield. The natural factors have greater influence on sediment yield of western region than that of eastern region. Generally, the natural factors play a dominant role on sediment yield in the Upper Yangtze River.

Runoff Simulation of Three Gorges Area in the Upper Yangtze River during 1998 Flood Season

The contribution of areal precipitation of the catchment from Cuntan toYichang (Three Gorges area) to eight flood peaks of the Upper Yangtze River (the upper reaches ofthe Yangtze River) is diagnosed for 1998 flood season. A rainfall-runoff model is employed tosimulate runoffs of this catchment. Comparison of observed and simulated runoffs shows that therainfall-runoff model has a good capability to simulate the runoff over a large-scale river and theresults describe the eight flood peaks very well. Forecast results are closely associated with thesensitivity of the model to rainfall and the calibration processes. Other reasons leading tosimulation errors are further discussed.

Analysis on River Sediment Changes of the Upper Reaches of Yangtze River

The sediment load and river sedimentation of the upper reaches of YangtzeRiver has been undergoing constant changes as complex landform, large mountain area and plentifulprecipitation make the drainage area of Yangtze River very vulnerable to watererosion and gravityerosion. Through analyzing the hydrological and sediment load statistics recorded by majorhydrological stations along Yangtze River since 1950s, and editing the accumulation graph of annualrunoff volume and annual sediment load, wefind out that the suspended-sediment of Yangtze river hasbeen decreasing year by year in Wulong Hydrological Station on Wujiang River, Beibei HydrologicalStation on Jialingjiang River, Lijiawan Hydrological Station on Tuojiang River and GaochangHydrological Station on Minjiang River, Yichang Hydrological Station, Cuntan Hydrological Stationalong Yangtze River mainstream share the same experience too. But the statistics obtained atPingshan Hydrological Station on Jinshajiang River shows the sedimentload there has increased.Taking ecological construction, hydraulic engineering construction and precipitation changes intoconsideration, the thesis analyses the causes for the sediment load decrease of Jialingjiang River,Tuojiang River, Minjiang River and Wujiang River and provides us both scientific foundation forfurther study of river sediment changes of the upper reaches of Yangtze River, and measures tocontrol river sedimentation.

Features and Sources of the Anomalous Moisture Transport for the Severe Summer Rainfall over the Upper Reaches of the Yangtze River

Using the daily NCEP/NCAR reanalysis dataset and the observation rainfalldata in China for the 1980-1997 period, features of severe summer rainfall over the upper reaches ofthe Yangtze River are investigated and then sources of moisture contributing to severe rainfallover eastern and western Sichuan Province (ES and WS for short) are examined with particularreference. It turns out that the severe rainfall occurring locally dominates summer rainfall overthe upper reaches of the Yangtze River. Climatological rainfall and anomalous one constitute severerainfall, but the latter accounts much for severe rainfall. The meridional moisture transportdominates the composite moisture transport on the occurrence day for ES region, while the zonal isequivalent to the meridional for WS region. Correlation between the moisture transport fluxes overthe two regions of severe rainfall and other regions, the anomaly and variation of the moisturetransport day by day during the period of severe rainfall lend a support for the conclusion that themeeting of the moisture from the West Pacific through the South China Sea (SCS) and the one fromnorthwestern China exerts a vital effect on the occurrence of severe rainfall, which can not beneglected.

Hazards Assessment of Regional Debris Flows Based on Geographic Information Science

Supported by the spatial analysis feature of geographic information science and assessment model of regional debris flows, hazards degrees of the debris flows in the Upper Yangtze River Watershed （UYRW） are divided into five grades based on grid cell. The area of no danger, light danger, medium danger, severe danger and extreme severe danger regions respectively are 278 000, 288 000, 217 000, 127 000, 15 000 km^2. Furthermore, the counties in the UYRW are classified into four classes based on the hazards degrees in each county. The number of severe danger, medium danger, light danger and no danger counties respectively are 49, 82, 77 and 105. The assessment results will be provided for the hazards forecasting and mitigation in the UYRW and ongoing regionalization of Main Function Regions in China as data and technique framework.

^(137)Cs Finger Printing Technique for Erosion and Sedimentation Studies

137 Cs is an artificial radionuclide with a half-life of 30.2 years,which was released into the environment as a byproduct of atmospheric testing of nuclear weapons during the period of 1950s to 1970s with a peak deposition in 1963.137 Cs fallout was strongly and rapidly adsorbed by soil particles when it deposited on the ground mostly with precipitation.Its following movements will associate with the adsorbed particles.137 Cs tracing technique has been widely used in soil erosion and sedimentation studies since 1980s.This paper introduces the basis of the technique and shows several case studies of assessment of soil erosion rates,investigation of sediment sources and dating of reservoir deposits by using the technique in the Loess Plateau and the Upper Yangtze River Basin.