Yan Dongchun

Simulation of Morphological Development of Soil Cracks in Yuanmou Dry-hot Valley Region, Southwest China

Soil cracking is an important process influencing water and solutes transport in the Yuanmou Dry-hot Valley region of Southwest China. Studying the morphological development of soil cracks helps to further reveal the close relationship between the soil cracking process and water movement in such semi-arid regions. Here we report regular changes on surface morphology of soil cracks with decreasing water in four different soils (Typ-Ustic Ferrisols, Ver-Ustic Ferrisols, Tru-Ustic Vertisols and Typ-Ustic Vertisols) through simulation experiments. Our results indicate the following: 1) Different soils ultimately have different development degrees of soil cracks, according to their various values of crack area density. Soil cracks in Typ-Ustic Ferrisols can only develop to the feeble degree, while those in the other three soils are capable of developing into the intensive degree, and even into the extremely intensive degree. 2) Soil crack complexity, as expressed by the value of the area-weighted mean of crack fractal dimension (AWMFRAC), is found to continuously decrease as a whole through the whole cracking process in all the studied soils. 3) Soil crack connectivity shows a uniform trend in the studied soils, that is to say, connectivity gradually increases with soil crack development.

Influencing factors of sediment deposition and their spatial variability in riparian zone of the Three Gorges Reservoir, China

Many factors can affect the sediment deposition and soil erosion process in riparian zone, including terrain, sediment transport and water level fluctuations. Clarifying the factors influencing sediment deposition process in the riparian zone of the Three Gorges Reservoirs is an important problem to determine the key area of sediment deposition and its trend of development in the study area. In order to reveal the influence of these environmental factors on the sediment deposition in riparian zone of the Three Gorges Reservoir, this study investigated 1) the amount of deposited sediment in different environmental conditions, 2) the potential factors affecting sediment deposition in riparian zone of the Three Gorges Reservoir, 3) the relationship between the deposited sediment amount and these factors previously mentioned using correlation analysis, and 4) the influence of human activities considered as an additional factor. This study found that 1) slope gradient, elevation, inundating duration and human activities were the main factors influencing sedimentation in riparian zone of the Three Gorges Reservoir, and 2) the impact of each factor varied with spaces. Specifically, in the upper reach from Jiangjin to Fuling, human activities such as gravel dredging, bank revetment and ports and wharfs constructing disturbed considerable amounts of deposited sediment, as a result, there was no natural law to dictate the distribution. In the middle reach from Fuling to Fengjie, slope gradient and inundating duration were the controlling factors, and the sediment deposition amount was greater in the areas with a gentler slope or lower elevation. Water flow on gentler slopes generally had lower velocity, resulting in more sediment to deposit. Sites with lower elevations would be drowned by sediment-laden flow with a longer duration resulting from hydrologic regime controlled by the operating strategy of the Three Gorges Reservoir, leading to a larger amount of sediment deposition. In the lower reach from Fengjie to Zigui, slope gradient was similar to the middle reach, performing a primary factor, while other factors showed little relationship with sediment amount.

Using 137Cs tracing methods to estimate soil redistribution rates and to construct a sediment budget for a small agricultural catchment in the three gorges reservoir region, China

Soil erosion and associated off-site sedimentation are threatening the sustainable use of the Three Gorges Dam. To initiate management intervention to reduce sediment yields, there is an increasing need for reliable information on soil erosion in the Three Gorges Reservoir Region (TGRR). The purpose of this study is to use 137Cs tracing methods to construct a sediment budget for a small agricultural catchment in the TGRR. Cores were taken from a pond and from paddy fields, for 137Cs measurements. The results show that the average sedimentation rate in the pond since 1963 is 1.50 g cm−2 yr−1 and the corresponding amount of sediment deposited is 1,553 t. The surface erosion rate for the sloping cultivated lands and the sedimentation rate in the paddy fields were estimated to be 3,770 t km−2 yr−1 and 2,600 t km−2 yr−1, respectively. Based on the estimated erosion and deposition rates, and the area of each unit, the post 1970 sediment budget for the catchment has been constructed. A sediment delivery ratio of 0.5 has been estimated for the past 42 years. The data indicate that the sloping cultivated lands are the primary sediment source areas, and that the paddy fields are deposition zones. The typical land use pattern (with the upper parts characterized by sloping cultivated land and the lower parts by paddy fields) plays an important role in reducing sediment yield from agricultural catchments in the TGRR. A 137Cs profile for the sediment deposited in a pond is shown to provide an effective means of estimating the land surface erosion rate in the upstream catchment.

137Cs and 210Pbex as soil erosion tracers in the hilly Sichuan Basin and the Three Gorges area of China

Accelerated soil erosion and land degradation represent major environmental problems for agricultural lands. Reliable information on the rates of soil loss is urgently needed. The traditional techniques for documenting rates of soil loss may meet this need, but face many limitations. The fallout radionuclides, especially 137Cs and 210Pbex, are increasingly used as effective tracers to quantify soil erosion rates, and they represent a valuable complement to the existing classical methods. This paper aims to introduce the basis for assessing soil erosion rates on cultivated and uncultivated slopes by using 137Cs and 210Pbex measurements, to compare the 137Cs and 210Pbex reference inventories, and to report several case studies undertaken in the hilly area of Sichuan Basin and the Three Gorges area of China.