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Effects of Spatial Scale and Scaling on Soil and Water Loss in Three Gorges Reservoir Area

Author: NiJiuPai
Tutor: WeiChaoFu;XieDeTi
School: Southwest Agricultural University
Course: Utilization of Agricultural Resources
Keywords: Soil and water loss "Source/sink" landscape erosion units Effect of spatial scale Multi-hierarchy spatial discretization Three Gorges Reservoir Area
CLC: S157.1
Type: PhD thesis
Year: 2005
Downloads: 891
Quote: 6
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Abstract


Effect of spatial scale on soil and water loss came of the discrepancy of the result between observation and simulation. The quantitative evaluation on effect of spatial scale on soil and water loss is the key issue of "scale, pattern and process" of soil and water loss because soil and water Loss is the representative process with variability in multi-scale, but it is unsatisfactory that the spatial variability of pivotal parameters are described and the scale variability of soil and water loss is analyzed in the present model of soil and water Loss. For the study of soil and water loss is concerned, some critical issues should be brought forward with the study in great scale, but the solutions are acquired with the study in small scale by all means, so it is essential that the quantitative evaluation on effect of spatial scale and scaling are carried out.Three Gorges Reservoir is regarded as the strategic reservoir of water resource and the security of water environment is life-and-death, but the serious loss of runoff, sediment and nutrient is a tremendous threat to the security of water environment in Three Gorges Reservoir Area. The theory of "scale, pattern and process" on soil and water loss is likely to be developed with the study of effect of spatial scale in Three Gorges Reservoir Area based on the analysis of "source/sink" landscape erosion units. In a word, effect of spatial scale on soil and water loss is not only a scientific challenge, but also a practical issue of prevention and cure. It is significant that the soil and water loss of region is quantitatively estimated based on the applied observation datum and model in small scale by means of the model of scaling.The scheme of the quantitative evaluation on effect of spatial scale on soil and water loss was set up based on multi-hierarchy spatial discretization, plotting out of "source/sink" landscape erosion units and identifying of spatial pattern of "source/sink" landscape erosion units in this paper. Subsequently, spatial variation and scale variation of soil and water loss were quantitatively analyzed in four sequential spatial scale of slope, sub-watershed, watershed and region, the models of the quantitative evaluation on effect of spatial scale and scaling up were established and the intellective evaluation system on effect of spatial scale was put forward on soil and water loss in Three Gorges Reservoir Area. Following is the main achievements obtained in the works.(1) Runoff process of purple soil slope matched well to kinematical wave model. The flux, depth and velocity of flow increased all together according with the increasing of rainfall density and slope length and the flux and depth of flow decreased according with the increasing of gradient. The velocity of flow attained to maximum when gradient was 52.91 ° and subsequently decreased according with the increasing of gradient. Similar to velocity of flow, the runoff erosion capacity of purple soil slope attained to maximum when the gradient was the critical slope gradient of erosion. The critical slope gradient of erosion was a variable and decreased according with the increasing of rainfall density and slope length, but increased according with the increasing of the soil grain. The critical slope gradient of erosion for the sloping crop land has been estimated with its range between 40.91 ° ~ 41.68° and for sloping non-crop land, it was with its range between 41.11° ~ 47.24° by using the wallop theory; The critical slope gradient of erosion for the sloping crop land has been estimated with its range between 45.13° ~ 45.81 ° and for sloping non-crop land, it was with its range between45.13 ° ~ 45.84° by using kinematical wave theory. Above-mentioned the critical slope gradient of erosion was in the range between 31.79° ~ 48.08° which was acquired by means of the formula of sediment transportation for symmetrical flows. The gradient was the controlling factor of runoff and sediment form up-slope on down-slope erosion process. Generally speaking, erosion and aggradations occurred at the same time and the quantity of aggradations was great than that of erosion, moreover the surpassing was rested with the ratio of slope length and gradient of up-slope to that of down-slope when the gradient of down-slope was smaller than that of up-slope. The sediment of up-slope was entirely conveyed and the additional sediment came into being which was root in the runoff of up-slope, moreover the quantity of additional sediment was rested with the ratio of slope length and gradient of up-slope to that of down-slope in the same way when the gradient of down-slope was greater than that of up-slope.(2) The discrepancy of "source/sink" landscape erosion units was resemble to that of geographical entities and the marked tolerance was weakened piece by piece form center to boundary. So the transitional zone was consequentially present between two landscape erosion units in which had different texture and the borderline is the nonobjective figure of the transitional zone. The water system of Wu-Qiao-He watershed was consist of three grades watercourse and the number of 1 level, 2 level and 3 level was compliably 1, 5, 22 based on the law of watercourse configuration which was set up by Horton and Strahler. The number of sub-watershed which distributed in different level watercourse was 1 level >3 level>2 level, but the distributed area was sharply descended according with the increasing of watercourse level. The ratio of embranchment, length, slope and area were compliably 4.51, 2.51, 4.17 and 5.78 and the dimension of water system was 1.64. The inner evenness of "source/sink" landscape erosion units increased accordingwith the increasing of grid scale based on transforming datum with central attribute. The effects of grid scale on the partition of "source/sink" landscape erosion units what distributed in different level watercourse had prominent discrepancy. In brief, the proportion of "source" landscape erosion units sharply decreased in the sub-watershed distributed in low-grade watercourse according with the increasing of grid scale and the degressive trend was more and more indistinctive according with the ascending of watercourse grade.(3) "source/sink" landscape erosion units of soil and water loss was the muster of plots which were characterized with similar topography, vegetation and soil. The soil and water loss system of region was provided with multi-hierarchy and multi-nesting structure consisting of "source/sink" landscape erosion units, sub-watershed, watershed and region form down to up. The quantitative evaluation on effect of spatial scale and scaling up on soil and water loss could be achieved by means of two approaches. The quantitative evaluation models of soil and water loss effect of "source/sink" landscape erosion units were set up for multi-scale in the first approach and the spatial pattern indexes of "source/sink" landscape erosion units were set up based on the Lorenz Curve theory and the characters of water system were analyzed based on GIS in the second approach. The first approach was feasible when the studied area was small and the spatial pattern of "source/sink" landscape erosion units was simple and the second approach was feasible in large scale accordingly. In practice, the above two approaches should be close integrated for the prediction of soil and watershed loss in large scale.(4) When spatial variation and scale variation of soil and water loss were analyzed in watershed by means of multi-hierarchy spatial discretization, for the sub-watershed was concerned, the scheme of spatial discretization was sub-watershed to "source/sink" landscape erosion units. For the sub-watershed was concerned, the scheme of spatial discretization was watershed to sub-watershed to "source/sink" landscape erosion units. Sanhe small watershed was consist of 77 "source/sink" landscape erosion units in Fengdou county. The total loss quantity of soil, organic matter, total N, total P and total K were compliably 5839.0U, 197.94t, 7.42t, 3.91t and 174.00t and the loss modulus of soil, organic matter, total N, total P and total K were compliably 5167.27t km’2, 175.17t km"2, 6.57t km’2, 3.46 t km’2 and 153.98 t km’2 in "source/sink" landscape erosion units scale. The total loss quantity of soil, organic matter, total N, total P and total K were compliably 5839.01t, 197.94t, 7.42t, 3.91t and 174.00t and the loss modulus of soil, organic matter, total N, total P and total K were compliably 5167.27t km’2, 175.17t km"2, 6.57t km’2, 3.46 t km’2 and 153.98 t km"2 in sub-watershed scale. The quantity of loss sediment and nutrient of Sanhe small watershed reduced by 32.61% form "source/sink" landscape erosion units scale to sub-watershed scale.(5 ) It was the feasible approach for the quantitative evaluation of spatial variation and scalevariation on soil and water loss in large area that the large area was divided to a mass of watersheds and the characteristics of water system were analyzed based on spatial variation and scale variation of soil and water loss in representative watershed and the spatial pattern indexes of "source/sink" landscape erosion units. Shi-Wan-Xi watershed was consist of 13 sub-watershed in Kai county. The quantity of loss sediment and nutrient of Shi-Wan-Xi watershed reduced by 32.59% form sub-watershed scale to watershed scale. Kai county was consist of 55 watersheds and the area discrepancy of 21268.47 hm2 was existent between Kai county area and 55 watersheds area. The quantity of loss sediment and nutrient of Kai county reduced by 45.46% form watershed scale to region scale.(6) Effect of spatial scale on soil and water loss was very complex phenomenon. In past, a phenomenon was described in macrostructure or a law was opened out on the study of soil and water loss, but it was rarely interpreted that how the macroscopic phenomenon occurred based on the interaction of microcosmic laws, so the transition bridge was devoid between macroscale and microscale. At present, the transition bridge might be set up with the developing of the intellective evaluation system based on agents technique. The system of soil and water loss was multi-hierarchy agents system and was consist of "source/sink" landscape erosion unit agents, sub-watershed agents, watershed agents and region agents form down to up according to the studied scale of soil and water loss. The dynamic evolution of soil and water loss system came into being because of complicated interaction among these agents. Compared with other agent-based simulation, the agent-based intellective evaluation of effect of spatial scale on soil and water loss was more complicated and the distributed artificial intelligence, multi-agents system and GIS should be integrated with reason.(7) The poverty and degeneration of ecological environment co-existed in the watersheds of Three Gorges Reservoir Area. For the sustainable development of the watersheds was concerned, the essential approach was attribute to the breaking of swart circle between poverty and degeneration of ecological environment. Anti-poverty and ecological sustainability needed to be considered at least when the programming of sustainable development was framed in these watersheds, so the multi-objective programming was the feasible means with which the indeterminacy, multi-objective and complexity of the system could be represented. The optimized scheme of landuse structure and regulating of landscape pattern was the key of the programming of sustainable development. With the optimized scheme of landuse structure and regulating of landscape pattern, the Shi-wan-xi watershed was divided into three regions: functional region of agricultural landscape, resuming and harmonious region of ecology and controlling region of ecology. The inlaid combination of landscape was set up among and in three regions accordingly and the heading of domain in three regions was put forward. The case showed that: The spatial

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CLC: > Agricultural Sciences > Agriculture as the foundation of science > Soil > Soil and Water Conservation > The causes and prevention of soil erosion
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