Spatial Distribution Of Soil Water Content And Sediment In The Dam Farmlands In A Small Catchment Of The Loess Plateau

The Loess Plateau of China was more famous with the most serious soil loss in the world than the biggest area. Lots of check-dams were built to hold sediment and conserve water widely and quickly. As a consequence, the widespread dam farmlands, which were believed to be to the last barrier to control soil and water losses, were formed quickly due to the serious soil and water losses. In addition, the dam farmland also can be grown plant to increase the crop yield with higher value than the sloping farmland. By 2002, there were about 113,500 check-dams which formed 3,340 km2 dam farmland, held 2.1×1010 m3 of sediments, and controlled 9,247 km2 drainage areas on the Loess Plateau of China. The number of check-dams and the area of dam farmlands are expected to be doubled by 2020. Many plans also showed that more check-dam should be built to conserve the soil and water for their great achievement. Consequently, it is necessary to pay more attention to the dam farmland for the aim of serving agriculture production and ecological protection.The spatial distribution of the soil water content and sediment in the dam farmland is very important to the crop yield, flood control, and water consuming and so on. The objectives of this dissertation were to explore the spatial distribution of soil water content and sediment in the dam farmlands. The study dam farmland located in the Liudaogou catchment of the northwest of Shaanxi province. The soil water content was measured by neutron tube and soil auger. The soil textures of the samples were measured by Mastersizer2000. Classic and geostatistic tools were combined to study the soil water content in the dam farmlands. In addition, fractal theory and parametric model were also introduced to study the soils taken from the dam farmlands.The major results of the present study in the dam farmland on the Loess Plateau of China are listed below: (1) The soil water content (0-6 cm) of the surface layer showed good spatial structure in the dam farmland. Gaussan model could fit the semivariance well. However, the semivariance showed anisotropism. The results showed that the Kriging and condition simulation could be well used in the study of soil water content of surface layer in the dam farmland.(2) The soil texture in the study dam farmland included silt loam, sandy loam, loamy sand, and sand. The soil water contents showed layered distribution corresponding to the particle size. The funnel flow was only found in the shallow soils with layered soil structure of the dam farmland after rainfall. The instability of an unsaturated wetting front as it passes from fine to coarse layers was demonstrated in the dam farmland. The soil water moved laterally in the dam farmland proved by the spatial correlation analysis. However, the water movement mainly happened on the first day after rainfall with a velocity of about 7 m day-1 which was quicker than the infiltration rate.(3) Analysis using Pearson correlation coefficients showed that the spatial variation in KS (at the observation scale) was significantly correlated with the spatial distributions of soil texture, saturated water content, and bulk density at both the filled dam farmland and the silting dam farmland under natural silting conditions. The areas with lowest KS values corresponded to the areas with the highest clay content, silt content, and saturated water content, indicating that the uplands of the dam farmlands are more prone to surface runoff. The area with the highest KS value was found at the middle portion of the dam farmlands with relative higher sand content and bulk density values (not the highest) which can benefit floodwater and runoff control.(4) The relationship between fractal feature and soil texture of the soils in dam farmlands was shown by sigmoid curve. However, the multifractal tool was invalid to predication soil degradation or soil desertification for the disordered intrinsic particle-size information. In addition, Weibull model was recommended to interpolate the soil particle-size distribution for the soils in the dam farmlands from the comparison among 14 models.(5) The dam farmlands without tillage contributed to the plant biodiversity. The plant species in dam farmland was similar with the sloping land. The collected data showed that soil water and sand fraction were important determinants of plant species. The local environment condition was very important for the establishment of the biodiversity in dam farmland.Some methods related to auto-correlation could be used to study topsoil water content for exploring more information. The existence of wetting front instability, carrying water rapidly and deeply in layered soils, might have great practical importance to the hydrologic process and environment protection. Not only the preferential flow, but also the layered water flow should be considered in case of possible contamination in the dam farmland. More attention should be paid to the preferential flow and layered flow aiming at water resource management and contamination control. As the water cycling of the dam farmland was more active due to the acceptation of runoff, the protection and the study was more urgent and important. Moreover, our results suggest focusing on environmental site conditions in order to better explain plant diversity in dam farmland. To the forage resource in filled dam farmland without tillage should be given more attention for it is an additional and potential resource. To encourage the local farmers to graze at a low stocking rate in dam farmland of no tillage could contribute to the biodiversity. If the terrain of dam farmland is too steep to decline the accessibility, advisable mowing is also encouraged.The spatial distribution of soil water and sediment content in the dam farmlands had important effects on the environmental impact assessment of check-dam, crop yield, water quality, and the layout of the check-dam. Based on the field observation, a little work about the soil water and sediment in the dam farmland was studied. The results were expected to be helpful on the related research about dam farmland. However, some science problems were still unclear, such as the accumulation of soil organic carbon, crop yield, the quality and quantity of the groundwater, the transport of the chemicals, and the relationship between sediment control and vegetation in the dam farmland. Those may be the direction of the study on the dam farmland in the future.