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
(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.