Chongfa Cai

Soil conservation planning at the small watershed level using RUSLE with GIS: a case study in the Three Gorge Area of China

Abstract The Three Gorge Project (TGP) of China necessitates the resettlement of over 1 million population (mostly farmers) to more rugged and isolated areas than their original settlements. Soil erosion is a serious environmental and production problem in this area. To decrease the risk on environmental impacts, there is an increasing demand for sound, and readily applicable techniques for soil conservation planning in the Three Gorge Areas (TGA). The objectives of the study were to develop and validate a soil erosion-predicting model based on the revised Universal Soil Loss Equation (RUSLE) in a geographic information systems (GIS) environment. The use of GIS to develop conservation-oriented watershed management strategies in the Wangjiaqiao watershed is presented. Data used for the RUSLE were either determined or taken from published literature pertaining to the Wangjiaqiao watershed. In combination with IDRISI, GIS software (Eastman, R.J., 1997. IDRISI for Windows: Users guide (Version 2.0). Clark University, Graduate School of Geography, Worcester, MA, Chapters 4–17) was used to evaluate different agricultural management strategies in terms of predicted soil loss in the watershed. This model allowed for easy assessment of soil erosion hazards under different crop and land management options over the entire watershed. The study revealed that the annual average soil loss rate from relatively flat agricultural land was approximately 26 t/ha, whereas 52 t/ha was found on the cultivated sloping lands, which constitutes a large proportion of soil loss in the watershed. In the watershed, approximately 38 ha of agricultural land had slopes >47% (25°) and should be reforested or returned to pasture. Contour tillage (CT) and contour farming with a seasonal no-till ridge (CTN) were most effective in reducing soil loss rates. If CT and CTN were implemented, approximately 31% and 70%, respectively, of the areas with soil loss > T EP would be reduced to T EP . T EP is soil loss tolerance for economic planning and was set at ≤10 t/ha year. In addition to soil loss reduction, the CTN has the potential to increase crop yield. Soil erosion hazards may be alleviated in over 91% of the agricultural lands if combined conservation measures including terraces, CTN, CT, and crop rotations were implemented in the watershed. The results of the study indicate that the RUSLE-GIS model is a useful tool for resource management and soil conservation planning. This technology is readily transferable and accessible to other land managers and agronomists in the TGA.

Effects of vegetation on runoff generation, sediment yield and soil shear strength on road-side slopes under a simulation rainfall test in the Three Gorges Reservoir Area, China.

Vegetation recolonization has often been used to control roadside slope erosion, and in this paper, four restoration models - Natural Restoration, Grass, Grass & Shrub, Sodded Strip - were chosen to recolonize the plants on a newly built unpaved roadside slope in the Three Gorges Reservoir Area. After eight months growth, eight rainfall simulations (intensity of 90 mm h(-1) for 60 min) and in-situ soil shear strength test were then carried out to identify the impacts of vegetation on roadside slope erosion and soil shear strength. The erosion on cutslopes was higher than that on fillslopes. The runoff coefficient and soil detachment rate were significantly lower on the Grass & Shrub model (4.3% and 1.99 g m(-2) min(-1), respectively) compared with the other three, which had the highest surface cover (91.4%), aboveground biomass (1.44 kg m(-2)) and root weight density (3.94 kg m(-3)). The runoff coefficient and soil detachment rate on roadside slopes showed a logarithmic decrease with the root weight density, root length density and aboveground biomass. The soil shear strength measured before and after the rainfall was higher on Grass & Shrub (59.29 and 53.73 kPa) and decreased on Grass (46.93 and 40.48 kPa), Sodded Strip (31.20 and 18.87 kPa) and Natural Restoration (25.31 and 9.36 kPa). Negative linear correlations were found between the soil shear strength reduction and aboveground biomass, root weight density and root length density. The variation of soil shear strength reduction was closely related to the roadside slope erosion, a positive linear correlation was found between runoff coefficient and soil shear strength reduction, and a power function was shown between soil detachment rate and soil shear strength reduction. This study demonstrated that Grass and Grass & Shrub were more suitable and highly cost-effective in controlling initial period erosion of newly built low-volume unpaved road.

Wetting rate and clay content effects on interrill erosion in ultisols of southeastern China.

Abstract An aggregate stability test and a simulated rainfall test were conducted on four representative Ultisols from southeastern China. The soils selected, with clay contents ranging between 117 and 580 g kg−1, were derived from shale and Quaternary red clay. The stability of aggregates (2–5 mm in diameter) obtained from the soil samples were determined by the Le Bissonnais method. For determination of infiltration, runoff, and erosion, the soil samples were packed in 30 cm × 60 cm trays, wetted at rates of 2, 10, and 60 mm h−1, and then exposed to simulated rainfall at 60 mm h−1 for 1 h. The results indicated that both aggregate stability and slaking caused by fast wetting increased with increasing clay content. The effect of wetting rate (WR) on infiltration and seal formation varied with clay contents. In the soil with low clay content (sandy loam), the infiltration rate was affected slightly by WR due to low aggregate stability and slaking. In the soils with medium clay content (silt clay loam and clay), WR affected infiltration significantly due to the high aggregate slaking force. In the soil with high clay content, the effect of WR on infiltration was significant, but not as evident as in the soils with medium clay content, which may be related to high aggregate stability by wetting partially compensating for slaking force. The effect of WR on soil loss was similar to that of runoff, but more pronounced. The findings from this study indicated that the relationship between wetting rate and clay content should be considered when predicting interrill erosion in Ultisols.

The effects of land use change on environmental quality in the red soil hilly region, China: A case study in Xianning County

Land use and land cover change is gaining recognition as a key driver of environmental change. Rapid change of land use has occurred in the red soil hilly region of southeast China in the past three decades due to rural land reform, population pressure and rapid economic growth. In this paper, land use change and its impacts on environmental quality of Xianning County were investigated using technologies of remote sensing (RS), geographic information systems (GIS), and spatial principal components analysis (SPCA). It was indicated that the area of paddy field and upland decline rapidly while the area of woodland, horticulture farm and residential land increased sharply between 1978 and 2002. The driving forces probably were land use policy of the State government, structural adjustment of agricultural sectors and implementation of environmental conservation measures. Based on the assessment of environmental implications of land use practices, the environmental quality of some land use types (i.e. Woodland, sparse woodland, and paddy field) tended to improve, while horticulture farms, water body, and residential land showed a deterioration trend of environmental quality during 1978–2002. Perhaps the most significant finding of this study is that environment appears a negative tendency in the study area from 1991 to 2002, because little attention has been paid to the status and management of agricultural lands, which account for a substantial proportion of total land area. Therefore, for further improving environment in red soil hilly region in China, urgent action is required to improve the advisory services support to farmers on new tillage practices, soil and water conservation practices, raising the efficiency of fertilizer and manure use.

Responses of Runoff and Soil Erosion to Vegetation Removal and Tillage on Steep Lands

Abstract Land use and land cover change is a key driver of environmental change. To investigate the runoff and erosion responses to frequent land use change on the steep lands in the Three Gorges area, China, a rainfall simulation experiment was conducted in plots randomly selected at a Sloping Land Conversion Program site with three soil surface conditions: existing vegetation cover, vegetation removal, and freshly hoed. Simulated rainfall was applied at intensities of 60 (low), 90 (medium), and 120 mm h−1 (high) in each plot. The results indicated that vegetation removal and hoeing significantly changed runoff generation. The proportion of subsurface runoff in the total runoff decreased from 30.3% to 6.2% after vegetation removal. In the hoed plots, the subsurface runoff comprised 29.1% of the total runoff under low-intensity rainfall simulation and the proportion rapidly decreased with increasing rainfall intensity. Vegetation removal and tillage also significantly increased soil erosion. The average soil erosion rates from the vegetation removal and hoed plots were 3.0 and 10.2 times larger than that in the existing vegetation cover plots, respectively. These identified that both the runoff generation mechanism and soil erosion changed as a consequence of altering land use on steep lands. Thus, conservation practices with maximum vegetation cover and minimum tillage should be used to reduce surface runoff and soil erosion on steep lands.

Effects of land uses on soil physic-chemical properties and erodibility in collapsing-gully alluvial fan of Anxi County, China

Abstract As a special kind of soil erosion that severely deteriorates the qualities of soil in granite regions, collapsing gully is widely distributed in the tropical and subtropical areas of South China. In particular, collapsing-gully erosion produces a large amount of sediment deposited on the plough layer of alluvial fan farmland and causes increase of desertification, great reduction of nutrients and rapid enhancement of erodibility in the soil. This study was designed to evaluate the effects of different land uses on the soil physic-chemical properties and erodibility of collapsing-gully alluvial fan. Our results show that the physical properties of soil in alluvial fan were greatly improved with smaller bulk density, increased soil porosity, strengthened water holding capacity and a higher particulate matter content. The chemical properties of soil were also significantly improved, including pH, cation exchange capacity, and the contents of organic matter, total and available nitrogen, potassium and phosphorus. All the land uses were proved to improve the soil properties, especially in the tea garden, vegetable land and paddy field. The results of correlation analyses among the properties demonstrate that the physical and chemical properties were significantly correlated, indicating that improving the soil physical properties is an effective method to increase the soil nutrient in the farmland of collapsing-gully alluvial fan. In addition, the application of land uses included grass land, eucalyptus forest land, vegetable land, tea garden, sweet potato land, and paddy field reduced the erodibility of the soil compare to bare land. The erodibility K values of soils in grass land, eucalyptus forest land, vegetable land, tea garden, sweet potato land, and paddy field were 14.43, 16.97, 45.45, 33.12, 18.94, and 34.01% lower than those of bare land, respectively. The results of multiple linear regression analysis show that the soil erodibility had a significant correlation with the physic-chemical properties, especially the soil texture and the content of organic matter. In conclusion, our results demonstrate that different land use patterns are effective to improve the quality of the soil in collapsing-gully alluvial fan, including the improvements of the soil structure, nutrients and anti-erosion ability. Our findings provide important implications for the soil improvement in the farmland of collapsing-gully alluvial fan.

Hydrological Response of Sloping Farmlands with Different Rock Fragment Covers in the Purple Soil Area of China

In the mountainous area of China, the rock fragments in top soils are often removed by farmers because of their side effects on tillage. To understand hydrological processes and to assess the risk of soil erosion in purple soil areas with sloping farmlands, this study investigated the effects of rock fragment cover on surface runoff, infiltration, subsurface runoff, and erosion under field conditions using a portable rainfall simulator. Experimental twin plots (two subplots, 1 m wide by 2 m long, 32% slope) with different rock fragment covers ranging from 0 to 42% were exposed to four rainfall intensities: 45.5 � 1.9 mm=h (I1), 60.3 � 3.6 mm=h (I2), 92.0 � 4.1 mm=h (I3), and 123.9 � 9.4 mm=h (I4). Surface runoff, subsurface runoff, soil moisture, and sediment were measured. The results show that the hydrological response was influenced by the rock fragment cover and rainfall intensity. The rate of surface runoff in bare soils was affected by the rainfall intensity and increased with increasing rainfall intensity. The values of the surface runoff rates in bare soils were lower than 70 mm=h under the I1, I2, and I3 rainfall intensities. As the rock fragment cover increased, the surface runoff rate decreased while the infiltration rate, the subsurface runoff rate and the deep percolation coefficient increased. On the other hand, when the surface runoff rates in the bare soils were higher than 70 mm=h (in rainfall of I4), there were no significant differences in these hydrological variables among soils with different rock fragment covers. The differences in hydrological variables among soils with varying rock fragment covers decreased with increasing rainfall intensity. The rock fragment cover determined the erosive response. As the rock fragment cover increased, the sediment concentration de- creased. The presence of surface rock fragments significantly reduces soil erosion, and the relationship between the soil erosion rate and rock fragment cover can be expressed by an exponential function with a high degree of reliability for different rainfall intensities. However, the effectiveness of the rock fragment cover in reducing soil erosion decreased with increasing rainfall intensity. DOI: 10.1061/(ASCE)HE .1943-5584.0000576.

Landscape pattern and transition under natural and anthropogenic disturbance in an arid region of northwestern China

Abstract There is a pressing need to determine the relationships between driving variables and landscape transformations. Human activities shape landscapes and turn them into complex assemblages of highly diverse structures. Other factors, including climate and topography, also play significant roles in landscape transitions, and identifying the interactions among the variables is critical to environmental management. This study analyzed the configurations and spatial-temporal processes of landscape changes from 1998 to 2011 under different anthropogenic disturbances, identified the main variables that determine the landscape patterns and transitions, and quantified the relationships between pairs of driver sets. Landsat images of Baicheng and Tekes from 1998, 2006 and 2011 were used to classify landscapes by supervised classification. Redundancy analysis (RDA) and variation partitioning were performed to identify the main driving forces and to quantify the unique, shared, and total explained variation of the sets of variables. The results indicate that the proportions of otherwise identical landscapes in Baicheng and Tekes were very different. The area of the grassland in Tekes was much larger than that of the cropland; however, the differences between the grassland and cropland in Baicheng were not as pronounced. Much of the grassland in Tekes was located in an area that was near residents, whereas most of the grassland in Baicheng was far from residents. The slope, elevation, annual precipitation, annual temperature, and distance to the nearest resident were strong driving forces influencing the patterns and transitions of the landscapes. The results of the variation partitioning indicated complex interrelationships among all of the pairs of driver sets. All of the variable sets had significant explanatory roles, most of which had both unique and shared variations with the others. The results of this study can assist policy makers and planners in implementing sustainable landscape management and effective protection strategies.

Fractal features of soil particle size distribution under different land-use patterns in the alluvial fans of collapsing gullies in the hilly granitic region of southern China

Collapsing gullies are among the most severe soil erosion problems in the tropical and subtropical areas of southern China. However, few studies have examined the relationship of soil particle size distribution (PSD) changes with land-use patterns in the alluvial fans of collapsing gullies. Recently, the fractal method has been applied to estimate soil structure and has proven to be an effective tool in analyzing soil properties and their relationships with other eco-environmental factors. In this study, the soil fractal dimension (D), physico-chemical properties and their relationship with different land-use patterns in alluvial fans were investigated in an experiment that involved seven collapsing gully areas in seven counties of southern China. Our results demonstrated that different land-use patterns of alluvial fans had a significant effect on soil physico-chemical properties. Compared to grasslands and woodlands, farmlands and orchards generally contained more fine soil particles (silt and clay) and fewer coarse particles, whereas significant differences were found in the fractal dimension of soil PSD in different land-use patterns. Specifically, the soil fractal dimension was lower in grasslands and higher in orchards relative to that of other land-use patterns. The average soil fractal dimension of grasslands had a value that was 0.08 lower than that of orchards. Bulk density was lower but porosity was higher in farmlands and orchards. Saturated moisture content was lower in woodlands and grasslands, but saturated hydraulic conductivity was higher in all four land-use patterns. Additionally, the fractal dimension had significant linear relationships with the silt, clay and sand contents and soil properties and exhibited a positive correlation with the clay (R2 = 0.976, P<0.001), silt (R2 = 0.578, P<0.01), organic carbon (R2 = 0.777, P<0.001) and saturated water (R2 = 0.639, P<0.01) contents but a negative correlation with gravel content (R2 = 0.494, P<0.01), coarse sand content (R2 = 0.623, P<0.01) and saturated hydraulic conductivity (R2 = 0.788, P<0.001). However, the fractal dimension exhibited no significant correlation with pH, bulk density or total porosity. Furthermore, the second-degree polynomial equation was found to be more adequate for describing the correlations between soil fractal dimension and particle size distribution. The results of this study demonstrate that a fractal dimension analysis of soil particle size distribution is a useful method for the quantitative description of different land-use patterns in the alluvial fans of collapsing gullies in southern China.

Variation of Soil Aggregation along the Weathering Gradient: Comparison of Grain Size Distribution under Different Disruptive Forces

The formation and stabilization of soil aggregates play a key role in soil functions. To date, few studies have been performed on the variation of soil aggregation with increasing soil weathering degree. Here, soil aggregation and its influencing factors along the weathering gradient were investigated. Six typical zonal soils (derived from similar parent materials) were sampled from temperate to tropical regions. Grain size distribution (GSD) in aggregate fragmentation with increasing disruptive forces (air-dried, water dispersion and chemical dispersion) was determined by laser diffraction particle size analyzer. Different forms of sesquioxides were determined by selective chemical extraction and their contributions to soil aggregation were identified by multiple stepwise regression analysis. The high variability of sesquioxides in different forms appeared with increasing free oxide content (Fed and Ald) from the temperate to tropical soils. The transformation of GSD peak to small size varied with increasing disruptive forces (p<0.05). Although in different weathering degrees, zonal soils showed a similar fragmentation process. Aggregate water stability generally increased with increasing soil weathering (p<0.01), with higher stability in eluvium (A) horizon than in illuvium (B) horizon (p<0.01). Crystalline oxides and amorphous iron oxides (Feo), especially (Fed-Feo) contributed to the formation of air-dried macroaggregates and their stability against slaking (R2 = 55%, p<0.01), while fine particles (<50μm) and Feo (excluding the complex form Fep) played a positive role in the formation of water stable aggregates (R2 = 93%, p<0.01). Additionally, water stable aggregates (including stability, size distribution and specific surface area) were closely related with pH, organic matter, cation exchange capacity (CEC), bulk density (BD), and free oxides (including various forms) (p<0.05). The overall results indicate that soil aggregation conforms to aggregate hierarchy theory to some extent along the weathering gradient and different forms of sesquioxides perform their specific roles in the formation and stabilization of different size aggregates.