Zhengan Su

Critical topographic threshold of gully erosion in Yuanmou Dry-hot Valley in Southwestern China

Gully initiation has been a subject of much discussion in the field of gully erosion. Gully initiation could be examined by the S–A relationship (S = aA − b ) between the critical slope of the gully head (S) and the upstream drainage area (A). Gully erosion plays an important role in sediment yield and land degradation in the Dry-hot Valley region of Southwest China; however, little research has focused on gully development in this region. This study investigated gully morphology and the critical topographic threshold of permanent gullies in Yuanmou Dry-hot Valley. Data for 36 gully heads were derived from the gully digital elevation model created by high-precision real-time kinematic GPS (RTK GPS). A significant relationship between gully length and volume (r 2 = 0.77, p < 0.01) was found to exist, indicating that gully length can be used to estimate the gully erosion rate. Additionally, as vegetation coverage inside the gully increases, the gully morphology tends to be stable. The S–A relationship was S = 0.5195A −.0899, while AS 2 values ranged between 4.2 and 758.5 m2. The low value of constant b (0.0899) indicates that collapse of the gully wall is the dominant process of gully erosion in Yuanmou Dry-hot Valley.

Impact of grass belt position on the hydraulic properties of runoff in gully beds in the Yuanmou Dry-hot valley region of Southwest China

Gully erosion is one of the major contributors to severe land degradation in the Yuanmou Dry-hot valley region of Southwest China. Planting vegetation in gully beds has proven to be an effective measure in controlling the development of gullies. However, the inner mechanism for this process remains unclear. In this paper, in situ scouring experiments were performed to reveal the influence of grass belt position on the hydraulic properties of runoff in gully beds. The results demonstrate that (1) grass belt position can have a significant effect on reducing runoff erosivity (runoff shear stress, stream power) and sediment content in gully beds, and that the reduction may be strengthened as the distance between the gully headwall and grass belt (DHG) decreases; (2) grass belts could play a positive role in enhancing the surface resistance and increasing flow energy consumption, as resistance and flow energy consumption increase as DHG increases; and (3) there were similar trends in the temporal variations within the hydraulic properties, which changed sharply in the early stage of the experiment and then tended to stabilize, with their time to stability continuously increasing as DHG decreased.

Changes in SOC and nutrients under intensive tillage in two types of slope landscapes

The net effect of tillage erosion on soil properties would be associated with the spatial variation in soil constituents, and therefore plays an important role in ecological agriculture. We conducted a consecutive tillage by hoeing 15 times during a period with no rainfall in the two slope landscapes (a linear slope and complex slope) of the Yangtze Three Gorges reservoir areas, to examine the relationship between soil erosion rates and the variations in soil chemical properties and compare the effects of soil redistribution on SOC and nutrients between the linear and complex slopes. After the simulated tillage, notable changes in 137Cs inventories of the soil occurred in the summit and toeslope positions on the linear slope, while there were significant changes in 137Cs inventories at convex and concave positions on the complex slope. Soil profile disappeared at the summit slope boundary, with the exposure area of 16.0% and 7.6% of the experimental plot, respectively, for the linear and complex slopes due to no soil replacement. Soil organic C and nutrients were completely depleted with the disappearance of soil profiles at soil eroding zones, whereas a remarkable increase in SOC, total N and available nutrient concentrations of the post-tillage surface soil and a decrease in total nutrient concentrations (P and K) were found at depositional zones on the linear slope. For the complex slope, however, changes in SOC and nutrient concentrations of the post-tillage surface soil exhibited a pattern different from that on the linear slope, which showed a remarkable decrease in SOC and total nutrient concentrations but a slight increase in available nutrient concentrations after tillage in the toeslope position. Due to the gradual increase in soil depth from top to bottom of the slope, SOC and nutrient inventories in the soil profiles were significantly correlated with soil redistribution rates on both the linear and complex slopes. Tillage causes remarkable changes of soil chemical properties in the surface soil layer and soil profile, and increases SOC and nutrient inventories for the soil profile downslope in steeply sloping landscapes.

Effects of initial step height on the headcut erosion of bank gullies: a case study using a 3D photo-reconstruction method in the dry-hot valley region of southwest China

Abstract Headcut erosion has been recognized as one of the main processes involved in gully development in the dry-hot valley region of southwest China. To examine the effect of initial step height on headcut erosion processes, three headcuts were constructed ranging in height from 0.75 to 1.25 m on an active bank gully head, and a series of scouring experiments were conducted under a flow discharge of 120 L min−1. The morphological evolutions of the plunge pools and soil loss volume were estimated by three-dimensional photo-reconstruction methods (3D-PR). As the step height increased, the experimental results showed that: (1) the transformed potential energy and shear stress would increase by approximately 4.89 J s−1 and 26.4 Pa on average when the step height increased 0.25 m; (2) the mean depth and width of the plunge pool exhibited obvious growth, and the morphology of the cross-section developed from approximately V-shaped to U-shaped; and (3) soil loss volume increased logarithmically, with total soil loss volumes of 0.076, 0.105 and 0.116 m3, respectively. Although the significant effects of the initial step height on headcut erosion were verified, further quantitative studies are required to quantify the mechanism of headcut erosion, especially for plunge pool erosion.

Effects of vegetation coverage and seasonal change on soil microbial biomass and community structure in the dry-hot valley region

Soil microorganisms are sensitive indicator of soil health and quality. Understanding the effects of vegetation biomass and seasonal change on soil microorganisms is vital to evaluate the soil quality and implement vegetation restoration. This study analyzed the soil phospholipid fatty acids (PLFAs) in fresh and withered Kudzu (Pueraria montana var. lobata) vegetation conditions in different seasons. The results showed that vegetation biomass and seasonal change significantly affected microbial biomass and its community structure. Both fresh and withered Kudzu cover significantly increased soil microbial biomass, and the growth effect of microbes in the soil with fresh Kudzu cover was more obvious than that with withered Kudzu cover. Compared with the dry season, the rainy season significantly increased the microbial biomass and the B/F (the ratio of bacterial to fungal PLFAs) ratio but dramatically reduced the G+/G- (the ratio of gram-positive to gram-negative bacteria PLFAs). Kudzu cover and seasonal change had a significant effect on microbial structure in soil covered by higher vegetation biomass. Furthermore, soil temperature and moisture had different correlations with specific microbial biomass in the two seasons. Our findings highlight the effect of Kudzu vine cover on the soil microenvironment and soil microhabitat, enhancing the soil quality in the Dry-hot Valley of Jinsha River, Southwest China.

Impacts of headcut height on flow energy, sediment yield and surface landform during bank gully erosion processes in the Yuanmou Dry-hot Valley region, southwest China: Changes in flow energy, sediment yield and surface landform

To quantify the changes in flow energy, sediment yield and surface landform impacted by headcut height during bank gully erosion, five experimental platforms were constructed with different headcut heights ranging from 25 to 125cm within an in situ active bank gully head. A series of scouring experiments were conducted under concentrated flow and the changes in flow energy, sediment yield and surface landform were observed. The results showed that great energy consumption occurred at gully head compared to the upstream area and gully bed. The flow energy consumption at gully heads and their contribution rates increased significantly with headcut height. Gully headcuts also contributed more sediment yield than the upstream area. The mean sediment concentrations at the outlet of plots were 2.3 to 7.3 times greater than those at the end of upstream area. Soil loss volume at gully heads and their contribution rates also increased with headcut height significantly. Furthermore, as headcut height increased, the retreat distance of gully heads increased, which was 1.7 to 8.9 times and 1.1 to 3.2 times greater than the incision depth of upstream area and gully beds. Positive correlations were found between energy consumption and soil loss, indicating that energy consumption could be used to estimate soil loss of headcut erosion. Headcut height had a significant impact on flow energy consumption, and thus influenced the changes in sediment yield and landform during the process of gully headcut erosion. Headcut height was one of the important factors for gully erosion control in this region. Further studies are needed to identify the role of headcut height under a wide condition. Copyright (c) 2018 John Wiley & Sons, Ltd.

Geotouristic Value of Badlands

Abstract Badlands occur in dissected landscapes on all continents and are of interest to tourists for their landscape and aesthetic value. People choose these eroded landforms as tourist destinations to pursue active tourism, movie tourism and geotourism. The geotouristic value of badlands varies across the world: some of them are listed as UNESCO World Heritage Sites and attract hundreds of thousands of tourists per year; others, located in less accessible areas without tourist infrastructure are only known to scientists and adventure tourists. Global geotourism is now developing primarily within geoparks, some of which contain a small number of badlands. Establishing new geoparks containing badland areas will increase the attraction and conservation of these unique geomorphic features on Earth.

Variation in the vertical zonality of erodibility and critical shear stress of rill erosion in China's Hengduan Mountains: Variation in the vertical zonality of erodibility-Su et al

Soil erodibility is an essential parameter used in soil erosion prediction. This study selected the Liangshan town watershed to quantify variation in the vertical zonality of rill erodibility (k(r)) in Chinas ecologically fragile Hengduan Mountains. Soil types comprised of yellow-brown (soil A), purple (soil B), and dry-red (soil C) in a descending order of occurrence from the summit to the valley, which roughly corresponds to the vertical climate zone (i.e. cool-high mountain, warm-low mountain, and dry-hot valley sections) of the study area. With elevation, vertical soil zonality varied in both soil organic matter (SOM) content and soil particle-size fractions. A series of rill erosion-based scour experiments were conducted, using water discharge rates of 100, 200, 300, 400, 500, and 600 mL min(-1). Additionally, detachment rates (Dr) were measured under three hydrological conditions (the drainage, saturation, and seepage treatments). Results show that both Dr and flow shear stress (&) values increased as discharge increased. As elevation increased, the k(r) values decreased, while the vertical zonality of critical shear stress (&(c)) values showed no obvious variation. The highest k(r) values were observed during the seepage treatment, followed by the saturation treatment then drainage treatment, indicating that variation in vertical hydraulic gradients could significantly alter k(r) values. This study also found that land-use types could also alter k(r) and &(c) values. Further research, however, is necessary to better quantify the effects of subsurface hydrological conditions and land-use types on k(r) under different soil zonalities in Chinas Hengduan Mountains. (c) 2018 John Wiley & Sons, Ltd.

Assessment of soil erosion by compensatory hoeing tillage in a purple soil

This study explores the role of a traditional tillage method, i.e., compensatory hoeing, for sustainable agro-ecosystem management in the hilly areas of the Chongqing municipality, south-western China. To validate the effects of compensatory tillage on the terraced slopes, the tillage method of noncompensatory hoeing was conducted on a linear slope. To acquire information about 137Cs inventories and soil texture, soil samples were collected by a core sampler with a 6.8-cm diameter at 5.0-m intervals along the toposequence and the linear slope in the dry season (March) of 2007. Meanwhile, a tillage erosion model was used for evaluating the spatial pattern of tillage erosion. The 137Cs data showed that on the terraced slope, soil was lost from the upper slope, and soil deposition occurred at the toe slope positions on each terrace. As a result, abrupt changes in the 137Cs inventories of soil were found over short distances between two sides of terrace boundaries. Results obtained from the tillage erosion model and the 137Cs data indicate that soil redistribution mainly results from tillage erosion in the terraced landscape. Consecutive non-compensatory tillage caused soil redistribution on the linear slope, resulting in thin soil profile disappearing at the top and soil accumulating at the bottom positions of the linear slope. This result further validates that compensatory tillage could avoid the complete erosion of the thin soil layer at the summit position. Therefore, this traditional tillage method, i.e., compensatory tillage, has maintained the soil quality at the summit of the slope in the past decades.