Anbang Wen

Sediment source identification by using137Cs and210Pb radionuclides in a small catchment of the Hilly Sichuan Basin, China

Double radionuclide tracers of137Cs and210Pb were used to investigate sediment sources in the Wujia Gully, a small catchment in the Hilly Sichuam Basin in 2002. Average137Cs and210Pb concentrations in the source soils of the steep forest slopes, gentle cultivated terraces and bare slopes were 7.15 ±0.40 and 162.01 ±3.86 Bq·kg-1, 4.01 ±0.31 and 70.96 ±2.65 Bq · kg-1, and 0 and 15.12 ±1.22 Bq · kg-1, respectively, while those concentrations in the recently deposited sediments in the reservoir were 3.06 ±0.23 and 72.66 ±1.61 Bq · kg-1, respectively. By using the mixing model, the relative sediment contributions from steep forest slopes, gentle cultivated terraces and bare slopes (including channel banks) were estimated to be 18%, 46% and 36%, respectively. Cultivated terraces and bare slopes (including channel banks) were the first and the second important sediment sources in catchment. Specific sediment yield in the catchment was 642 t · km-2 · a-1 from the deposited sediment volume in the reservoir since 1956. Soil erosion rates for the forest slopes and cultivated terraces, which accounted for 2/3 and 1/3 of the drainage area in the catchment, were estimated to be 173 and 886 t · km-2 · a-1, respectively.

Comparison of the soil losses from 7Be measurements and the monitoring data by erosion pins and runoff plots in the Three Gorges Reservoir region, China

The potential for using (7)Be measurements to document soil redistribution associated with a heavy rainfall was estimated using (7)Be method on a bare purple soil plot in the Three Gorges Reservoir region of China. The results were compared with direct measurement from traditional approaches of erosion pins and runoff plots. The study shows that estimation of soil losses from (7)Be are comparable with the monitoring results provided by erosion pins and runoff plots, and are also in agreement with the existing knowledge provided by 137Cs measurements. The results obtained from this study demonstrated the potential for using (7)Be technique to quantify short-term erosion rates in these areas.

Changes in land use and agricultural production structure before and after the implementation of grain for green program in Western China - taking two typical counties as examples

Soil erosion becomes a serious environmental problem in the world, especially in western China. An effective management practice called the Grain for Green Program (GGP), which was launched in 1999, aims to reduce soil and water loss and alleviate the ecological environment problem in western China. Two typical counties in western China, the Zhongxian (in Chongqing Municipality) and Ansai (in Shaanxi Province) were chosen to evaluate the dynamic changes of land use and agricultural production structure before and after the implementation of the Program in this paper. The results showed that the cultivated land area was reduced by 7.08% from 1989 to 2003. The cultivated land per person was decreased by 8.42% during 1999–2003. Moreover, the stability index of the secondary sector of the economy was increased from 0.91 in the period 1990–1999 to 0.94 in the following ten years. In addition, the stability index of tertiary economic sector increased from 0.88 to 0.92 in Zhongxian county. Meanwhile, the cultivated land area was reduced by 15.48% from 1990 to 1999. The soil erosion modulus was decreased by 33.33% from 1999 to 2006. Also, the stability index of secondary and tertiary economic sectors was 0.86 in the period 1998–2002. However, it decreased by 77% during 2002 to 2007 in Ansai County. These results imply that the Grain for Green Program had different impact on the two regions. Several effective strategies of soil and water conservation have been carried out to ameliorate the sustainable development of ecological environment and economy in these two counties of western China.

Sediment transfer at different spatial and temporal scales in the Sichuan Hilly Basin, China: Synthesizing data from multiple approaches and preliminary interpretation in the context of climatic and anthropogenic drivers

Quantifying sediment production and transfer at different spatial and temporal scales in a changing environment is critical in understanding the potential effects of climatic and anthropogenic drivers. Accordingly, estimates of soil erosion and sediment production at hillslope field, first-order small catchment (<0.25km2) and river basin scales in the Sichuan Hilly Basin of Southwestern China, generated using a variety of techniques, including fallout radionuclide tracing, runoff plot observations, core chronology dating and conventional sediment flux monitoring, were synthesized and interpreted in the context of potential climatic and human controls. Mean annual soil erosion rates ranged from 800Mg·km-2·yr-1 to 4500Mg·km-2·yr-1 on the basis of fallout radionuclide tracing and from 600Mg·km-2·yr-1 to 3300Mg·km-2·yr-1 using runoff plot monitoring on selected cultivated hillslopes. A high slope-channel sediment delivery ratio was observed, meaning that a substantial proportion of eroded sediment was delivered into downstream drainage channels. An obvious temporal trend of decreasing sediment transfer to the river channels in the first-order catchments was identified, which may be driven by change in regional precipitation regime and the implementation of multiple soil conservation and reforestation practices over recent decades.

Spatial distribution, sources and ecological risk assessment of heavy metals in Shenjia River watershed of the Three Gorges Reservoir Area

Surface soil/sediment samples were collected from the Water-Level Fluctuation Zone (WLFZ), cultivated land and forest land at 50 different grid points from Shenjia watershed, the Three Gorges Reservoir area in August 2013. The spatial distribution, sources and ecological risk assessment for Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Nickel (Ni), Lead (Pb) and Zinc (Zn) were analyzed in this study. The results showed all tested metals had similar distribution patterns except Ni and Cr, with areas of high concentrations distributed in the southwest (WLFZ and watershed outlet) of the study area. Ni and Cr, which were highly positively correlated and present in high concentrations, were primarily distributed in the south and middle zones of the study area. Lower concentration areas of all metals were uniformly distributed west of the high-elevation zones and forest land. Factor analysis (FA) and factor analysis-multiple linear regression (FA-MLR) showed that the major sources of Cd were fertilizer and traffic sources, which together accounted for 87% of Cd. As, Zn and Cu levels were primarily supplied by industrial and domestic sources, accounting for 76% of As, 75% of Cu and 67% of Zn. Surface soils/sediments of the study watershed contaminated by Cd represent a high ecological risk, whereas other metals represent low ecological risks. The potential ecological risk index (PERI) analysis indicated that it had a low (wide-range) ecological risk and a moderate (small-range) ecological risk primarily distributed in the outlet of the study watershed. Fertilizers and traffic are the primary sources of Cd pollution, which should be more closely controlled for the purposes of water quality and ecological conservation.

Particle size differentiation explains flow regulation controls on sediment sorting in the water-level fluctuation zone of the Three Gorges Reservoir, China

The Three Gorges Dam has significantly interrupted fluvial continuity and modified the mass transfer regime along river continuums. Flow regulation following regular dam operations drives dramatic hydrological regime shifts, which facilitates sediment dispersal in the water-level fluctuation zone over episodic inundation periods. How flow regulation modulates sediment redistribution, however, remains unclear. In this study, we depict absolute particle size composition of suspended sediment and sink sediment in the water-level fluctuation zone, and these are interpreted in the context of flow regulation controls on sediment sorting. Multiple sampling strategies were applied at different spatial and temporal scales, to overcome limitations of labour and cost input in a large-scale field study and to collect representative samples. The results revealed a longitudinal fining trend and seasonal variability in particle size composition for suspended sediment. Sink sediment collected from the water-level fluctuation zone during a single summer flood event displayed a similar longitudinal fining trend, reflecting preferential settling of coarser fractions in the backwater reaches where flow velocity declines sharply. Surface sediment demonstrated a laterally coarsening trend with increasing elevations along a slope profile. Flooding duration, frequency and timing represent key factors in determining the elevation-dependent variations in the magnitude of sedimentation and its source inputs. Relatively longer flooding duration and frequent intermediate summer floods with high suspended sediment flux are responsible for high sedimentation rates in the lower portions with distal upstream source inputs, while low sedimentation rates in the upper portions are principally associated with water impoundment and sediment produced from local bank erosion. Vertical particle size variability was observed along a sedimentary core profile, which most likely reflects seasonal differences in source supply with contrasting particle size characteristics. We conclude that absolute particle size differentiation explains flow regulation controls on sediment sorting in the water-level fluctuation zone of the Three Gorges Reservoir.

Assessment of sediment yield in a small karst catchment by using 137Cs tracer technique

There is very thin soil layer in karst rocky desertification areas in Southwest China, sediment deposition and sediment yield in the karst area affects the development of vegetation greatly. In the present study, the Cs-137 technique was used to assess the rate of sediment deposition and sediment yield in a small karst catchment. The Cs-137 inventory within the depression varied between 800 m(-2) and 2,200 Bq m(-2), with the mean value of 1,500.1 Bq m(-2). The Cs-137 reference inventory at a nearby reference site was 805.9 Bq m(-2). It could be inferred to that sediment deposition had occurred in the catchment. The mean depth of sediments deposition in the depression was 6 cm and the deposition rate was approximately 0.13 cm yr(-1). The analysis of the topographic characteristics of the catchment revealed that the sediment deposition occurred mainly at the lower part of the small catchment. Although, there was a sinkhole in the depression, little sediment had drained out with runoff through the sinkhole, because the local people built ridges around the sinkholes for storing water. According to this, sediment yield rate in the small catchment was estimated to be approximately 19.25 to 27.5 t km(-2) yr(-1), and the extremely low sediment yield was maybe the main obstacle to vegetation restoration in karst rocky desertification areas.

Tempo-spatial variations of sediment-associated nutrients and contaminants in the Ruxi tributary of the Three Gorges Reservoir, China

Ruxi River is a tributary of the Three Gorges Reservoir. This study examined the temporal and spatial dynamics in particle size characteristics and the associated nutrients and contaminants of the fluvial suspended and deposited sediments along the Ruxi River. Temporal variations in the particle size distribution of the suspended sediment are controlled mainly by differences in sediment source during different seasons. Total organic carbon (TOC), total nitrogen (TN) and total phosphorous (TP) in the < 62 μm fraction of the suspended sediment exhibit considerably higher concentrations in spring, indicating high probability of algal blooms in the backwater areas. Downstream trends in the nutrient contents of < 62 μm deposited sediments imply the greatest potential for eutrophication in the backwater ends, where highest nutrient concentrations were detected. Assessment of metal contamination shows that the sediments deposited in the water-level fluctuation zone were moderately to strongly contaminated by Cadmium (Cd), with a considerably high potential ecological risk. The findings reported have emphasized the impacts of reservoir impoundment on aquatic and/or terrestrial environment in this region. More information on physical, chemical and biological processes of sediment and sediment-associated materials are needed for developing environmentally and ecologically sound policies of water and sediment management.

Understanding the effects of contour hedgerow and terracing hedgerow on soil and water conservation in the remote mountainous regions of Southwest China

BackgroundSoil and water loss is a serious worldwide environmental problem, for example, soil loss from the sloping cultivated lands in the remote mountainous regions of Southwest China due to the abundant precipitation and steep sloping cultivated lands in these areas. Various soil protection techniques have been adopted to prevent soil loess, the local people did not admire the introduced contour hedgerow measurement from abroad; however, they have been actively accepted another traditional one, the terracing hedgerow. The purpose of this work is to understand the differences on soil and water conservation between contour hedgerow and terracing hedgerow. MethodsThe different structure between four treatments of contour hedgerow and terracing hedgerow were analysed. Meanwhile, three heights (0 cm, 10 cm, and 15 cm) of lynchet were set, and the soil erosion module, runoff efficiency, ratio of output and input, economic effects among the different treatments were compared. The soil anti-scourability was calculated using the equation, and analysis of variance (ANOVA) was performed with SPSS 11.5 and Microsoft Excel software. ResultsThe obvious difference was that terracing hedgerow with a certain height of walkway was more convenient for farming activities and effectively reduced soil and water loss. The treatment of terracing hedgerow 2 (H15H) reduced runoff 55.56%±6.25% and reduced erosion modulus 79.26%±3.50% when compared to the sloping cultivated land plots with no lynchet and no hedgerow (CK). The contributions of the independent variables on runoff, soil erosion reduction and soil anti-scouring were in the following order: terracing hedgerow 2 (H15H) > terracing hedgerow 1 (H10H) > contour hedgerow (H0H) > non-hedgerow (CK). The value of ratio of output to input of H15H treatment was 1.52, which was the highest value among different treatments. ConclusionsAlthough the treatment of terracing hedgerow 2 (H15H) needs more labour force, however, results in the higher hedgerow yield and ratio of output to input than other treatments as well as stronger anti-scourability. Thus, the traditional terracing hedgerow with 15 cm lynchet is recommended in the remote mountainous regions of Southwest China, even should be recommended often and used extensively in the similar climatic regions in other countries.