Xinbao Zhang

Sedimentation and associated trace metal enrichment in the riparian zone of the Three Gorges Reservoir, China

Impoundment of the Three Gorges Reservoir has created an artificial riparian zone with a vertical height of 30 m and a total area of 349 km(2), which has been subjected to seasonal inundation and exposure due to regular reservoir impoundment and the occurrence of natural floods. The significant alteration of hydrologic regime has caused numerous environmental changes. The present study investigated the magnitude and spatial pattern of sedimentation and metal enrichment in a typical section of the riparian zone, composed of bench terraces with previous agricultural land uses, and explored their links to the changed hydrologic regime. In particular, we measured the total sediment depths and collected surface riparian sediments and down-profile sectioned riparian soils (at 5 cm intervals) for trace metal determination. Our analysis showed that the annual average sedimentation rates varied from 0.5 to 10 cm·yr(-1) and they decreased significantly with increasing elevation. This lateral distribution was principally attributed to seasonal variations in water levels and suspended sediment concentrations. Enriched concentrations of trace metals were found both in the riparian sediments and soils, but they were generally higher in the riparian sediments than in riparian soils and followed a similar lateral decreasing trend. Metal contamination assessment showed that the riparian sediments were slightly contaminated by Ni, Zn, and Pb, moderately contaminated by Cu, and moderately to strongly contaminated by Cd; while riparian soils were slightly contaminated by As, and moderately contaminated by Cd. Trace metal enrichment in the riparian sediments may be attributed to external input of contaminated sediments produced from upstream anthropogenic sources and chemical adsorption from dissolved fractions during pure sediment mobilization and after sink for a prolonged flooding period due to reservoir impoundment.

A Simplified 137Cs Transport Model for Estimating Erosion Rates in Undisturbed Soil

(137)Cs is an artificial radionuclide with a half-life of 30.12 years which released into the environment as a result of atmospheric testing of thermo-nuclear weapons primarily during the period of 1950s-1970s with the maximum rate of (137)Cs fallout from atmosphere in 1963. (137)Cs fallout is strongly and rapidly adsorbed by fine particles in the surface horizons of the soil, when it falls down on the ground mostly with precipitation. Its subsequent redistribution is associated with movements of the soil or sediment particles. The (137)Cs nuclide tracing technique has been used for assessment of soil losses for both undisturbed and cultivated soils. For undisturbed soils, a simple profile-shape model was developed in 1990 to describe the (137)Cs depth distribution in profile, where the maximum (137)Cs occurs in the surface horizon and it exponentially decreases with depth. The model implied that the total (137)Cs fallout amount deposited on the earth surface in 1963 and the (137)Cs profile shape has not changed with time. The model has been widely used for assessment of soil losses on undisturbed land. However, temporal variations of (137)Cs depth distribution in undisturbed soils after its deposition on the ground due to downward transport processes are not considered in the previous simple profile-shape model. Thus, the soil losses are overestimated by the model. On the base of the erosion assessment model developed by Walling, D.E., He, Q. [1999. Improved models for estimating soil erosion rates from cesium-137 measurements. Journal of Environmental Quality 28, 611-622], we discuss the (137)Cs transport process in the eroded soil profile and make some simplification to the model, develop a method to estimate the soil erosion rate more expediently. To compare the soil erosion rates calculated by the simple profile-shape model and the simple transport model, the soil losses related to different (137)Cs loss proportions of the reference inventory at the Kaixian site of the Three Gorge Region, China are estimated by the two models. The over-estimation of the soil loss by using the previous simple profile-shape model obviously increases with the time period from the sampling year to the year of 1963 and (137)Cs loss proportion of the reference inventory. As to 20-80% of (137)Cs loss proportions of the reference inventory at the Kaixian site in 2004, the annual soil loss depths estimated by the new simplified transport process model are only 57.90-56.24% of the values estimated by the previous model.

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.

Flow regulation manipulates contemporary seasonal sedimentary dynamics in the reservoir fluctuation zone of the Three Gorges Reservoir, China

Since the launch of the Three Gorges Dam on the Yangtze River, a distinctive reservoir fluctuation zone has been created and significantly modified by regular dam operations. Sediment redistribution within this artificial landscape differs substantially from that in natural fluvial riparian zones, due to a specific hydrological regime comprising steps of water impoundment with increasing magnitudes and seasonal water level fluctuation holding a range of sediment fluxes. This study reinterpreted post-dam sedimentary dynamics in the reservoir fluctuation zone by stratigraphy determination of a 345-cm long sediment core, and related it to impact of the hydrological regime. Seasonality in absolute grain-size composition of suspended sediment was applied as a methodological basis for stratigraphic differentiation. Sedimentary laminations with relatively higher proportions of sandy fractions were ascribed to sedimentation during the dry season when proximal subsurface bank erosion dominates source contributions, while stratigraphy with a lower proportion of sandy fractions is possibly contributed by sedimentation during the wet season when distal upstream surface erosion prevails. Chronology determination revealed non-linear and high annual sedimentation rates ranging from 21.7 to 152.1cm/yr. Although channel geomorphology may primarily determine the spatial extent of sedimentation, seasonal sedimentary dynamics was predominantly governed by the frequency, magnitude, and duration of flooding. Summer inundation by natural floods with enhanced sediment loads produced from upstream basins induced higher sedimentation rates than water impoundment during the dry season when distal sediment supply was limited. We thus conclude that flow regulation manipulates contemporary seasonal sedimentary dynamics in the reservoir fluctuation zone, though little impact on total sediment retention rate was detected. Ongoing reductions in flow and sediment supply under human disturbance may have profound implications in affecting sedimentary equilibrium in the reservoir fluctuation zone. The results herein provide insights of how big dams have disrupted the sediment conveyance processes of large scale fluvial systems.

Farmer’s adaptive strategies on land competition between societal outcomes and agroecosystem conservation in the purple-soiled hilly region, southwestern China

The worldwide extension and intensification of farming during the last century has led to ecosystem degradation and caused a series of environmental problems. Conservation of ecosystem services in agricultural regions has been implemented by top-down government actions or initiated by resilience scientists in the developed countries, but little attention was paid in the developing countries, especially in some remote mountainous regions. The present paper presents a case study showing how local farmers obtained both maximal societal outcomes and agroecosystem conservation interests in the absence of distinct boundaries between agricultural and protected ecological areas in the densely populated purple-soiled hilly region of southwestern China. The local community (Yanting County) has developed a mosaic agricultural-forestry-fishery-stock breeding system with spatially targeted land uses, diverse agricultural productions and multiple ecological partnerships. It indicates that the local farmers have hereditarily perceived sound strategies on maximizing sustainable societal outcomes and optimizing tradeoffs among macro-market, state policy, new technological facility and ecological reinforcement.

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.

Principal Denudation Processes and Their Contribution to Fluvial Suspended Sediment Yields in the Upper Yangtze River Basin and Volga River Basin

This paper synthesized the principal land denudation processes and their role in determining riverine suspended sediment yields (SSY) in two typical geographical environments of the Upper Yangtze River Basin in China and the Volga River Basin in Eastern Europe. In the Upper Yangtze River Basin, natural factors including topography, climate, lithology and tectonic activity are responsible for the spatial variation in the magnitude of denudation rates. Human disturbances have contributed to the temporal changes of soil erosion and fluvial SSY during the past decades. On one hand, land use change caused by deforestation and land reclamation has played an important role in the acceleration of sediment production from the central hilly area and lower Jinsha catchment; On the other hand, diverse soil conservation practices (e.g., reforestation, terracing) have contributed to a reduction of soil erosion and sediment production since the late 1980s. It was difficult to explicitly decouple the effect of mitigation measures in the Lower Jinsha River Basin due to the complexity associated with sediment redistribution within river channels (active channel migration and significant sedimentation). The whole basin can be subdivided into seven sub-regions according to the different proportional inputs of principal denudation processes to riverine SSY. In the Volga River Basin, anthropogenic sheet, rill and gully erosion are the predominant denudation processes in the southern region, while channel bank and bed erosion constitutes the main source of riverine suspended sediment flux in the northern part of the basin. Distribution of cultivated lands significantly determined the intensity of denudation processes. Local relief characteristics also considerably influence soil erosion rates and SSY in the southern Volga River Basin. Lithology, soil cover and climate conditions determined the spatial distribution of sheet, rill and gully erosion intensity, but they play a secondary role in SSY spatial variation.

QUANTITATIVE ASSESSMENT OF SEDIMENT REDISTRIBUTION IN THE SICHUAN HILLY BASIN AND THE CENTRAL RUSSIAN UPLAND DURING THE PAST 60 YEARS

Agricultural lands around the globe have been seriously affected by soil erosion and resultant on- and off-site eco-environmental problems. Quantitative assessment of sediment redistribution allows for explicit understanding the effects of natural and anthropogenic agents on catchment soil erosion and sediment delivery. To this end, sediment redistribution at field and catchment scales in two agricultural regions of the Sichuan Hilly Basin in southwestern China and the Central Russian Upland was comprehensively assessed using multiple approaches including 137Cs tracing, soil morphology comparison, empirical-mathematic modeling, sediment budgeting, discharge and sediment monitoring, and sediment dating. Field measurements were undertaken in the zero-order small catchments (with drainage area less than 0,25 km2), and soil erosion rates were found to be 6-7 t ha-1-yr-1. Long-term repeated measurements indicated that both precipitation changes and conservation practices had contributed to the alleviation of soil erosion on hillslopes. However, eroded sediment was transferred from hillslopes to streams through different pathways for both regions. High slope-channel connectivity and substantial proportions of sediment delivery were observed in the Sichuan Hilly Basin. Changes of riverine suspended sediment yield were indicative of soil erosion and sediment delivery on upland catchments. Large quantity of sediment was redeposited on first-order dry-valley bottoms and only 4-12% of the gross sediment load was delivered into adjacent river channels in the Central Russian Upland.

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.