The contribution of gully erosion to total sediment production in a small watershed in Southwest China

Determining the contribution of gully erosion to the total sediment yield is important for enhancing decision-making regarding sediment abatement and soil conservation measures. The dry-hot valley region of the Jinsha River is one of the most intensive areas of gully erosion in China. However, the contribution of gully erosion to total sediment production in this region is unclear. Instead of using the source tracing technique commonly employed in other studies, this study provides a method of rapidly reconstructing historical sediment yields based on the properties of sediment that accumulated in an undrained pond at a small gully watershed outlet. We produced a three-dimensional model in geographical information system software to calculate the volume of each layer of accumulated pond deposits from 2006 to 2011. Moreover, we estimated sheet erosion amounts using the Universal Soil Loss Equation. The results showed that sheet erosion accounted for between 14.28 and 23.39% of the total sediment yield in the watershed in the case that the sediment delivery ratio was 100%. Therefore, gully erosion is the major source of total soil loss in this area, and effective soil measures of gully erosion mitigation are crucial for managing the sediment yield.     

河北坝上农田、退耕地和天然草地土壤的可风蚀性

对河北坝上农田、退耕地和天然草地土壤干团聚体稳定性（DASt）、土壤可蚀度（EF）和理化性质进行分析,讨论了这3种土地利用类型土壤风蚀可蚀性的差异及其与土壤理化性质的关系。结果表明：农田和天然草地EF与DASt值差异性显著（P<0.05）,天然草地和退耕地、退耕地和农田差异性不显著,对于EF和DASt,天然草地 > 退耕地> 农田;对于平均粒径,农田≈退耕地>天然草地;对于土壤机械组成黏粒与粉粒含量,天然草地 > 退耕地 > 农田;对于砾石含量,农田 > 退耕地 > 天然草地;对于土壤有机质和全氮含量,天然草地 > 退耕地 > 农田。农田、退耕地、天然草地的DASt、EF系数与砾石含量、平均粒径有着显著相关性,有机质、全氮含量和DASt在天然草地上极显著负相关;EF与有机质、全氮含量在3种地类显著负相关。坝上地区草地和退耕地土壤肥力较高,但表层土壤具有很高的风蚀潜力,因此要限制开垦草地,在必要的地区继续退耕还林还草,提高地表植被的覆盖度,防风固沙,提高土壤肥力,减少土壤风蚀。     

自动连续称重式集沙仪设计及野外应用

集沙仪是定量监测风沙活动的重要工具。设计了一种自动连续称重式集沙仪,可以根据试验需要采集不同高度沙样,每分钟收集一次质量数据,实现高频连续集沙。试验结果显示,自动连续称重式集沙仪在15、30、60 cm高度的集沙量满足随高度增加呈指数减少的规律;在15 cm高度,风速与输沙通量呈指数函数关系(y=6E-05·e0.8417x,R2=0.6042),随集沙高度升高输沙通量与风速相关性减弱。自动连续称重式集沙仪与新型平口式集沙仪所测的输沙通量具有很好的一致性(y=0.8119x+0.2645,R2=0.7481)。自动连续称重式集沙仪虽然在稳固性方面需要改良,但能够很好地捕捉高度在15 cm以下沙物质的输移信息,野外应用效果良好,是一种适合野外高精度风沙活动自动监测的新型测量工具。     

库布齐沙漠110MW光伏基地次生风沙危害的动力学机制

为了解释光伏设施干扰下沙质地表侵蚀堆积的动力机制,在库布齐沙漠110MW光伏电站选取2个断面,利用MetOne 014A/024A型自动风速仪,同步观测了光伏板前后不同位置20、50、100、200cm高度的1min平均风速,分析了光伏板干扰下流场格局变异及其与地表蚀积态势的关系。结果表明:(1)光伏设施强烈干扰流场格局,产生流场分异区:板下集流加速区、板前板后遇阻减速区、板面抬升区和板间恢复区,但光伏基地上风向边缘区域与腹地的流场分布格局存在显著差异;(2)空间流场特征区发育程度所指示的动力分布与地面侵蚀堆积态势基本对应;(3)植被可有效增大地面粗糙度,降低0~50cm近地表过境风速,减缓地表蚀积过程的发生与加剧;(4)光伏板下的集流加速和板前板后的反向涡旋,是塑造地表风蚀沟(坑)及积沙带发育的主要动力。     

Sediment delivery from hillslopes and the Universal Soil Loss Equation: some perceptions and misconceptions

The Universal Soil Loss Equation (USLE) or the revised USLE (RUSLE) are often used together with sediment delivery ratios in order to predict sediment delivery from hillslopes. In using sediment delivery ratios for this purpose, it is assumed that the sediment delivery ratio for a given hillslope does not vary with the amount of erosion occurring in the upslope area. This assumption is false. There is a perception that hillslope erosion is calculated on the basis that hillslopes are, in effect, simply divided into 22·1 m long segments. This perception fails to recognize the fact the inclusion of the 22·1 m length in the calculation has no physical significance but simply produces a value of 1·0 for the slope length factor when slopes have a length equal to that of the unit plot. There is a perception that the slope length factor is inappropriate because not all the dislodged sediment is discharged. This perception fails to recognize that the USLE and the RUSLE actually predict sediment yield from planar surfaces, not the total amount of soil material dislocated and removed some distance by erosion within an area. The application of the USLE/RUSLE to hillslopes also needs to take into account the fact that runoff may not be generated uniformly over that hillslope. This can be achieved by an equation for the slope length factor that takes account of spatial variations in upslope runoff on soil loss from a segment or grid cell. Several alternatives to the USLE event erosivity index have been proposed in order to predict event erosion better than can be achieved using the EI₃₀ index. Most ignore the consequences of changing the event erosivity index on the values for the soil, crop and soil conservation protection factors because there is a misconception that these factors are independent of one another. Copyright © 2007 John Wiley & Sons, Ltd.     

Nereis diversicolor and copper contamination effect on the erosion of cohesive sediments: A flume experiment

The effect of bioturbation on the erodability of natural and manipulated copper spiked sediments (3 μmol Cu g⁻¹ dw) was investigated using sediments collected in the Tagus estuary and Nereis diversicolor (900 ind m⁻²). The input of particulate matter and Cu into the water column as a result of erosion was quantified in an annular flume at 7 shear velocities (1–13 cm s⁻¹). The biogeochemical characteristics of the sediment were analysed in depth down to 8 cm. Cu contamination elicited lower levels of eroded matter and lower shear strength profiles. Eroded matter and sediment shear strength values were higher (up to 1.7 kg m⁻²) in the presence of N. diversicolor, whose effect was less pronounced under contamination. Sediment erodability was not only related to hydrodynamics but was highly affected by the biogeochemical characteristics and contamination of the sediments.     

Developing a sediment budget for a small drainage basin in Australia

A sediment budget was developed for the 1.7 km² Maluna Creek drainage basin located in the Hunter Valley, New South Wales, Australia, for the period 1971-86. the impact of viticulture, which commenced at Maluna in 1971, was studied using erosion plots, with caesium-137 as an indicator of both soil erosion and sedimentation. Two methods were used to estimate vineyard soil losses from caesium-137 measurements. Sediment output from the catchment was measured for three years, and extrapolated from readings taken at a nearby long-term stream flow gauging station for the remaining 13 years. Relative amounts of soil loss from forest (60 per cent basin area), grazing land (30 per cent) and vineyards (10 per cent) were calculated. Soil losses by rain splash detachment were ten times greater from bare/cultivated sufaces than from the forest. Erosion plots of area 2 m² showed no significant differences in soil loss between forest and grassland but, under bare soil, losses were 100 times greater. the ¹³⁷Cs method was employed to calculate net soil loss from all vineyard blocks using both a previously established calibration curve and a proportional model. the latter method gave estimates of soil loss which were 3-9 times greater than by the calibration curve, and indicated that average soil losses from the vineyard were equivalent to 62 t ha^?1 y^?1 (1971-86). It was estimated that the forest contributed 1-8 per cent, the grazing land 1.6 per cent, and the vineyard 96.6 per cent of the total soil loss during that period. Sediment storages within the fluvial system adjacent to the vineyard ws 9460 t for the period, whereas sediment output was equivalent to 215 t km^?1 y^?1. Independent measurements of soil erosion, storage, and output showed that 56 per cent of the eroded sediment remained in the catchment, and 34 per cent was transported out by Maluna Creek. the budget was able to be balanced to within 10 per cent.     

A quantitative study of sediment delivery and stream pollution from different forest road types

The objective of this paper is to quantify, and enable the prediction of, sediment delivery and water pollution impacts from a spectrum of forest roads. Ten 100–200 m long sections of forest road were selected to incorporate a wide range of the key physical site factors that are likely to affect the rate of sediment generation. Each road section was permanently instrumented for 1 year to measure rainfall and runoff continuously. Suspended load, bedload, and traffic were integrated measurements over 2‐ to 3‐week site‐service intervals. Total annual sediment load (normalized for slope) varied about 25‐fold, from 216 mg m^?2 per millimetre of rain for a high‐quality gravel surfaced road with minimal traffic to 5373 mg m^?2 per millimetre of rain for an unsurfaced road on an erodible subsoil with moderate light‐vehicle traffic. For the seven gravel‐surfaced roads in this study, truck traffic (axles/week) explained 97% of the variation in annual sediment delivery (per unit of rainfall) from the road. Equations are proposed that allow annual sediment delivery rates to be estimated when net rainfall, road slope, road area, and truck traffic are known. Roads produce runoff rapidly and were found to deliver sediment for about the same duration as rainfall is falling, in this study varying between 5 and 10% of the time. The patterns of sediment delivery measured from the experimental roads (frequency, duration, and intensity) in this study are similar to levels that have been shown to alter the composition of in‐stream macroinvertebrate communities in small (e.g. <10 l s^?1), clean, mountain streams. However, in larger well‐mixed streams (e.g. >500 l s^?1), dilution is sufficient to prevent concentrations reaching critical levels that are likely to result in biological impacts. Copyright © 2006 John Wiley & Sons, Ltd.     

Post‐wildfire changes in suspended sediment rating curves: Sabino Canyon,Arizona

Wildfire has been shown to increase erosion by several orders of magnitude, but knowledge regarding short‐term variations in post‐fire sediment transport processes has been lacking. We present a detailed analysis of the immediate post‐fire sediment dynamics in a semi‐arid basin in the southwestern USA based on suspended sediment rating curves. During June and July 2003, the Aspen Fire in the Coronado National Forest of southern Arizona burned an area of 343 km². Surface water samples were collected in an affected watershed using an event‐based sampling strategy. Sediment rating parameters were determined for individual storm events during the first 18 months after the fire. The highest sediment concentrations were observed immediately after the fire. Through the two subsequent monsoon seasons there was a progressive change in rating parameters related to the preferential removal of fine to coarse sediment. During the corresponding winter seasons, there was a lower supply of sediment from the hillslopes, resulting in a time‐invariant set of sediment rating parameters. A sediment mass‐balance model corroborated the physical interpretations. The temporal variability in the sediment rating parameters demonstrates the importance of storm‐based sampling in areas with intense monsoon activity to characterize post‐fire sediment transport accurately. In particular, recovery of rating parameters depends on the number of high‐intensity rainstorms. These findings can be used to constrain rapid assessment fire‐response models for planning mitigation activities. Copyright © 2006 John Wiley & Sons, Ltd.     

Vulnerability of water resources,vegetation productivity and soil erosion to climate change in Mediterranean watersheds

Climate change is expected to increase temperatures and lower rainfall in Mediterranean regions; however, there is a great degree of uncertainty as to the amount of change. This limits the prediction capacity of models to quantify impacts on water resources, vegetation productivity and erosion. This work circumvents this problem by analysing the sensitivity of these variables to varying degrees of temperature change (increased by up to 6·4 °C), rainfall (reduced by up to 40%) and atmospheric CO₂ concentrations (increased by up to 100%). The SWAT watershed model was applied to 18 large watersheds in two contrasting regions of Portugal, one humid and one semi‐arid; incremental changes to climate variables were simulated using a stochastic weather generator. The main results indicate that water runoff, particularly subsurface runoff, is highly sensitive to these climate change trends (down by 80%). The biomass growth of most species showed a declining trend (wheat down by 40%), due to the negative impacts of increasing temperatures, dampened by higher CO₂ concentrations. Mediterranean species, however, showed a positive response to milder degrees of climate change. Changes to erosion depended on the interactions between the decline in surface runoff (driving erosion rates downward) and biomass growth (driving erosion rates upward). For the milder rainfall changes, soil erosion showed a significant increasing trend in wheat fields (up to 150% in the humid watersheds), well above the recovery capacity of the soil. Overall, the results indicate a shift of the humid watersheds to acquire semi‐arid characteristics, such as more irregular river flows and increasingly marginal conditions for agricultural production. Copyright © 2007 John Wiley & Sons, Ltd.