Seasonal variations in soil erosion resistance during concentrated flow for a loess-derived soil under two contrasting tillage practices

Soil erodibilty during concentrated flow (K_c) and critical flow shear stress (τ_cr), both reflecting the soil's resistance to erosion by concentrated runoff, are important input parameters in many physically-based soil erosion models. Field data on the spatial and temporal variability of these parameters is limited but crucial for accurate prediction of soil loss by rill or gully erosion. In this study, the temporal variations in K_c and τ_cr for a winter wheat field on a silt loam soil under three different tillage practices (conventional ploughing, CP; shallow non-inversion tillage, ST; deep non-inversion tillage, DT) in the Belgian Loess Belt were monitored during one growing season. Undisturbed topsoil samples (0.003 m³) were taken every three weeks and subjected to five different flow shear stresses (τ = 4–45 Pa) in a laboratory flume to simulate soil detachment by concentrated flow. To explain the observed variation, relevant soil and environmental parameters were measured at the time of sampling. Results indicated that after two years of conservation tillage, K_c(CP) > K_c(DT) > K_c(ST). K_c values can be up to 10 times smaller for ST compared to CP but differences strongly vary over time, with an increasing difference with decreasing soil moisture content. The beneficial effects of no-tillage are not reflected in τ_cr. K_c values vary from 0.006 to 0.05 sm⁻¹ for CP and from 0.0008 to 0.01 sm⁻¹ for ST over time. Temporal variations in K_c can be mainly explained by variations in soil moisture content but consolidation effects, root growth, residue decomposition and the presence of microbiotic soil crusts as well play a role. τ_cr values increase with increasing soil shear strength but K_c seems more appropriate to represent the temporal variability in soil erosion resistance during concentrated flow. The large intra-seasonal variations in K_c, which are shown to be at least equally important as differences between different soil types reported in literature, demonstrate the importance of incorporating temporal variability in soil erosion resistance when modelling soil erosion by concentrated flow.     

Tillage and crop management effects on soil erosion in central Croatia

Soil erosion continues to be a primary cause for soil degradation and the loss of soil quality throughout the world. Our objectives were to quantify soil erosion (referred to as erosional drift) and to assign erosion risk to six tillage and crop management treatments evaluated from 1995 to 1999 for a 5-year maize (Zea mays L.), soybean (Glycine hyspida L.), winter wheat (Triticum aestivum L.), oil-seed rape (Brassica napus var. oleifera L.), and spring barley (Hordeum vulgare L.) plus double-crop soybean rotation on Stagnic Luvisols in central Croatia. Standard black fallow (tilled, unsown, and without any vegetative cover) Universal Soil Loss Equation (USLE) plots were used to establish the erosion potential associated with the rainfall pattern for each year. Soil loss from the check plots was several times greater than the T value, which is estimated to be 10 t ha⁻¹ per year. During the 2 years when spring seeded maize or soybean were grown (1995 and 1996) erosion risk was extremely high, especially for treatments where tillage and planting (row direction) were up and down the slope. When autumn seeded winter wheat or oil-seed rape were grown (1996/1997 or 1997/1998), soil erosion was insignificant. Also, except when plowing and sowing were up and down slope, erosion loss for the spring barley plus double-crop soybean crops in 1999 was insignificant. With no-tillage, soil erosion from the maize and soybean crops was reduced 40 and 65% compared to plowing up and down slope, even though the planting direction was still up and down the slope. With the exception of maize in 1995, erosion losses were moderate to insignificant when plowing and planting were performed across the slope. We conclude that erosion risk can be used as a reliable indicator of sustainable land management and that using no-tillage or plowing and planting perpendicular to the predominant slope are effective soil conservation practices for this region.     

Cover crops and their erosion-reducing effects during concentrated flow erosion

Cover crops are a very effective erosion control and environmental conservation technique. When cover crops freeze at the beginning of the winter period, the above-ground biomass becomes less effective in protecting the soil from water erosion, but roots can still play an important role in improving soil strength. However, information on root properties of common cover crops growing in temperate climates (e.g. Sinapis alba (white mustard), Phacelia tanacetifoli (phacelia), Lolium perenne (ryegrass), Avena sativa (oats), Secale cereale (rye), Raphanus sativus subsp. oleiferus (fodder radish)) is very scarce. Therefore, root density distribution with soil depth and the erosion-reducing effect of these cover crops during concentrated flow erosion were assessed by conducting root auger measurements and controlled concentrated flow experiments with 0.1 m topsoil samples. The results indicate that root density of the studied cover crops ranges between 1.02 for phacelia and 2.95 kg m^− 3 for ryegrass. Cover crops with thick roots (e.g. white mustard and fodder radish) are less effective than cover crops with fine-branched roots (e.g. ryegrass and rye) in preventing soil losses by concentrated flow erosion. Moreover, after frost, the erosion-reducing potential of phacelia and oats roots decreased. Amoeba diagrams, taking into account both below-ground and above-ground plant characteristics, identified ryegrass, rye, oats and white mustard as the most suitable species for controlling concentrated flow erosion.     

植被叶面积指数与覆盖度定量表征红壤区土壤侵蚀关系的对比研究

选择较合理的植被指标,对提高植被水土保持效益评价精度具有重要理论与现实意义。本研究基于福建省长汀县河田镇12个土壤侵蚀试验小区2007年和2008年2 a的降雨、径流、泥沙数据及各小区的植被叶面积指数(LAI)、植被覆盖度(VFC)资料,利用对比与统计分析方法,研究了2种植被覆盖类型(纯马尾松林、马尾松林草)和不同覆盖度(80%、60%、45%、30%、15%和5%)的红壤水土流失特点,探讨了LAI与VFC定量表征水土流失关系的稳定性和可靠性。结果表明,与裸地小区比较,马尾松纯林覆盖一定程度上降低了土壤侵蚀模数,对径流的减少作用并不明显;而马尾松林草覆盖可显著减少25%左右的径流,土壤侵蚀模数减少也在90%以上,林草结合的植被覆盖结构具有更强的水土保持功能。以LAI表征2种植被覆盖类型的土壤侵蚀模数,均能达到显著水平(p0.05);而以VFC来表征土壤侵蚀模数,仅马尾松林草覆盖类型达到了显著水平。选择LAI表征土壤侵蚀模数来评价植被水土保持效益,将更为稳定和可靠。     

The significance of drilling date and crop cover with reference to soil erosion by water,with implications for mitigating erosion on agricultural land in South East England

Early drilling of autumn‐planted cereals is strongly advised in UK government publications targeted at farmers, in part as a measure to combat soil erosion by water. However, in years when rainfall is heavy in early autumn, this strategy is ineffective. Late drilling of autumn‐planted cereals also increases the risk of erosion, but for a different reason: crop cover develops more slowly in cooler weather, resulting in a longer exposure of nearly bare ground. The crucial factor affecting both strategies is the timing of autumn and early winter rainfall. We discuss a conceptual model based on the notion of a ‘window of opportunity’ for erosion, comprising the relationship between drilling date, date of attainment of a sufficiently protective crop cover and the timing of rainfall; variations are presented for different weather conditions and management choices. Of these three factors, only the date of drilling can be chosen by the farmer. The date of attaining a sufficiently protective crop cover can only be predicted approximately. The timing of rainfall cannot be predicted. Thus, erosion control advice to farmers, which is based on choice of date of drilling to minimize erosion during the ‘window of opportunity’, is both difficult to formulate and likely to be ineffective. Sites at risk of erosion need to have better thought‐out mitigation measures in place, rather than relying on a fortuitous temporal pattern of autumn and winter rainfall to minimize the risk of erosion.     

集中水流内红壤分离速率与团聚体特征及抗剪强度定量关系

为明确红壤结构特征与抗剪强度对坡面土壤分离能力的影响,选取8种典型红壤为研究对象,通过团聚体稳定性分析,饱和抗剪强度测定,和室内模拟冲刷试验,就各参数间定量关系进行了初步探讨。研究结果表明:团聚体稳定性特征参数As集合了红壤团聚体破碎的主要机制,与不同水流剪切力中红壤分离速率有较好的相关性;红壤饱和抗剪强度(σs)与集中水流内临界水流剪切力(τc)呈较好的线性关系。基于WEPP细沟侵蚀模型,将团聚体稳定性特征参数As代替可蚀性因子Kc,饱和抗剪强度(σs)代替临界水流剪切力(τc),得出新的预测方程。结果显示预测方程能准确的预测坡面集中水流内红壤分离速率(R2=0.887 1)。该结果为深入研究红壤侵蚀机理提供了新思路,对完善侵蚀物理过程模型具有重要的意义。     

Effects of freshly incorporated straw residue on rill erosion and hydraulics

Rill hydraulics (and hence, flow detachment) are modified by the presence of incorporated vegetation residue. Typically, water flow in the rill is retarded due to the extra shear stress generated by the residue. The main objective of this study was to develop an approach to predict soil detachment by rill flow in the presence of freshly incorporated vegetation residue that is compatible with our current understanding of rill hydraulics and requires no additional information on rill geometry. Laboratory experiments were carried out to collect a dataset on rill flow detachment on surfaces with incorporated straw that was compatible with existing dataset on bare soils (Giménez and Govers, 2001). Interaction between bed roughness and flow hydraulics in eroding rills. Water Resources Research, Vol. 37 No. 3, 791–799).     

Investigating source areas of eroded sediments transported in concentrated overland flow using rare earth element tracers

Rare earth element oxides (REOs) have excellent potential for use as tracers in erosion studies. Using laboratory and field experiments we aimed to develop and test a simple application method for spreading REOs and to use REOs to determine the source of sediment to concentrated overland flow paths.     

Runoff- and erosion-reducing effects of vegetation on the loess hillslopes of China under concentrated flow

Evaluating the effects of revegetation on runoff and erosion reduction is essential for studying soil and water conservation on the Loess Plateau after implementation of China's Grain for Green Project. However, quantifying the influence of revegetation on the erosion caused by concentrated runoff in extreme rainstorms is still challenging. To evaluate this influence, scouring-erosion experiments were implemented in situ on the vegetated hillslope plots (GR) and bare hillslope plots (CK). The runoff-reducing effects of grass (GRR) averaged 31%, 20% and 8%, and the erosion-reducing effects of grass (GER) averaged 93%, 95% and 93% on the 5°-plots, 10°-plots and 18°-plots, respectively. The ratios of GRR to GER were 0.09–0.33, implying that the ability of vegetation to reduce erosion was greater than its ability to reduce runoff. The GRR and GER obviously decreased as the inflow rate increased, and the GRR decreased as the hillslope gradient increased, but there were no obvious differences in the GER between hillslope gradients. Vegetation could decrease the ability of the concentrated flow to carry and transport sediment and increase the energy consumption of the concentrated flow in response to hydraulic resistance. Vegetation also significantly reduced the degree of rill development. The degree of rill dissection on the GR (0.054–0.087 m² m^?2) was lower than that on the CK (0.061–0.184 m² m^?2). Our findings provide an essential reference for ecological environment and vegetation restoration on loess hillslopes.     

小麦/玉米免耕处理对产量及土壤水分和风蚀的影响

通过3年田间定位试验,研究在小麦、玉米间作条件下,免耕高留茬、免耕秸秆覆盖、免耕秸秆覆盖+播前耙地、传统耕作4种耕作处理对小麦/玉米产量、土壤含水率、土壤风蚀的影响.结果表明:1)免耕处理小麦玉米间作混合产量比传统耕作产量增产2.6％～6.6％,其中玉米产量比传统耕作处理增产8.6％ ～ 13.2％,小麦产量比传统耕作降低5.2％～8.8％;2)免耕处理可增加耕作层土壤蓄水量13.4％～15.9％,降低土壤风蚀量55.6％～100.0％.说明小麦秸秆高茬收割覆盖、玉米秸秆立地过冬的间作免耕技术可在甘肃河西、沿黄地区及国内同类型的小麦玉米间作地区进行示范推广.