Runoff and sediment losses from rough and smooth soil surfaces in a laboratory experiment

Soil surface roughness may significantly impact runoff and erosion under rainfall. A common perception is that runoff and erosion are decreased as a function of roughness because of surface ponding and increased hydraulic roughness that reduces effective flow shear stress. The objective of this study was to measure the effects of initial surface roughness on runoff and erosion under controlled laboratory conditions. Initially, rough and smooth surfaces were exposed to five simulated rainfall applications at 5% and 20% slopes. In all cases, runoff was delayed for the case of the initially rough surface; however, this effect was temporary. Overall, no statistical differences in either total runoff or erosion were measured on the 20% slope. At 5% slope, runoff was less on the rough surface for the first rainfall application but greater on the final three, probably due to the formation of a depositional seal in that case. This resulted in an overall insignificant difference in runoff for the sum of the five rainfall applications. Erosion was greater on the rougher slope at 5% steepness, probably due to concentration of flow as it moved around the roughness elements on the rougher slope. These results indicate that commonly held perceptions of the impact of soil surface roughness on runoff and erosion may not be entirely correct in all cases.     

The impact of sowing density of small grains on rill and ephemeral gully erosion in concentrated flow zones

Despite the fact that soil erosion by water causes considerable on-site and off-site problems, farmers in Europe are reluctant to adopt prevention and control measures when such measures require additional labour and material inputs. This paper documents the impact of multiple sowing of small grains on concentrated flow erosion rates and grain production for a winter triticale field (X Triticosecale Wittmack ex. A. Camus) in the Belgian loess belt. Multiple sowing refers to drilling more than once in zones of concentrated flow erosion in order to increase the total root mass in this zone. Multiple sowing strongly altered the morphology of erosion channels and reduced soil loss significantly. Statistical analysis confirmed that differences in channel dimensions could be explained by the seedling density. Doubling the root mass in the topsoil by multiple sowing, resulted on average in a reduction of soil loss by 42% for the whole growing season. For the winter period, soil loss reduction, mainly attributed to the triticale roots, amounted even to 53%, showing the tremendous impact of seedling roots on soil erosion by concentrated overland flow in the early stages of vegetation growth. Furthermore, total grain yield in the multiple drilled zones was not significantly smaller compared to the conventionally drilled parts of the field. Grain size of cereals was slightly smaller. The results of this case study indicate that double sowing in concentrated overland flow zones may be a viable soil erosion control technique.     

Effects of 30-year-old Aleppo pine plantations on runoff,soil erosion,and plant diversity in a semi-arid landscape in south eastern Spain

Forest management policies in Mediterranean areas have traditionally encouraged land cover changes, with the establishment of tree cover (Aleppo pine) in natural or degraded ecosystems for soil conservation purposes: to reduce soil erosion and to increase the vegetation structure. In order to evaluate the usefulness of these management policies on reduced erosion in semi-arid landscapes, we compared 5 vegetation cover types (bare soil, dry grassland, shrublands, afforested dry grasslands and afforested thorn shrublands), monitored in 15 hydrological plots (8 × 2 m), in the Ventós catchment (Alicante, SE Spain), over 4 years (1996 to 1999). Each cover type represented a different dominant patch of the vegetation mosaic on the north-facing slopes of this catchment. The results showed that runoff coefficients of vegetated plots were less than 1% of the precipitation volume; whereas runoff in denuded areas was nearly 4%. Soil losses in vegetation plots averaged 0.04 Mg ha^− 1 year^− 1 and increased 40-fold in open-land plots. The evaluation of these forest management policies, in contrast with the natural vegetation communities, suggests that: (1) thorn shrublands and dry grassland communities with vegetation cover could control runoff and sediment yield as effectively as Aleppo pine afforestation in these communities, and (2) afforestation with a pine stratum improved the stand's vertical structure resulting in pluri-stratified communities, but reduced the species richness and plant diversity in the understorey of the plantations.     

Comparison of concentrated-flow detachment equations for low shear stress

Several equations exist to describe the relationship between concentrated-flow detachment and shear stress (τ). However, the advantages and disadvantages of these equations for specific circumstances remain unclear. This study examines the performance of linear and power equations with and without a critical shear stress (τ_c) term for concentrated-flow detachment at low shear stress. Equations were fit to data collected from experiments on five midwestern US soils using flume experimental data at low shear stress levels. Field experimental data were also available for these soils. The linear equation was simple to use and parameter values were easily estimated with linear regression. However, significant lack of fit was found when the linear equation was applied to data collected from low to medium shear stress levels. The value of soil erodibility (K) for a soil varied by a factor of 3 and critical shear stress (τ_c) varied by a factor of 2.5. The linear equation prediction underestimated detachment (D) by 25% at high shear stress and overestimated detachment by 30% at low shear stress. In contrast, the power equations gave more stable erodibility parameters because these equations reduced the systematic nature of the observation residuals found with the linear equation. Correlation between rill detachment D and τ was generally lower with the linear compared to the power equations for conditions tested. For higher shear stresses and longer slopes, the linear equation may be acceptable where field experiments show a linear trend. It is suggested that τ_c only be used when it has a value significantly different from zero.     

宜兴茶园土壤侵蚀及生态影响

茶园是苏南丘陵坡地的一种典型利用方式 ,其上发生的土壤侵蚀及产生的生态影响不容忽视。而要测度土壤侵蚀 ,137Cs示踪法是目前使用较多的一种有用工具 ,但要用此法 ,需找到研究区的137Cs背景值 ,并建立合适的估算模型。本文在对宜兴茶园研究时 ,确定了这里的137Cs背景值为 2 2 0 0Bqm- 2 ,并建立了估算耕作土壤的较合理的模型h =Hc× (Cref-Ct) /(Cref-Cin)。在此基础上衡量了研究对象的土壤侵蚀量 ;并进一步探讨了由此引发的生态影响 ,包括土层减薄、土壤质地改变和养分流失这样的直接影响 ,以及与之相应的一些间接影响 ,如能耗增加、水体富营养化及土地适宜性变化等 ;并且 ,还采用等值侵蚀模数这个指标 ,在与其他地区尤其是黄土和红壤区比较中揭示了苏南地区土壤侵蚀的生态危害的严重性     

缙云山不同林地类型土壤特性及其水源涵养功能

通过分析不同林地类型土壤特性及林地枯落物水文特性,对缙云山4种类型(针阔混交林、常绿阔叶林、楠竹林和灌木林)林地涵养水源功能进行了研究。结果表明,灌木林表土层有机质含量为楠竹林的3.75倍,针阔混交林有机质含量为楠竹林的2.22倍,常绿阔叶林有机质含量为楠竹林的1.53倍。枯落物蓄积量为16.21~32.42t/hm2,枯落物最大持水率次序为针阔混交林>灌木林>常绿阔叶林>楠竹林,最大持水量为灌木林>针阔混交林>常绿阔叶林>楠竹林。土壤非毛管持水量由大到小依次是灌木林(66.2 mm)>针阔混交林(57.52 mm)>常绿阔叶林(47.99 mm)>楠竹林(46.98 mm)。灌木林土壤饱和蓄水量最好,为266.48 mm;针阔混交林较好,为190.4 mm;常绿阔叶林次之,为186.8 mm;楠竹林最差,为1 74.8 mm。灌木林和针阔混交林较楠竹林有更好的涵养水源功能。在今后的森林经营中,应考虑营造灌木林和混交林。     

Gully erosion and environmental change: importance and research needs

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales.Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates.In this respect, various research questions can be identified. The most important ones are:

What is the contribution of gully erosion to overall soil loss and sediment production at various temporal and spatial scales and under different climatic and land use conditions?

What are appropriate measuring techniques for monitoring and experimental studies of the initiation and development of various gully types at various temporal and spatial scales?

Can we identify critical thresholds for the initiation, development and infilling of gullies in different environments in terms of flow hydraulics, rain, topography, soils and land use?

How does gully erosion interact with hydrological processes as well as with other soil degradation processes?

What are appropriate models of gully erosion, capable of predicting (a) erosion rates at various temporal and spatial scales and (b) the impact of gully development on hydrology, sediment yield and landscape evolution?

What are efficient gully prevention and gully control measures? What can be learned from failures and successes of gully erosion control programmes?

These questions need to be answered first if we want to improve our insights into the impacts of environmental change on gully erosion. This paper highlights some of these issues by reviewing recent examples taken from various environments.     

珠江源头区不同地类人工恢复植被树种选择及生态效益研究

选择珠江源头区主要的6个造林树种,通过设置固定观测样地和观测设施,对不同地类人工恢复植被树种和其生态效益进行了4a对比分析。（1）树高生长以云南松为最慢,旱冬瓜最快,旱冬瓜、圆柏、藏柏、柳杉、川滇桤木树高分别是云南松的4.9,2.8,2.5,2.3,1.7倍。胸径生长依次为旱冬瓜〉圆柏〉藏柏〉柳杉〉川滇桤木〉云南松。从长势来看,旱冬瓜、圆柏、藏柏、柳杉、云南松为珠江源头区适宜的造林树种。（2）从6个树种的香浓—威纳指数来看,依次为阴坡退耕地〉荒草地〉退化林地〉阳坡耕地。均匀度指数则依次为阴坡退耕地〉阳坡耕地〉荒草地〉退化林地。（3）各地类的容重均减小,土壤毛管孔隙度比造林恢复前明显提高,表明土壤的结构得到了明显改善,耕地的板结状况也得到显著改善。（4）试验区的平均土壤侵蚀模数降为1087t/（km^2·a）,土壤侵蚀减少了38.48%,水土流失面积由188.27hm^2减少到104.83hm^2,水土流失面积减少44.32%。     

滇中本地和外来植物群落对紫色土理化性质的影响

在同一"气候-母质"条件下,本文研究了滇中飒马场流域4种处于不同演替阶段的本地植被群落和1种外来植被群落对酸性紫色土理化性质的影响。结果表明:随着灌草丛、云南松、针阔混交林到次生常绿阔叶林的本地植物群落演替过程,土壤的容重、pH、水溶性盐阳离子含量明显降低,而土壤粘粒、水解性酸、活性铝、有机碳、全氮含量显著增加,同时表层土壤有机碳和全氮占其剖面总含量的比例有降低的趋势。次生常绿阔叶林土壤0~150 cm土层的有机碳、全氮贮存量比灌草丛土壤的分别高出35 Mg hm-2和1.2 Mg hm-2,而其pH则比灌草丛土壤的低0.33。这反映出酸性紫色土的富铝化过程、生物富集过程和粘化过程随着植被演进而加强。桉树林土壤有机碳、全氮贮量和pH分别为93.04 mg hm-2、2.45 mg hm-2和4.49,与针阔混交林土壤的有机碳、全氮贮量和pH没有显著差异,但是其水解性酸度和活性铝含量均显著高于针阔混交林土壤的,表明长期种植桉树有加速土壤酸化的可能。     

半干旱黄土丘陵区不同植被类型的土壤水分特征及其稳定性

[目的]研究5种不同植被类型土壤水分动态特征,为区域内优化配植模式提供理论依据。[方法]以甘肃省定西市半干旱黄土丘陵区5种典型的植被类型为研究对象,使用土壤水分速测仪对2017,2018年0—200 cm土层土壤含水量进行观测,并对不同植被类型土壤水分特征及其稳定性进行分析。[结果]①不同植被类型土壤水分呈现出:山杏×侧柏混交林地(11.24%)退耕苜蓿草地(10.97%)自然荒地(10.66%)退耕林地(9.99%)柠条林地(9.55%)。②5种植被类型在0—200 cm土层依据有序聚类法可分为动态变化层(0—40 cm),利用层(40—100 cm)和弱利用层(100—200 cm)3个层次。在动态变化层(0—40 cm),土壤水分呈现出:自然荒地(14.23%)最高,柠条林地(11.04%)最低;在利用层(40—100 cm):退耕苜蓿草地(12.16%)最高,柠条林地(9.15%)最低;在弱利用层(100—200 cm):自然荒地(10.81%)最高,退耕林地(8.61%)最低。③不同植被类型土壤水分土层稳定性(变异系数)呈现出:退耕林地(19.9%)山杏×侧柏混交林地(21.3%)自然荒地(21.9%)退耕苜蓿草地(22.6%)柠条林地(23.6%)。④不同植被类型土壤水分时间稳定性(变异系数)呈现出:自然荒地(25%)退耕林地(25.2%)山杏×侧柏混交林地(26.5%)柠条林地(27.1%)退耕苜蓿草地(31.9%)。[结论]柠条林地土壤水分含量最低,在土层和时间上不稳定,山杏×侧柏混交林地较其他植被类型土壤水分含量更高,更具有稳定性。