土壤理化及力学性质对干热河谷台地边坡沟蚀发育的影响

金沙江干热河谷土地整理台地边坡沟蚀发育严重,水土流失强烈,高投入的土地整理工程效益难以发挥,严重威胁着该区的生态安全和社会经济发展。为查明干热河谷土地整理台地边坡沟蚀发育关键影响因子、防治台地边坡沟蚀发育,该研究采用野外实地测量法和实验室测定的方法,研究了干热河谷土地整理台地边坡的沟蚀发育形态特征,系统分析了土壤理化性质和力学性质对台地边坡沟蚀发育形态特征的影响。结果表明:1)非毛管孔隙度是影响台地边坡沟蚀发育形态特征的主要指标,但非毛管孔隙度对台地边坡沟蚀发育宽度、深度、截面积、密度以及割裂度的影响形式和影响程度有较大差异。2)抗剪强度仅对台地边坡沟道深度和截面积影响显著,而对沟道宽度、密度和割裂度的影响甚微。3)分散率对台地边坡沟道宽度和割裂度的影响较为明显,对沟道深度、截面积和密度无显著影响。4)沟道宽度和密度均主要受非毛管孔隙度控制,割裂度受分散率的影响最大,但沟道深度和沟道截面积受土壤性质的影响相对较弱。     

晋西黄土区蔡家川小流域切沟的空间分布及形态特征

为了掌握切沟的空间分布及发育特点,实现小流域综合治理中切沟治理措施的精准对位配置,该研究以晋西黄土区吉县蔡家川小流域为对象,利用Quickbird影像和数字高程模型（DEM）对流域内370条切沟的形态特征和空间分布进行了提取和解译,并对其中40条切沟进行实地调查验证,采用核密度指标探讨切沟在小流域内的空间分布特征,分析坡度、坡向、高程等地形因子对切沟空间分异特征的影响。结果表明：1）蔡家川小流域内农地、果园的主要分布区（东北部和东南部）是切沟的高聚集区,而在人工林为主的中上游区属于中密集区;2）切沟数量、密度、频度和规模均随坡度增加呈现先增大后减小的趋势,坡度阈值为25°。 5°～25°的坡面切沟广泛发育,数量占研究区的71.5%,密度、频度也远高于其他坡度的坡面;3）切沟密度和频度的坡向分异与流域走向有关。切沟密度北坡最大（3.03 km/km2）,东坡最小（0.86 km/km2）,切沟频度西北坡最大（12.7条/km2）,东坡最小（6.1条/ km2）。南北向的切沟规模明显大于东西向;4）切沟数量、密度、频度随海拔增加呈现先增大后减小的趋势,在海拔为1 000～1 200 m的坡面上切沟数量占77%,海拔为1 100～1 200 m的坡面上切沟密度最大（2.17 km/km2）,海拔为1 000～1 100 m的坡面上切沟频度最高（15.5条/ km2）。随海拔升高切沟逐渐向大型化发展,沟道越发平缓,下切侵蚀逐渐减弱。研究结果可揭示流域切沟发育的重点地形以及在地形上的共性特点,为黄土区小流域切沟治理提供依据,可为科学认知切沟空间分布提供参考。     

Gully Head Retreat in the Sub‐Humid Ethiopian Highlands: The Ene‐Chilala Catchment

In the northern highlands of Ethiopia, gully erosion is severe. Despite many efforts to implement gully prevention measures, controlling gully erosion remains a challenge. The objective is to better understand the regional gully erosion processes and to prevent gully head retreat. The study was conducted in the Ene‐Chilala catchment in the sub‐humid headwaters of the Birr River located southwest of Bahir Dar, Ethiopia. Twelve gully heads were monitored during the 2014 and 2015 rainy monsoon phase. We measured gully head morphology and retreat length, soil shear strength, ground water table levels, and catchment physical characteristics. Two active gully head cuts were treated in 2014 and an additional three head cuts in 2015 by regrading their slope to 45° and covering them with stone riprap. These treatments halted the gully head advance. The untreated gullies were actively eroding due to groundwater at shallow depths. The largest head retreat was 22.5 m, of which about half occurred in August of the first year when the surrounding soil was fully saturated. Lowering both the water table and protecting the gully heads can play a key role in reducing gully expansion and soil loss due to gully erosion in the Ethiopian highlands. Copyright © 2016 John Wiley & Sons, Ltd.     

Morphological Similarity of Channels: From Linear Erosional Features (Rill,Gully) to Alpine Rivers

The geometric characteristics of incised features such as channels, rills, ephemeral gully, gully, represent the erosional transport regime and the fluvial dynamic equilibrium, and thus it is critical for the understanding of the long‐term evolution of natural, agricultural, and anthropogenic landscapes. This paper examines the morphological similarity of channelized erosion in two different environments such as Alpine landscapes and cultivated hillslopes. The first dataset comprises six rivers in the Italian Alps, three in the Carnia region and three in the Dolomites, where erosion is mainly the effect of discharges with high sediment loads or landslides and debris flows. The agricultural areas dataset includes rills, ephemeral gullies, and gullies surveyed in literature. This research highlights that the eroded volume in Alpine rivers is in line with that of agricultural landscapes or badlands around the world. Dolomites rivers of colluvial origin, flowing on soils that are not particularly deep and subject to natural disaggregation, tend to behave similarly to ephemeral gullies. Contrarily, channels that exhibit evident alluvial morphologies and coarse grain sizes are more similar to gully erosion. At different spatial scales, the results demonstrated that length–volume equations calibrated on rills, ephemeral gullies, gullies and badlands, might be feasible also for Alpine channels. The research areas present soils and bedrock lithology that differs from those in literature, thus suggesting that the morphology of linear erosion is independent of the intrinsic soil characteristics. Differences emerged between Dolomites and Carnia rivers: this highlights the importance of taking into account in future analyses other forcing factors (e.g. climate) on land degradation processes. Copyright © 2017 John Wiley & Sons, Ltd.     

土壤侵蚀过程中坡面流水力学特性及侵蚀动力研究评述

土壤侵蚀动力过程是水流和土壤两者相互作用的复杂物理过程,而含沙水流是土壤侵蚀的主要动力,深入理解坡面流水力学特征及侵蚀动力是研究土壤侵蚀动力学规律的基础.本文从坡面流水力学特性及侵蚀动力,包括:流速、水深、流态、阻力规律以及坡面流的切应力、冲刷动力、运动能量等多方面对国内外关于土壤侵蚀动力过程研究进行了系统深入的评述,并探讨了研究中存在的问题,重点指出:坡面流作为三维、非恒定非均匀沿程变量流,流动形态千变万化,坡面状况较为复杂,其均匀流理论远不能真实反应自然界复杂地表状况下的水流水力学特性及其变化规律.开展复杂地表的水流运动过程、水力学参数变化规律及坡面侵蚀水动力过程研究是今后土壤侵蚀水动力学研究的重要方向,这对于深入了解土壤侵蚀水动力过程的内在机制、构建物理侵蚀模型具有重要的研究意义.     

Equation for high-rate gully erosion

An approach to gully erosion is presented in this paper. The approach is based on general equations derived from theoretical considerations. The equations apply to a situation of intense erosion rate, such as at peak discharge during the few critical rainstorms, able to generate or to widen gullies.Equations linking gully widening to gully deepening are derived. They do not depend on the way in which concentrated flow aggressiveness is estimated. The equation expressing gully width/depth relationship was successfully compared with data from the literature.When runoff aggressiveness was estimated through unit stream power and bottom flow shear stress, the width/discharge relationships found were similar to those expected on the basis of previous studies (e.g., Leopold and Maddock [U. S. Geol. Surv. Prof. Pap. 252 (1953) 57 pp.]) even if slope gradient explicitly appears in contrast with empirical evidence. Only threshold conditions for gullies indicate that flow shear stress (for laminar flow conditions) can explain the observed trends. This astonishing result most probably indicates that gully initiation needs more complex contexts to be explained than the one here used (based on a Montgomery and Dietrich [1994. Landscape dissection and drainage area–slope thresholds. In: M.J. Kirkby (ed.), Process Models and Theoretical Geomorphology. Wiley, 221–246] approach).A selection of the proposed equations have been arranged into a research model and an example of the outcome has been given for two situations typical of cropland in southern Tuscany (Italy). The results indicate that the spatial distribution of soil characteristics and of land use influences significantly gully generation and evolution. This further confirms that gully morphological thresholds cannot be explained by simple approaches.     

Hyporheic flow as a potential geomorphic agent in the evolution of channel morphology in a gravel-bed river

Morphologic and hydrologic properties of head-cut gullies formed in meandering bends of a gravel-bed river in northern Japan were examined to investigate their topographic development. head-cut gullies had incised to 2 m below the floodplain surface and had perennial seepage outflows. Because no surface runoff was observed across the meandering necks, we hypothesized that hyporheic flow erosion cutting into the down-river edge of the meandering necks is one factor for the development of head-cut gullies. Several topographic features caused by sapping and piping erosion were observed within the head-cut gullies. A tracer experiment and examination of the water table, water chemistry, and water table responses in wells on the floodplain and within the head-cut gully revealed that significant preferential hyporheic flow occurred between coarse cobble and fine sediment layers of the floodplain materials. During a storm event, water tables around the head-cut gully quickly responded to changes in the water table of the stream channel; this result also suggests that hyporheic flow occurred across the meandering bends. Hyporheic flow around the head-cut gullies had higher hydraulic conductivities and preferential flow paths that may relate to buried beds of paleochannels. An erosion model suggests that it is unlikely that hyporheic flow alone had enough energy to account for all the soil erosion. Thus, a combination of hyporheic flow (sapping erosion) and channel erosion (bank erosion) appeared to contribute to the formation of the head-cut gullies. Hyporheic flow is an important component in the geomorphic evolution of channels and the floodplain in a gravel-bed river.     

黄土坡面片蚀过程动力学机理试验研究

片蚀是坡面侵蚀演变的初始形态,揭示片蚀动力学机理,找出与片蚀关系最密切的水动力学参数对有效防治片蚀具有重要意义。该文采用人工模拟降雨方法,对黄土坡面片蚀过程动力学机理进行了研究,结果表明:1)一次降雨过程的黄土坡面片蚀模数对水动力学参数平均值响应关系的相关系数大小顺序为平均断面单位能量>平均水流功率>平均单位水流功率>平均水流切应力;2)降雨过程中黄土坡面片蚀率对水动力学参数瞬时值响应关系的相关系数大小顺序是断面单位能量>水流功率>单位水流功率>水流切应力;3)水流断面单位能量是与试验条件下黄土坡面片蚀动力学过程关系最密切的水动力学指标,是描述试验条件下片蚀动力学过程最好的动力学参数。试验条件下黄土坡面片蚀动力学过程的发生发展根源于坡面片蚀中水流断面单位能量的动力作用。     

冻融坡面水蚀动力参数动态响应时空演化过程

为揭示冻融作用对坡面土壤水蚀的影响,探究春季解冻期坡面土壤的水蚀动力参数动态响应时空演化过程,采用2个坡度(10°,15°)、4个流量(4.5,6.5,8.5,10.5L/min)和4个起始解冻深度(2,5,10,15cm),进行野外径流冲刷试验,系统地分析冻融坡面水蚀动力参数雷诺数、弗劳德数、流速、水流剪切力、水流功率和单位水流功率在不同起始解冻深度、不同流量和不同坡度条件下的时空演化过程。结果表明:冻融坡面水蚀动力参数雷诺数、流速、水流剪切力和水流功率随流量的增加呈增加趋势;水流剪切力、水流功率与单位水流功率随坡度增加而增大;水流剪切力和水流功率随起始解冻深度的加深而增大;雷诺数、弗劳德数、流速和单位水流功率随起始解冻深度的变化趋势不明显,其起始解冻深度为5cm时,水蚀动力参数随时间变化最为剧烈;建立了冻融坡面水蚀动力参数雷诺数(R~2=0.728)、水流剪切力(R~2=0.644)、水流功率(R~2=0.721)、流速(R~2=0.533)和单位水流功率(R~2=0.553)的幂函数预测方程。     

The onsite cost of gully erosion and cost‐benefit of gully rehabilitation: A case study in Ethiopia

Gully erosion often indicates extreme land degradation. In the highlands of Ethiopia, wide and deep gullies are common features, significantly affecting land used for agriculture. However, little action is being taken to address the problem. Where rehabilitation actions in Ethiopia are taking place, they are rarely studied by financially quantifying their costs and benefits. Therefore, this study developed methods of quantifying gully erosion costs and rehabilitation costs and benefits by considering the soil components of four model rehabilitated gullies in Ethiopia. Data were obtained from a physical survey supported by secondary information from relevant agricultural offices. Quantification was achieved by monetizing gully erosion cost through yield loss and gully rehabilitation calculations according to expenditure, while rehabilitation benefits were obtained by monetizing estimated deposited soil nutrients using fertilizer prices. Cost‐benefit values show that investment in gully rehabilitation can be an economically viable proposition in some instances. Stakeholders involved in gully rehabilitation should continue to invest in appropriate techniques of gully rehabilitation and management to ensure continued benefits from rehabilitated gullies and use of surrounding farmlands. Copyright © 2010 John Wiley & Sons, Ltd.     

Assessing the potential of changing land use for reducing soil erosion and sediment yield of catchments: a case study in the highlands of northern Ethiopia

Reservoir sedimentation is the most serious threat for water harvesting schemes and hydroelectric power dams in Ethiopia. Designing watershed conservation strategies and management is crucial to reduce the rate of sedimentation. Because different landscape types have varying potentials for enhancing erosion processes, site‐ and process‐specific conservation measures are needed to target an appropriate intervention to the most needed locations. In this study, a GIS‐based distributed soil erosion/deposition model was used to simulate the potential of land‐use and cover (LUC) changes and conservation measures for reducing water induced soil erosion and potential sediment yield for two catchments in northern Ethiopia. LUC change and conservation measures targeted gully and stream buffers, protection of steep slopes and protection of areas with soil loss greater than a given threshold. The results show that land management measures targeted at hot‐spot areas of erosion and gully formation could reduce potential annual sediment yield from catchments by approximately 60% compared with the current losses. The study demonstrates the potential of a GIS‐based LUC‐redesign approach as a tool for optimizing land‐use and management strategies to reduce run‐off and erosion rates in the highlands of northern Ethiopia.     

黄土区坡耕地耕作对浅沟径流产沙及其形态发育特征的影响

为研究耕作对浅沟径流产沙及形态发育特征的影响,在野外调查的基础上,设计坡度(15°、20°、25°)、雨强(1.0、1.5、2.0 mm/min)及放水流量(7.53~23.45 L/min)3个处理,采用室内模拟降雨和放水冲刷的方法,测定了不同处理下浅沟径流量、产沙量。结果表明:1)2种浅沟水流均为紊流,耕作使浅沟水流雷诺数和弗劳德数分别减小0.95%~30.77%、2.64%~39.14%,阻力系数和糙率系数分别增加4.01%~58.82%、0.88%~27.87%;2)试验条件下,耕作使浅沟土壤剥蚀率增大9.48%~37.87%,未耕作与耕作浅沟土壤剥蚀率分别与坡度—流量交互作用、雨强—坡度交互作用呈极显著线性关系,土壤剥蚀率与径流剪切力、径流功率及单位径流功率均呈显著的线性关系,未耕作浅沟发生剥蚀的临界剪切力、临界功率及临界单位径流功率分别为17.576 N/m2、5.036 W/(m2·s)、0.0381 m/s,耕作浅沟为10.585 N/m2、3.544 W/(m2·s)、0.0277 m/s;3)耕作使浅沟宽度增加1.98%~31.79%,浅沟面积增大0.84%~32.03%,下切深度降低2.82%~26.67%;4)耕作使浅沟土壤侵蚀量增加0.91%~22.80%,未耕作和耕作浅沟土壤侵蚀量分别占坡面土壤侵蚀总量的44.09%~74.16%和42.44%~56.44%,与雨强—流量交互作用均呈极显著的线性函数关系。结果可为该区浅沟侵蚀预测模型的建立及农业生态环境安全与保护提供科学依据。     

Modelling Soil Detachment of Different Management Practices in the Red Soil Region of China

Soil erosion from cropland is a primary cause of soil degradation in the hilly red soil region of China. Soil characteristics and the resistance of soil to erosion agents can be improved with appropriate management practices. In this study, hydraulic flume experiments were conducted to investigate the effects of five management practices [manure fertilizer (PM), straw mulch cover (PC), peanut–orange intercropping (PO), peanut–radish rotation (PR) and traditional farrow peanut (PF)] on soil detachment. Based on the results, three conservation management practices (PC, PM and PO) increased the resistance of soil to concentrated flow erosion. The rill erodibility of different treatments was ranked as follows: PC (0·001 s m⁻¹) < PM (0·004 s m⁻¹) < PO (0·007 s m⁻¹) < PF (0·01 s m⁻¹) < PR (0·027 s m⁻¹). The rill erodibility was affected by soil organic content, aggregate stability and bulk density. The soil detachment rate was closely correlated with the flow discharge and slope gradient, and power functions for these two factors were developed to evaluate soil detachment rates. Additionally, the shear stress, stream power and unit stream power were compared when estimating the soil detachment rate. The power functions of stream power and shear stress were equivalent, and both are recommended to predict detachment rates. Local soil conservation can benefit from the results of this study with improved predictions of erosion on croplands in the red soil region of China. Copyright © 2016 John Wiley & Sons, Ltd.     

甘肃省黄土高原典型小流域侵蚀沟道特征

[目的]对甘肃省黄土高原典型小流域侵蚀沟道的分级分类进行简要研究,为进一步推动该区水土保持和生态治理工作提供理论基础。[方法]在甘肃省第一次全国水利普查结果的基础上,运用GIS技术,采用地貌几何定量数学模型分级方法,定量化分级沟道,并从地质、地貌、大小以及形状对侵蚀沟道进行分类,由此来推断黄土高原区侵蚀沟道的特征。[结果]甘肃黄土高原典型小流域侵蚀沟道主要以Ⅰ级沟道为主,其中丘陵沟壑区主要以半开析、中度割裂、半主沟型为主;高塬沟壑区主要以半开析、中度割裂、支沟型为主。[结论]丘陵沟壑区第五副区沟壑狭窄陡峭,沟道破碎化程度高,水土流失量高,治理难度大,沟道难以利用。     

东北黑土区不同季节侵蚀沟形态发育比较分析

侵蚀沟形态特征是了解沟蚀发生、发育的关键性因素,探究不同季节侵蚀沟形态参数变化特征对深入理解侵蚀沟的侵蚀过程和侵蚀沟的治理工作具有重要意义。选取黑龙江省海伦市光荣小流域内的3条侵蚀沟为研究对象,探究季节变化下侵蚀沟长度、面积、体积等形态参数值的差异。结果表明:(1)不同季节侵蚀沟横断面形态由沟头到沟尾均呈现出由"V"形向"U"形转变;(2)侵蚀沟在降雨期由沟头前进和沟尾后退所引起的长度增加量大于融雪期由沟头前进所引起的长度增加量;(3)侵蚀沟面积在降雨期的增加量大于融雪期,但不同时期侵蚀沟面积变化位置不同,降雨期侵蚀沟面积增加主要是由长度变化和沟头周围发生大规模崩塌引起的,而融雪期则是由长度变化和沟岸扩张引起侵蚀沟面积增加;(4)侵蚀沟体积在降雨期变化量大于融雪期,这与侵蚀沟长度、面积和径流的剪切能力有关,降雨径流的下切侵蚀能力大于融雪径流。研究结果可为侵蚀沟防治提供理论依据。     

东北典型黑土区侵蚀沟形态及分布特征

东北黑土区是中国最重要的粮食生产基地之一,同时也是中国四大水蚀区之一,其中沟道侵蚀是水蚀发生的主要形式,掌握东北黑土区沟道侵蚀的基本情况,剖析其形态及分布特征,对于分析东北黑土区沟道侵蚀的成因及水土流失治理至关重要。该研究以典型黑土区嫩江县为研究区域,利用航空遥感影像及1∶5万DEM对区域内侵蚀沟进行普查统计,并分析侵蚀沟的沟宽、沟长、纵比降等形态特征,以及海拔、坡度、坡向等地形因素对侵蚀沟分布的影响,为黑土区侵蚀沟的治理提供理论依据。研究结果表明:1)沟道侵蚀主要分布在丘陵区,沟道类型以切沟为主,冲沟影响面积仅次于切沟,研究区以小于500 m的侵蚀沟为主,数量及面积分别占总数的96.67%、61.99%;2)侵蚀沟的沟宽与沟长为正相关,纵比降与沟宽、沟长为负相关;3)坡向对各类型侵蚀沟的分布影响较小,坡度对侵蚀沟的分布有一定影响,细沟和浅沟主要分布在2~6°和6~15°的坡面上,而切沟和冲沟则主要分布在0~2°、2~6°的坡面上,90%以上的细沟和浅沟、80%以上的切沟、冲沟发生在耕地上;4)嫩江县冲沟地形阈值模型拟合表明嫩江县冲沟发生的临界坡度较小,较小的坡度就能对坡面形成冲刷,出现冲沟的发生发育,而模型中主导径流过程的参数数值较小,表明嫩江县冲沟的产生可能受到地下过程的影响。     

西藏中南部侵蚀沟形态无人机航测与传统地面测量的对比分析

[目的]分析无人机和传统地面方法测量侵蚀沟形态的差异,研究无人机影像提取西藏地区侵蚀沟形态的适宜性,以期准确、快速获取西藏地区沟蚀参数,为该地侵蚀沟快速调查与防治提供基础资料。[方法]选取6个研究地点20条侵蚀沟,对实地测量与无人机正射影像提取的沟长、沟宽进行对比分析。[结果]与实地测量相比,影像提取侵蚀沟沟长的平均偏差集中在2%～5%;沟宽的平均偏差集中在0～40%,其中实测沟宽范围在400～1 000 cm时,提取值和实测值的偏离程度最低。在6个研究地点中,影像提取值的偏离程度和侵蚀沟所处位置没有明显的关系。沟缘土质、沟缘线附近的植被及放牧对沟缘的踩踏是影响侵蚀沟形态提取的主要因素。[结论]无人机遥感可为西藏地区沟蚀监测提供便捷、可靠的数据源。     

柔性植被倒伏对地表径流水力特性及侵蚀的影响

[目的] 为探讨柔性植被倒伏后形成的近地表覆盖减少径流和侵蚀量及调节地表径流水力特性的影响。[方法] 以流体力学和水力学基本理论为依据,通过5个覆盖度、5个坡度和3个流量组合的条件下进行室内放水试验,系统研究柔性植被倒伏下的侵蚀规律以及水动力学特征。[结果] (1)侵蚀量分布随坡度增加呈先上升后下降趋势,流量在高坡度下对侵蚀过程的影响更为显著;(2)随着柔性植被覆盖度增加,以50%覆盖度为分界线,侵蚀量基本呈先降低后增加趋势;(3)平均流速v随覆盖度增加而减小,下降趋势随覆盖度增加由急变缓,Fr变化趋势与平均流速相似;阻力系数f与覆盖度呈线性正相关,随坡度升高,阻力系数值逐渐降低;而雷诺数Re对土壤侵蚀的影响比Fr、v和f更敏感。(4)随着植被覆盖度增加,形态剪切力增大(颗粒剪切力减小),形态剪切力在总剪切力的变化中起决定性作用,总剪切力与植被覆盖度呈正相关。[结论] 柔性植被倒伏后形成的近地表覆盖可减少土壤侵蚀,研究可为坡面流植被侵蚀规律奠定一定的理论基础,促进明渠水力学理论在坡面水流方面的扩展。     

土壤结皮坡面流水动力学特征

为了深入探讨土壤结皮对侵蚀的影响机制以及两者之间的关系,以10°坡为例,在变流量(1.0,1.4,2.0,2.4和2.8 L/min)条件下进行室内冲刷试验,研究土壤结皮坡面径流水动力学特征(平均流速、平均径流深度、雷诺数、水流剪切力、水流功率、阻力系数)并分析坡面流水动力学参数与土壤侵蚀量的关系。结果表明,土壤结皮对坡面流水动力学参数影响显著。土壤结皮坡面雷诺数始终小于500,坡面流流态为层流;土壤结皮坡面具有较大坡面流流速,较小径流深度、水流剪切力和水流功率。结皮坡面的土壤侵蚀量明显低于无结皮坡面的土壤侵蚀量。土壤侵蚀量与坡面水动力学参数相关关系显著(相关系数R0.90),土壤侵蚀量与雷诺数呈线性正相关,与水流剪切力、水流功率的对数呈线性正相关,与阻力系数呈线性负相关。因此,在本研究中,单纯从径流冲刷侵蚀的角度土壤结皮的存在有利于减小坡面土壤侵蚀量。由于降雨因素对土壤结皮的侵蚀效应影响较大,将雨滴打击与径流冲刷相结合才能更好地研究土壤结皮对侵蚀的影响机制。