Gully development in the Moldavian Plateau of Romania

Gully erosion has been recognized as an important environmental threat in the Moldavian Plateau of Eastern Romania. The main objective of this study was to define the process-based gully development by providing quantitative information from long-term field measurements in small catchments.Three main areas of monitoring gullies were explored: aerial photographs of flights in 1960 and 1970, classical leveling and repeated survey through a particular close stakes grid after 1980. The Caesium-137 technique has been used effectively in the areas of deposition of gully sediments to obtain reliable information on dating specific levels of sediments and to provide chronological measures of gully development.Most of the discontinuous gullies exhibit both proper gullying, mainly if not exclusively by gully head advance, and aggradation of the gully basin floor. Results indicated that the mean gully head retreat was 0.92 m year^− 1 and the mean areal gully growth was 17.0 m² year^− 1. Both values indicate a slow erosion rate for this area. The average annual regime of gullying is pulsatory, one that is best described by great fluctuations. Conventional measurements on sedimentation over the period 1987–1997 indicate a higher rate of aggradation in the upper half of the gully floor. Information on the Caesium-137 depth profile was used to provide estimates of a mean sedimentation rate of 4.4 cm year^− 1 over the period 1963–1996 and 2.5 cm year^− 1 after 1986 for the short successive discontinuous gullies. A new classification of the discontinuous gullies based on two criteria, respectively, the field patterns and the rate of aggradation within the gully basin floor was established.For continuous gullies, linear gully head retreat, areal gully growth and eroded material rates were quantified for three periods (1961–1970, 1971–1980 and 1981–1990). Results indicate that gully erosion has decreased since 1960. This gullying decline is due to the rainfall distribution, and the increased influence of soil conservation practices. The mean gully head retreat of 12.5 m year^− 1 between 1961–1990 was accompanied by a mean areal gully growth of 366.8 m² year^− 1 and a mean erosion rate of 4168 t year^− 1. As with discontinuous gullies, continuous gullies showed pulsatory development.The critical period for gullying over 1981–1996 covers 4 months from Mid March to Mid July in an area with mean annual precipitation around 500 mm. Another main finding of this 16-year stationary monitoring was that 57% of the total gullying occurred during the cold season, with the remainder during the warm season. Of the total gully growth, 66% results from only four years (1981, 1988, 1991 and 1996), when a greater amount of precipitation fell.     

Historical evolution of permanent gullies in the Myjava Hill Land, Slovakia

The Myjava Hill Land is part of the area with the highest density of permanent gullies in Slovakia (up to 11 km km⁻²). Gullies are often 10–15 m deep, and occasionally extend >20 m. The pattern and density of gullies have been controlled primarily by the original land use from the pre-collectivization era. Old maps and historical sources indicate two to three periods of gully formation. The central part of the area, settled between the second half of the 16th and the beginning of the 19th centuries, was affected by gully formation in two periods, the first between the end of the 16th century and the 1730s and the second roughly between the 1780s and 1840s. Marginal parts of the study area to the west and south–west, which were settled earlier, may have been affected by disastrous gullying as early as the 14th century. Gullies were formed during periods of extensive forest clearance and expansion of farmland, but the triggering mechanism of gullying was extreme rainfalls during the Little Ice Age. However, periods of gully formation in the Myjava Hill Land were not fully synchronous with the well-known phases of gullying in other central European countries.     

Development of badlands and gullies in the Sneeuberg, Great Karoo, South Africa

The study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the Sneeuberg uplands of the Great Karoo. This is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation. Species diversity has declined and ground cover has been reduced, leading to a positive feedback loop which exacerbates the degradation. Many foot slopes developed in shales, clays and colluvium have extensive, incipient badland development with closely spaced gullying up to 1.5 m deep. In valley-bottom and valley-side depression locations gullies up to 8 m deep have developed, usually cut to bedrock through valley fills of mainly Holocene colluvium. Both badlands and gullies appear to have developed since European settlement and to be part of the same hydrological system with extensive areas of bare ground (badlands) feeding water to incising gullies. Experiments using simulated rainfall throw some light on current processes. Badland areas are active under high-frequency, low-magnitude rainfall events. Major gullies are likely to be the result of occasional, high-magnitude events, but these have not been observed. Overgrazing in the past is the most likely cause of the degradation.     

Evaluating gully erosion using Cs and Pb/Cs ratio in a reservoir catchment

Water erosion in the hilly areas of west China is the main process contributing to the overall sediment of the Yellow River and the Yangtze River. The impact of gully erosion in total sediment output has been mostly neglected. Our objective was to assess the sediment production and sediment sources at both the hillslope and catchment scales in the Yangjuangou reservoir catchment of the Chinese Loess Plateau, northwest China. Distribution patterns in sediment production caused by water erosion on hills and gully slopes under different land use types were assessed using the fallout ¹³⁷Cs technique. The total sediment production from the catchment was estimated by using the sediment record in a reservoir. Sediment sources and dominant water erosion processes were determined by comparing ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in surface soils and sub-surface soils with those of sediment deposits from the reservoir at the outlet of the catchment. Results indicated that landscape location had the most significant impact on sediment production for cultivated hillslopes, followed by the terraced hillslope, and the least for the vegetated hillslope. Sediment production increased in the following order: top>upper>lower>middle for the cultivated hillslope, and top>lower>upper>middle for the terraced hillslope. The mean value of sediment production declined by 49% for the terraced hillslope and by 80% for the vegetated hillslope compared with the cultivated hillslope. Vegetated gully slope reduced the sediment production by 38% compared with the cultivated gully slope. These data demonstrate the effectiveness of terracing and perennial vegetation cover in controlling sediment delivery at a hillslope scale. Averaged ¹³⁷Cs activities and ²¹⁰Pb/¹³⁷Cs ratios in the 0–5 cm surface soil (2.22–4.70 Bq kg⁻¹ and 20.70–22.07, respectively) and in the 5–30 cm subsoil (2.60 Bq kg⁻¹ and 28.57, respectively) on the cultivated hills and gully slopes were close to those of the deposited sediment in the reservoir (3.37 Bq kg⁻¹ and 29.08, respectively). These results suggest that the main sediment sources in the catchment were from the surface soil and subsoil on the cultivated slopes, and that gully erosion is the dominant water erosion process contributing sediment in the study area. Changes in land use types can greatly affect sediment production from gully erosion. An increase in grassland and forestland by 42%, and a corresponding decrease in farmland by 46%, reduced sediment production by 31% in the catchment.     

Recent history of sediment deposition in marl- and sand-based marshes of Belize, Central America

There are two contrasting types of wetlands in Belize: marl- and sand-based marshes. We measured accumulation rates of sediment in six cores from marl- and sand-based marshes of northern Belize and compared biogeochemical characteristics to assess recent wetland history (1850 to present). Sediment depth increments were analysed for bulk density, LOI, nutrients (C, N, P, S, Ca, Mg, K and Na) and snail shell density and species diversity. Cores were dated using ²¹⁰Pb and a constant rate of supply model. Unsupported ²¹⁰Pb inventories of the cores ranged from 6.16 to 8.92 pCi cm⁻². Marl-based marshes showed the maximum peak of ²¹⁰Pb activity from 4 to 10 cm below the sediment surface. ²¹⁰Pb peaks corresponded with the bottom of a marl layer containing chlorophyll a and we suggest that this relationship reflects the high growth and decomposition rates of cyanobacterial mat. Inorganic carbon, Ca, Mg, K, Na, S and Pb contents and accumulation rates were much greater in the marl-based marshes than in the sand-based marshes. Average dry mass accumulation rates in the six marshes ranged from 113 to 572 g m⁻² year⁻¹ over the past 100 years. Average linear sedimentation rates during the last 100 years in the two types were not significantly different (0.93 and 1.08 mm year⁻¹, respectively). Increased sediment accumulation by human activities such as soil washout from adjacent roads was recorded in a sand-based marsh near roads. Sediment cores in the marl-based marshes display changes of marsh vegetation, apparently caused by water level changes. The vegetation change occurred at the end of the 1800s and the beginning of 1900s and is represented by a band of dark peat in otherwise marl-dominated sediments. Overall, the sediment cores show that conditions were relatively undisturbed by human activities in the recent past.     

陇东黄土高塬沟壑区土壤侵蚀的137Cs法研究

该文通过运用铯－１３７法对陇东黄土高塬沟壑区土壤侵蚀进行研究，定量地分析了塬、坡、沟各土地利用类型的侵蚀及产沙。结果表明，通过生物及工程措施治理后的沟谷其侵蚀量都大大减小，然而塬面侵蚀问题却相对突出，塬面侵蚀量不可忽视。     

黄土丘陵区典型沟道侵蚀诱发的CO2通量估算

以黄土丘陵区典型侵蚀沟道为对象,基于沟道剖面有机碳和¹³⁷Cs数据,采用碳库重分布模型估算了典型沟道侵蚀诱发的CO₂通量,并通过检验模型预测效率、解析影响因子,提出了模型校正的思路。结果表明:(1)在长期侵蚀作用下,沟道侵蚀区和沉积区均表现为剧烈的侵蚀效应,侵蚀区侵蚀速率介于30.99~46.44 mm/a,沉积区侵蚀速率介于34.20~37.88 mm/a,沉积区土壤流失速率略小于侵蚀区;(2)碳库重分布模型估算显示,侵蚀区与沉积区均表现为较强烈的碳源效应,侵蚀区CO₂通量介于18.41~28.44 g/(m²·a),沉积区CO₂通量介于22.19~29.25 g/(m²·a);(3)侵蚀部位、土壤容重、有机碳含量、侵蚀量、沟道平均坡度、植被地上部与地下部生物量共同解释了碳库重分布模型预测效率的变异特征(R²=0.68),其中侵蚀部位、侵蚀量、有机碳含量、土壤容重、植被地下部对预测效率有强驱动效应;(4)引入被忽略的植被新输入有机碳库参数,有望校正碳库重分布模型,提升模型预测效率。该研究结果明确了碳库重分布模型在沟道侵蚀区相比沉积区有更高的CO₂通量预测效率,为进一步提高模型的预测精度,可以考虑引入植被输入有机碳库作为校正参数。     

Geomorphological investigation on gully erosion in the Rift Valley and the northern highlands of Ethiopia

Gully erosion phenomena are very common in Ethiopia. They affect large areas with different morphological, pedological and climatic characteristics. The amount of soil loss due to gullying has become a very serious problem in the recent decades as it was associated to remarkable depletion of cultivated land. Field investigations on gully morphology and its genetic processes were carried out in two study areas of Ethiopia, representative of different geo-environmental conditions: the Lakes Region in the Rift portion north of Shashamene and the area surrounding the town of Mekele in Tigray.Two main types of gullies were identified on the basis of their morphological and hydraulic geometry characteristics: (1) discontinuous gullies which generally develop on low gradient slopes (1–5° on average) and the hydraulic radius of which increases from an upstream minimum to a maximum, at approximately their mid length, and decreases again to a relative minimum at their downstream end; and (2) stream gullies, formed by deep erosion processes typically migrating upslope. In order to investigate the main causes originating the different types of gullies, shear stress data were collected in the field from their hydraulic geometry. Hypotheses on the mechanisms responsible for both discontinuous and stream gullies development and for their different characteristics are discussed considering the pattern of shear stress variation in the downstream direction.     

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.     

Rill and gully development during an extreme winter runoff event in Norway

Erosion on agricultural fields in Norway has earlier been reported to be mainly sheet and minor rill erosion. Exceptional rainfall events during January and February (230% of 30 years monthly mean) in 1990 led to extreme runoff and widespread erosion also on areas normally not considered to have a high erosion risk. This study presents the results of a field survey in three counties where rills and ephemeral gullies were measured after this event. Shifting weather conditions with freezing and thawing had given frozen subsoil. Topsoil conditions varied from ice- and snow-covered surface to thawed surface with frozen subsoil. In one of the areas with a high clay content, only the upper topsoil layer was unfrozen during this event. Extended sheet erosion and smaller rills occurred. In the southernmost locations, more of the topsoil profile was unfrozen. Combined with a high silt/sand content, more severe rilling and ephemeral gullies developed. Gullies developed down to the depth of the drainpipes in all locations. This erosion equals soil losses of more than 100 tons ha⁻¹ or 8–9 mm soil lost from the entire field area. Different mechanisms for gully development were registered. Gullies developed as enlarged rills with headward migration and sidewall sloughing. Knickpoints in the slope could also lead to gully development. Concentrated water flow entering the field from farmyards, roads, neighbouring fields and woods could be the starting point for uncontrolled rilling and ephemeral gullying. The combination of frozen subsoil, saturated soil with low strength and intense rainfall led to gully development also on areas with gentle slopes, especially sandy soils. Management practices like crop cover, tillage and lack of surface water control highly influenced the development of gullies.     

Threshold conditions for initiation of valley-side gullies in the Middle Veld of Swaziland

Gullied and ungullied catchments in four areas of deeply weathered saprolite in the Middle Veld of Swaziland can be successfully separated (p<0.01) using discriminant analysis based on area and slope of their basin heads. The discriminant scores for the catchments indicate that those on the Manzini land system differ in their behaviour from those on the Lobamba land system. No differences exist within each land system with respect to land use when the basins are classified as arable, rangeland or mixed. The discriminant functions indicate that gullies on the Lobamba land system have developed by Hortonian overland flow whereas those on the Manzini land system should be attributed to subsurface processes. Since no evidence exists for subsurface activity, it is suggested that gullying on the Manzini land system has been more active and, though initiated by overland flow, has developed to a stage where seepage at the base of the saprolite is now the main process. Erosion control measures should be based on dense ground vegetation to reduce flow velocity on the Lobamba land system and deep-rooted vegetation with high evapotranspiration rates to reduce flow volumes on the Manzini land system.     

Topographical thresholds for ephemeral gully initiation in intensively cultivated areas of the Mediterranean

Ephemeral gullying is an important erosion process in Mediterranean areas suffering from land degradation because of increased drought and human impact. An ephemeral gully is formed when a hydraulic threshold for incision into a resistant soil surface is exceeded. The flow intensity at the soil surface of any landscape position is controlled by topographical parameters, such as local slope (S) and drainage-basin area (A). The objective of this study was to investigate to what extent one could use field measurements of S and A to predict the location of ephemeral gully initiation in intensively cultivated Mediterranean environments. Two relatively homogeneous study areas in Southeast Spain and Northeast Portugal with widespread ephemeral gullying were selected. S and A were measured by standard methods at the initiation point of 50 ephemeral gullies in each study area. The relationship between S and A was statistically investigated. On a double logarithmic scale, the S and A data showed considerable scatter. Orthogonal regression resulted in the threshold relationship S=aA^−b with b=0.133 for Spain, and b=0.226 for Portugal. The analysis also showed that S and A were weakly correlated for the Spanish dataset, and more strongly correlated for the Portuguese dataset. The correlation was considerably strengthened by including additional information on planform curvature and on tillage direction relative to the gully direction. Greater soil stoniness and the presence of a less permeable horizon generally increased the threshold level in both study areas. The threshold relationships resulting from this study were compared with literature data. The indices A_cS and ln(A_c/S) were applied to our study sites but neither relationship individually described the general trend between the data of S and A. Therefore, an alternative index, A_c^bS, with 0.133≤b≤0.226, according to the empirical trend between drainage basin area and local slope, was proposed to describe the topographical threshold conditions for ephemeral gully initiation in similar Mediterranean areas.     

贵州省西南部麦岗水库沉积物的137Cs和210Pb测年与沉积速率研究

对贵州省西南部麦岗水库沉积物柱芯MG4-2,采用210Pb和137Cs法进行了测年与现代沉积速率研究.结果表明,该柱芯中存在这3个明显的137Cs蓄积峰,对应1964,1975,1986年3个计年时标.运用210Pb的恒定补给速率(CRS)模式计算得到的年代,总体上与137Cs法的结果吻合较好,但存在一定的偏差,特别是与1964年的计年时标的偏差较大,这可能是因为沉积物柱芯受到了一些人为扰动所致.两种核素测定的平均沉积速率在数值上有一定的偏差,但都反映了麦岗水库沉积物近几十年以来经历了从慢到快再到慢的沉积过程,初步推测,沉积速率的这种变化与人类活动有一定的关系.两种计年方法结合,相互印证,使计年和沉积速率的研究更为准确.     

东北黑土区沟道侵蚀现状及其防治对策

沟道侵蚀是土地退化最严重的表现形式,东北黑土区是中国除黄土高原外沟道侵蚀最为严重的区域,沟道侵蚀严重威胁东北区域农业生产和生态环境。该研究基于多年的研究积累,结合国家侵蚀沟治理专项调查和国家重点研发计划项目研究成果以及已有文献,系统总结归纳了东北黑土区沟道侵蚀特征、发展演变趋势及其危害;梳理出现有侵蚀沟治理措施的成功经验及失败教训,凝练提出区域侵蚀沟防治目标及4种主要侵蚀沟治理模式,并给出了适用范围及条件;同时提出了新时代东北黑土区侵蚀沟防治对策建议。该研究为区域现代农业发展和生态文明建设决策以及沟道侵蚀治理工程的实施提供了科技支撑。     

甘肃省小陇山林区降雨型浅层滑坡汇水面积-坡度关系及其影响因素

沟头发生侵蚀的地形临界模型可有效预测侵蚀沟的形成条件,浅层滑坡失稳形成的洼地也是沟头形成的方式之一。为探究浅层滑坡临界起动模型的特点,以甘肃省天水市小陇山林区的降雨型浅层滑坡为研究对象,运用汇水面积-坡度关系,构建临界起动模型,与黄土高原典型侵蚀沟(浅沟、切沟)的临界起动模型进行对比分析,并探讨土地利用类型、植被类型和土壤质地对该模型的影响。结果表明:(1)浅层滑坡临界起动模型为S=3.50A_s^-0.34,其侵蚀阈值为3.50,大于黄土高原典型浅沟(0.96)和切沟(1.54)的侵蚀阈值。研究区浅层滑坡一般发生于土层较薄的陡坡地带,其平均坡度(S=1.26)大于浅沟(S=0.35)与切沟(S=0.46),单位汇水面积(A=89.08 m²/m)小于浅沟(A=920.93 m²/m)和切沟(A=1 129.82 m²/m)。(2)汇水面积与坡度平方的乘积(AS²)代表了沟头产生侵蚀的能量指标值。研究区浅层滑坡AS²值在269.1~5 703.2 m²,平均值为1 772.97 m²,黄土高原浅沟AS²值在4.74~892.66 m²,切沟在41~814 m²,启动能量值方面,浅沟<切沟<浅层滑坡。(3)土地利用类型、植被类型和土壤质地通过影响土壤的抗冲力、渗透性和黏粒含量,从而对浅层滑坡的起动难易程度产生影响。在不同的土地利用方式中,农地最易发生侵蚀,其次是林地。油松林附近浅层滑坡的抗侵蚀能力高于日本落叶松林。研究结果为探究浅层滑坡的起动条件提供理论依据。     

基于Google Earth影像的横断山区沟蚀及侵蚀沟类型调查研究

[目的]探讨横断山区侵蚀沟的类型、分布特征及影响因素,为研究沟蚀对横断山区土壤侵蚀及土地退化的影响提供依据。[方法]基于Google Earth高清影像结合GIS的方法,基于可见性的基本原则,对横断山区开展侵蚀沟的抽样调查。共布设调查单元2 242个,每个单元尺寸1km×1km。[结果]共有571个调查单元发现有侵蚀沟分布,占调查单元总数的25.5%,平均沟壑密度为2.20km/km2。沟蚀在海拔低于1 500m的区域发育显著,沟壑密度随坡度增加而增长,与年降雨相关性较弱。草地是沟蚀发育的主要土地利用类型,燥红土的沟蚀分布率及平均沟壑密度显著高于其他土壤类型。通过总结国内外现有的侵蚀沟分类体系及其异同,结合调查结果,判定该次调查的横断山区侵蚀沟类型主要为切沟和冲沟(包括有泥石流事件的冲沟)。[结论]横断山区切沟和冲沟分布广泛,发育强烈,其分布受海拔、坡度、土地利用及土壤类型等因素的影响较为显著。     

The use of environmental radionuclides as tracers in soil erosion and sedimentation investigations: recent advances and future developments

Although much of the recent attention on the environmental problems has focused on climatic change, there is also increasing concern that accelerated soil erosion and associated land degradation represent a major problem for sustainable development and environmental protection. There is an urgent need to obtain reliable quantitative data on the extent and rates of soil erosion worldwide to provide a more comprehensive assessment of the magnitude of the problems and to underpin the selection of effective soil conservation measures. The use of environmental radionuclides, in particular ¹³⁷Cs, affords an effective and valuable means for studying erosion and deposition within the landscape. The key advantage of this approach is that it can provide retrospective information on medium-term (30–40 years) erosion/deposition rates and spatial patterns of soil redistribution, without the need for long-term monitoring programmes. Advantages and limitations of the technique are highlighted. The launching of two closely linked International Atomic Energy Agency (IAEA) research networked projects in 1996 involving some 25 research groups worldwide has made a major contribution to co-ordinating efforts to refine and to standardise the ¹³⁷Cs technique. The efficacy and value of the approach has been demonstrated by investigations in a number of environments. Significant developments that have been made to exploit its application in a wide range of studies are reported in this review paper. Other environmental radionuclides, such as unsupported ²¹⁰Pb and ⁷Be offer considerable potential for use in soil erosion investigations, both individually and complementary to ¹³⁷Cs. The IAEA through research networks and other mechanisms is promoting further development and applications of these radionuclides in soil erosion and sedimentation studies for a sustainable resource use and environmental protection.     

Historical and Geographical Trends in Sediment Chronology from Lakes and Marine Sites Along the Norwegian Coast

Sediment cores collected from 12 lakes and eight marine sites along the Norwegian and Svalbard coast as part of a project investigating polycyclic aromatic hydrocarbons (PAH) in sediments were dated radiometrically using ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am fallout radionuclides. In all lake cores, except on Svalbard, the ¹³⁷Cs activity versus depth profile appears to have been significantly modified by post-depositional migration within the sediment column. The relatively low ¹³⁷Cs inventories suggest that these sites were not significantly impacted by fallout from the 1986 Chernobyl accident. All the marine cores have ¹³⁷Cs inventories that are substantially lower than in lake sediments almost certainly due to leaching of ¹³⁷Cs from the marine sediments due to higher solubility in the seawater. In the marine surface sediments, the unsupported ²¹⁰Pb concentrations are up to an order of magnitude lower than in the corresponding lake sediments reflecting the higher (dry mass) sedimentation rate at the marine sites. Five of the cores including marine sites and lakes have unusual high ²¹⁰Pb flux most likely due to sediment focusing. Most of the irregularities in the ²¹⁰Pb records seem to be due to slump events but some patterns are also due to possibly increased accumulation rates. Three of the marine cores show systematic increase in their sedimentation rate from c.1960 towards the present while only one lake shows the same systematic increase.     

On-site effects of concentrated flow erosion in vineyard fields: some economic implications

Although it is well known that ephemeral gully erosion is a process of a recurrent nature and its contribution to total sediment production is far from negligible, there is a gap in the assessment of changes that the ephemeral gullying cycle produces in the fields' landscape and in the general economic balance of farms (the cost of erosion). The present paper applies a method for assessing the topographic changes produced by concentrated flow erosion, mainly ephemeral gullies, and their filling by farmers. The method is based on the comparison of multi-date detailed topographic data (digital elevation models, DEMs). In the case study, 0.20-m spatial resolution DEMs of March 2000, June 2000 and July 2002 were used to assess the changes in a vineyard field (located in the Penedès region, Catalonia, NE Spain), in which there are hillside ditches (broadbase terraces) that function as sediment traps to avoid major soil loss. The study period was divided into two, according to an extreme rainfall event that occurred on 10 June 2000, in which 214.6 mm fell in 1 day, representing 44% of the annual precipitation. This storm, with a return period in the area of 105 years, displaced 282±4 Mg ha⁻¹ of sediment, 58% of which was due to concentrated surface runoff that caused considerable surface lowering (ephemeral gullies) of up to 0.4–0.5 m deep in some parts of the field. During the second period, about 208±3 Mg ha⁻¹ of soil and sediment deposited in the hillside ditches was used to fill ephemeral gullies. The general sediment/soil balance at the locations where ephemeral gullies recurrently occur is negative (−74±3 Mg ha⁻¹), indicating that recurrent concentrated surface runoff is producing a progressive surface lowering of those zones. The research also assessed the efficiency of the hillside ditches as sediment traps. A total of 113±2 Mg ha⁻¹ was deposited in these structures during the period March 2000–June 2000 (in which the extreme rainfall event occurred), which represents 54% of the material used to fill ephemeral gullies in the period June 2000–July 2002. Finally, the cost of erosion, evaluated as the cost of the operations necessary to redistribute the sediment/soil over the field and to repair the hillside ditches, represented 5% of the income of the farms.     

Slope and rainfall effects on the volume of sediment yield by gully erosion in the Souar lithologic formation (Tunisia)

The Souar lithologic formation in semi-arid Tunisia is undergoing severe gully erosion which is threatening soil and water resources. Soil conservation strategies have focused more on terracing than on gully control techniques, since the contribution of gully sediment yield in the overall soil loss from watersheds is unknown. The paper reports investigations into the sediment yield provided by head-cut as well as sidewall–floor erosion of first order gullies on gentle and steep slope catchments underlined by the Souar lithologic formation. We measured mean field sediment volumes evacuated by different headward reaches of 10 and 9 gullies located on gentle and steep slope catchments, respectively. Two equations between the length of the gully head cutting and the corresponding volume of evacuated sediment were established. The treatment with a Geographic Information System (Arc View) of air photographs of six flights from 1952 to 2000 allowed the calculation of the volume of sediment provided both by head cutting and gully sidewalls–floor erosion through the following up of gully extension in eight catchments during the five periods separating the dates of these flights. Total gully erosion was on average 1.66 m³ ha^− 1 year^− 1 for the gentle slopes and 5.603 m³ ha^− 1 year^− 1 for the steep slopes. Sidewalls–floor contribution in total erosion was on average 81.5% for the gentle slopes and 77.8% for the steep slopes. We found out that the mean annual rainfall resulting from 40 mm daily rainfall threshold explained better the variation of annual head cutting sediment yield for these five periods than any other annual rainfall resulting from lower daily rainfall thresholds. Two equations between these two variables were established both for gentle and steep slope catchments.