Quantify Soil Loss Research Articles

Using 137Cs and 210Pbex measurements and conventional surveys to investigate the relative contributions of interrill/rill and gully erosion to soil loss from a small cultivated catchment in Sicily

In many cultivated areas in semiarid Mediterranean regions, soil erosion is responsible for problems related to both on-site and off-site impacts, including reduced crop productivity, water quality, and degradation of freshwater ecosystems. In some areas of Sicily, where intense short duration rainfall events are common, soil erosion is a very serious problem, especially on land subjected to continuous tillage operations. The rates of soil loss in these areas and their impact differ according to the dominant type of erosion. Several existing studies have focused on the impacts of either linear (gully- or ephemeral gully-erosion) or interrill–rill erosion, but to date the relative magnitude of these two different types of erosion, has rarely been assessed. This paper reports the results of a study aimed at comparing the relative contribution of interrill–rill erosion and gully erosion to soil loss from a small cultivated catchment located in Sicily (Italy). Surveys of ephemeral gullies (EG) in the study catchment carried out at the event scale since 1999 are used to quantify soil loss attributable to EG erosion. 137Cs and 210Pbex measurements are used to quantify the net soil loss from the catchment attributable to interrill–rill (IRR) erosion. The study demonstrates that EG formation occurred 7 years out of 10, with a mean soil loss averaged over a 10-year period equal to 26.5tha−1yr−1. The rates of IRR erosion estimated using 137Cs and 210Pbex measurements provided values of mean annual net soil loss of 38.8tha−1yr−1 and 34.2tha−1yr−1, respectively. The resulting ratios of soil loss attributable to EG to total soil loss (IRR+EG) were 0.41 and 0.44 for the 137Cs and 210Pbex measurements, respectively. The results suggest that the contributions of EG and IRR erosion are of a similar magnitude in semiarid regions of Sicily, although the precise value of the ratio is likely to vary both spatially and temporally in response to catchment morphology, soil erodibility and land use and inter-annual variability of rainfall magnitude and erosivity. The findings are consistent with those of other studies that have attempted to compare the relative efficacy of the two erosion types. The use of 137Cs and 210Pbex measurements in the study area provided important insights into the relative importance of IRR and EG erosion and the same approach could be employed in other locations where both forms of erosion occur and there is a need to quantify their relative importance.

Erosion in the Mediterranean: The Case of Olive Groves in the South of Spain (1752–2000)

Despite the key role that soil and its management have played in the sustainability of farming societies, few historical studies have been carried out on erosion and its effects. In Mediterranean agriculture, erosion from olive cultivation is currently one of the region's main environmental problems. This essay develops a case study from southern Spain to quantify soil loss in olive groves over three centuries, using empirical data and model analysis methods that enable us to distinguish between different levels of erosion throughout history. After identifying the main periods of soil erosion, we explore the social and institutional context of the region to explain the factors that changed management of olive groves and influenced soil losses. The main objective of this essay is to provide a historical perspective on soil erosion that considers social and institutional factors.

Water quality change and habitat potential in riparian ecosystems

Abstract Riparian ecosystems play a vital role in providing ecosystem services that include habitat support and protection of water quality. This study assessed the role of riparian ecosystems along the lateral and longitudinal dimensions of the watershed system. A reach-scale assessment, spatial analysis using GIS, and a dynamic simulation of interactions were used to evaluate riparian dynamics in the Westfield River Watershed of Massachusetts. Riparian and riverine characteristics (slope, soils, and flow regime), disturbance regimes (land use and bank impairment), bird habitat suitability, available niches for wildlife, and vegetation biomass were found to vary along the longitudinal dimension of the watershed. Habitat potential (suitability to sustain a species) declined in general (trend) with riparian distance and a maximum potential was observed at an intermediate level. Dynamic simulation was performed using the STELLA model to quantify interactions between different reaches, land conditions, and land use to quantify soil loss (RUSLE equation) and spatial accumulation. Simulations showed a spatial influence in sediment transfer and loading into the stream. Policy simulations on land use in specific riparian locations showed a significant impact of urbanization on the water quality of the river. Significant reductions in soil loss (3%–48% decrease, depending on the spatial location of the practice) could be achieved through implementation of Best Management Practices (BMPs). Optimal placement of BMPs and conservation efforts in the riparian zone could be used to protect habitat potential and water quality of the watershed.

Desintensificação do uso da terra e sua relação com a erosão do solo

A conservação dos recursos naturais tais como solo e água, deve ser preocupação de governos e instituições. Processos erosivos são responsáveis pela degradação desses recursos, em várias partes do globo. Objetivou-se, com este estudo, avaliar a dinâmica da cobertura da terra em uma região de topografia complexa e empregar a equação universal de perda de solos modificada (RUSLE) para quantificar a perda de solos em decorrência da erosão. O uso da terra no distrito de Vale Vêneto, município de São João do Polêsine na região central do Estado do Rio Grande do Sul, foi avaliado em imagens Landsat nos anos de 1988, 2000 e 2007. A análise espaço temporal do uso da terra indica que as áreas de floresta tiveram um aumento de 16,2% no período de 19 anos. As áreas de agricultura sofreram uma diminuição de 57,5%, no mesmo período. A erosão bruta específica sofreu uma redução de 44%, de 1988 a 2007. Verificou-se que a desintensificação do uso da terra leva a uma configuração mais favorável de conservação do solo e mitigação da erosão. É provável que as mudanças ocorridas no uso da terra estejam relacionadas à não reprodução das famílias rurais que, anteriormente, cultivavam áreas impróprias para a atividade agrícola nesta região.

Análise multivariada e variabilidade espacial na estimativa da erodibilidade de um argissolo vermelho-amarelo

A erodibilidade é um fator de extrema importância na caracterização da perda de solo, representando os processos que regulam a infiltração de água e sua resistência à desagregação e o transporte de partículas. Assim, por meio da análise de dependência espacial dos componentes principais da erodibilidade (fator K), objetivou-se estimar a erodibilidade do solo em uma área de nascentes da microbacia do Córrego do Tijuco, Monte Alto-SP, e analisar a variabilidade espacial das variáveis granulométricas do solo ao longo do relevo. A erodibilidade média da área foi considerada alta, e a análise de agrupamento k-means apontou para uma formação de cinco grupos: no primeiro, os altos teores de areia grossa (AG) e média (AM) condicionaram sua distribuição nas áreas planas; o segundo, caracterizado pelo alto teor de areia fina (AF), distribui-se nos declives mais convexos; o terceiro, com altos teores de silte e areia muito fina (AMF), concentrou-se nos maiores declives e concavidades; o quarto, com maior teor de argila, seguiu as zonas de escoamento de água; e o quinto, com alto teor de matéria orgânica (MO) e areia grossa (AG), distribui-se nas proximidades da zona urbana. A análise de componentes principais (ACP) mostrou quatro componentes com 87,4 % das informações, sendo o primeiro componente principal (CP1) discriminado pelo transporte seletivo de partículas principalmente em zonas pontuais de maior declividade e acúmulo de sedimentos; o segundo (CP2), discriminado pela baixa coesão entre as partículas, mostra acúmulo da areia fina nas áreas de menor cota em toda a área de concentração de água; o terceiro (CP3), discriminado pela maior agregação do solo, concentra-se principalmente nas bases de grandes declives; e o quarto (CP4), discriminado pela areia muito fina, distribui-se ao longo das declividades nas maiores altitudes. Os resultados sugerem o comportamento granulométrico do solo, que se mostra suscetível ao processo erosivo devido às condições texturais superficiais e à movimentação do relevo.

The suitability of using leaf area index to quantify soil loss under vegetation cover

Soil erosion by water under forest cover is a serious problem in southern China. A comparative study was carried out on the use of leaf area index (LAI) and vegetation fractional coverage (VFC) in quantifying soil loss under vegetation cover. Five types of vegetation with varied LAI and VFC under field conditions were exposed to two rainfall rates (40 mm h−1 and 54 mm h−1) using a portable rainfall simulator. Runoff rate, sediment concentration and soil loss rate were measured at relatively runoff stable state. Significant negative exponential relationship (p < 0.05, R2 = 0.83) and linear relationship (p < 0.05, R2 = 0.84) were obtained between LAI and sediment concentration, while no significant relationship existed between VFC and sediment concentration. The mechanism by which vegetation canopy prevents soil loss was by reducing rainfall kinetic energy and sediment concentration. LAI could better quantify such a role than VFC. However, neither LAI nor VFC could explain runoff rate or soil loss rate. Caution must be taken when using LAI to quantify the role of certain vegetation in soil and water conservation.

Uso da terra e perda de solo na Bacia Hidrográfica do Rio Colônia, Bahia

Mudanças no uso da terra muitas vezes potencializam a erosão hídrica acarretando perda de água, solo, nutriente e matéria orgânica dos sistemas agrícolas, razão por que se estimou a perda de solo na bacia hidrográfica do rio Colônia, na Bahia, nos últimos vinte e sete anos, utilizando-se o software SWAT (Soil and Water Assessment Tool). Para tal, procedeu-se à digitalização de mapas temáticos, interpretação de fotografias aéreas de 1975; classificação supervisionada de imagens de satélites de 2002 e produção de mapas de uso da terra. O SWAT foi utilizado na obtenção de mapas temáticos digitais por sub-bacia hidrográfica do rio Colônia, quantificação das perdas de solo em cada sub-bacia e nas formas de usos obtidos por conceito teórico, simulando as inclusões de áreas de preservação permanente, bem como mata em toda a superfície das sub-bacias. Estima-se que, entre 1975 e 2002, a média de perda de solo na bacia hidrográfica do rio Colônia foi de 47 t ha-1 ano-1 e em 2002 a estimativa de perda de solo foi de 46,64 t ha-1 ano-1. Na simulação de um cenário teórico de área de preservação permanente (APP) e mata, ocorreu diminuição da média da perda de solo em toda a bacia hidrográfica do rio Colônia de, respectivamente, 9,09 t ha-1 ano-1 e 20,91 t ha-1 ano-1.

Estimation of soil erosion on cultivated fields on the hilly Meghalaya Plateau, North-East India

The estimation of soil erosion rates in complex subtropical agricultural systems of hilly environment is difficult and most of the traditional methods have serious limitations. The 137Cs technique allows to obtain relatively quickly retrospective medium term soil erosion results. The objective of this study was using 137Cs approach to quantify soil loss under agricultural system which develops under growing human pressure on the hilly terrain of the Meghalaya Plateau. The measured values of caesium inventory for all sampling points are between 2% and 63% of the reference value of caesium inventory. The estimated annual soil loss for sampling points located on the slope are between 29 and 79 Mg·ha−1·yr−1 in the case of an improved mass balance model. It means that soil erosion in this manually tilled agricultural area cannot be neglected.

Scaling effect for the quantification of soil loss using GIS spatial analysis

Accurate estimation of soil loss/deposition forced by rainfall events plays a major role in water resources management, which directly affects the quality of agricultural land and water storage capacity in reservoirs. In this paper, the soil loss model, Geographic Information System (GIS) based Universal Soil Loss Equation (USLE) was used to quantify soil loss in a small basin located in the southern part of Korea. The surface characteristics, such as soil texture, elevation and vegetation type, are needed to run the USLE model. Geospatial data has been successfully used to derive suitable model factors for this purpose. However, it is difficult to select the grid size of elements for the best fit, which is often decided in a subjective and intuitive way. A GIS spatial analysis was performed to investigate the scaling effect to estimate the soil loss in the USLE model using remotely sensed geospatial data. The results showed that the slope length factor (L) and slope steepness factor (S) were sensitive to the grid size; the optimal resolution for quantifying soil loss in the USLE model for the study site was 125 m. This approach presents a method for the selection of a suitable scale for estimating soil loss using remotely sensed geospatial data, which eventually improves the prediction of soil loss on a basin scale.

Distribuição de 137Cs em três solos representativos do estado de pernambuco

O 137Cs tem sido usado para quantificar perdas ou deposição de solo, com base em valores das concentrações em áreas de referência que tiveram pouca perturbação nas últimas décadas. Determinações em áreas assim ainda não foram feitas na região Nordeste do Brasil. Em Pernambuco, foram selecionadas três áreas com topografia plana, vegetação nativa e solos não perturbados, em Goiana, Sertânia e Araripina. Em cada área, foram marcados quatro pontos, distantes 20 a 50 m um do outro, e em cada um retiradas amostras de solo, em camadas consecutivas de 3 cm de espessura, até 30 cm. As atividades de 137Cs foram determinadas usando espectrometria gama com detector de germânio hiperpuro (HPGe). Os estoques médios de 137Cs (Bq m-2) foram de 71,6 ± 6,3 em Goiana, 64,0 ± 13,8 em Araripina e 95,5 ± 9,8 em Sertânia. Valores nessa faixa têm sido relatados na Paraíba e na Bahia e, mais altos, na região Sudeste, confirmando a diminuição dos estoques em áreas mais próximas do Equador. As maiores atividades ocorreram nas camadas superficiais, variando de 0,5 a 1,6 Bq kg-1, e decresceram linearmente com a profundidade, até os limites de detecção: 18 cm em Araripina, 15 cm em Goiana e 9 cm em Sertânia. Em Goiana, acima dos horizontes inorgânicos havia um horizonte orgânico, com a mais alta atividade (2,06 Bq kg-1). Atividades e estoque altos e menor aprofundamento em Sertânia poderiam ser explicados pela predominância de argilominerais 2:1, em contraposição à de argilominerais 1:1 nos outros locais. Houve correlações positivas entre a atividade de 137Cs e os valores de pH em água e KCl e correlação negativa com as concentrações de Al3+. Confirma-se que, em áreas de referência, a maior atividade do 137Cs está na camada superior, com decréscimos regulares ao longo do perfil do solo.

Approaches to modelling land erodibility by wind

Land susceptibility to wind erosion is governed by complex multiscale interactions between soil erodibility and non-erodible roughness elements populating the land surface. Numerous wind erosion modelling systems have been developed to quantify soil loss and dust emissions at the field, regional and global scales. All of these models require some component that defines the susceptibility of the land surface to erosion, ie, land erodibility. The approaches taken to characterizing land erodibility have advanced through time, following developments in empirical and process-based research into erosion mechanics, and the growing availability of moderate to high-resolution spatial data that can be used as model inputs. Most importantly, the performance of individual models is highly dependent on the means by which soil erodibility and surface roughness effects are represented in their land erodibility characterizations. This paper presents a systematic review of a selection of wind erosion models developed over the last 50 years. The review evaluates how land erodibility has been modelled at different spatial and temporal scales, and in doing this the paper identifies concepts behind parameterizations of land erodibility, trends in model development, and recent progress in the representation of soil, vegetation and land management effects on the susceptibility of landscapes to wind erosion. The paper provides a synthesis of the capabilities of the models in assessing dynamic patterns of land erodibility change, and concludes by identifying key areas that require research attention to enhance our capacity to achieve this task.

A comparison of WEPP and SWAT for modeling soil erosion of the Zhangjiachong Watershed in the Three Gorges Reservoir Area

Soil and water conservation is important for the Three Gorges Reservoir Area in China, and quantification of soil loss is a significant issue. In this study, two widely used models – the Water Erosion Prediction Project (WEPP) and the Soil and Water Assessment Tool (SWAT) – were applied to simulate runoff and sediment yield for the Zhangjiachong Watershed in the Three Gorges Reservoir Area. The models were run and the simulated runoff and sediment yield values were compared with the measured runoff and sediment yield values. In the calibration period, the model efficiency ( E NS ) values for the WEPP and SWAT were 0.864 and 0.711 for runoff, and 0.847 and 0.678 for sediment yield, respectively. In the validation period, the E NS values for WEPP and SWAT were 0.835 and 0.690 for runoff, and 0.828 and 0.818 for sediment yield, respectively. The results of E NS and the other criteria indicate that the results of both models were acceptable. WEPP simulations were better than SWAT in most cases, and could be used with a reasonable confidence for soil loss quantification in the Zhangjiachong Watershed.

A quantitative model for predicting gully erosion risk in karstified Mediterranean environments: Lebanon case study

S oil erosion is a major land degradation problem in Mediterranean environments leading to great annual loss of natural resources. This has resulted in the development and/or testing of numerous models designed for quantifying soil loss or pinpointing to areas that are suffering from soil erosion. The advent of geographic information systems (GIS) has promoted an explosive growth in the number and variety of GIS-based model applications at catchment and larger scales during the past decades. Though a lot has been done and achieved in erosion research, there is no emphasis on predicting erosion hazard on the peculiar and attractive karst landscapes commonly distributed in the Mediterranean region (for example, 70% of the total area of Lebanon). These environments are particularly fragile ecosystems for several reasons: (1) the predominance of thin soils with low fertility, which readily succumbs to desertification when soil erosion takes place under improper land management; and (2) nonenvironmentally sensitive human activities such as deforestation, burning, and overgrazing in many karst areas leave only barren soils subjected to the activity of intense rainfall. We have used GIS to conceptualize a regional quantitative empirical model specified to predict erosion in dynamic karst landscapes at a scale of approximately 1:100,000.

Characteristics and factors controlling the development of ephemeral gullies in cultivated catchments of black soil region, Northeast China

Ephemeral gully erosion is an important process in the black soil region, Northeast China and can be responsible for severe damage to agricultural lands. However, little research on gully formation in this area has been published. The study described in this paper attempted to quantify soil losses, the spatial distribution and morphology of the gullies, and the factors that control their development. Ephemeral gullies were measured in spring and summer of 2005 in two small catchments. The critical periods for ephemeral gully formation were late spring and summer in the study area. Mean soil losses due to ephemeral gully erosion were 0.40 and 0.43 kg m −2 year −1 for only croplands despite low slope gradients, and this loss is above the tolerable erosion rates of 0.20 kg m −2 year −1. The erosion rates were greater in spring because the topsoil thawed before deeper layers, reducing infiltration into the soil, and the bare vegetation cover provided no barriers to surface flow. In contrast, summer erosion occurred primarily in response to intense rain events. Development of the gullies was promoted by freeze-thaw cycles in spring and was affected by the type of agricultural operations and crops in summer. A linear regression model for the prediction of ephemeral gully length at the catchment level was established using field data, and although it did not successfully predict the length of individual gullies, it explained 55% of the variation in ephemeral gully length.

Land Degradation Risk Assessment for Tuzla Creek Basin (Biga Peninsula) Using a GIS- Based RUSLE Model

The objectives of this study are to identify land degradation and erosion risk by using Geographical Information System (GIS) and Remote Sensing (RS) techniques, and to investigate impacts of land-use and land-cover (LULC) change on erosion risk in the Tuzla Creek Basin in the Biga Peninsula which is located NW part of Western Anatolia. In this paper, the soil loss model (Revised Universal Soil Loss Equation-RUSLE) was used to quantify soil loss in the basin. The dynamic landcover parameter in the model is carried out by using remotely sensed data from Landsat TM 1987 and Landsat ETM+ 2000 multi-spectral sensor systems. The erosion model predicts an increase in the amount of severe soil loss risk categories in the study area from 1987 to 2000, as a result of land cover changes. An increasing trend in land degradation has been observed along the Tuzla Creek Basin. If present landuse and land management practices continue, the land degradation will probably surpass critical level. This study shows that GIS and RS techniques are important for soil erosion risk studies.

Holocene alluvial sediment storage in a small river catchment in the loess area of central Belgium

Soil erosion and sediment deposition widely affect landscape development, particularly in erosion-prone areas with loess-derived soils. Nevertheless, until now, few attempts were made to quantify soil losses and sediment storage over long (centennial or millennial) timescales. In this study, the Holocene alluvial sediment storage in a small river catchment (52 km 2) of the Belgian loess belt is estimated, and a preliminary sediment budget for the catchment is presented. In the valley of the Nethen River (c. 13 km long), a detailed survey of the alluvial sediment archive was conducted. Hand augerings and percussion drillings were made along cross-valley transects at 12 locations in the catchment. AMS 14C dating of peat samples provided a temporal framework for the interpretation of the cores. Results show that the thickness of Holocene sediment deposits in the Nethen valley is 4 to 6 m, which corresponds to a total clastic sediment mass of ∼14 × 10 6 t stored in the valley bottom. Three alluvial units could be distinguished and associated with deposition phases from 9600 to 2900 B.C., 2900 B.C. to A.D. 1000 and A.D. 1000 to present. In contrast to the older sediments (units 1 and 2), deposits from the last 1000 year (unit 3) contain little organic matter. They are seldom intercalated with peat layers, and devoid of tufa. Unit 3 reaches a thickness of c. 2 m, thereby representing 50% of the Holocene sediment mass stored in the alluvial plain. The mean sedimentation rate in the alluvial plain for this last phase is ∼26 t ha − 1 a − 1 , which is about tenfold larger than the sedimentation rates calculated for the older Holocene sediment units. Sediment supply towards the alluvial plain has therefore increased tremendously since Medieval times. These results are in contrast to dating results obtained for colluvial sediments in a nearby dry valley within the catchment of the Nethen, where soil erosion and sediment deposition started in the early Iron Age and was already substantial during the Roman Age. This means that there is a time lag of about one millennium between the onset of high sedimentation rates in the upstream area and high deposition rates in the alluvial plain. This is probably caused by a change in coupling (sediment connectivity) between the plateau, slopes, and rivers. As soil erosion proceeds, first the dry zero-order valleys in the catchment act as sediment traps, and only after these are filled sediment reaches the floodplains. The preliminary sediment budget for the Nethen catchment illustrates that 50% of the sediment that was eroded during the Holocene was stored in colluvial deposits, which are mainly located on footslopes and in dry valley bottoms. Another 29% of the sediment mass is stored in the alluvial plain.

Assessment of gully erosion in eastern Ethiopia using photogrammetric techniques

Eastern Ethiopia is severely affected by soil erosion as it is revealed by the commonly occurring deep and wide gullies. These gullies are dynamically expanding into agricultural lands at an alarming rate. They are also the chief sources of sediment causing siltation of lakes that are valuable sources of both drinking and irrigation water in the Alemaya and Harar areas. A quantitative assessment of gully dynamics and the associated soil loss and a prognosis of areas susceptible to further gullying were deemed crucial. Aerial photographs from two different dates (1966 and 1996) with approximate scales of 1:45 000 were used to study the development and dynamics of gullies as indicated by volume changes between the two dates. They were digitized at a pixel size of 20 μm. Digital elevation models (DEM) with grid widths of 20 and 2 m were derived from stereo image pairs separately. Gully breaklines and borderlines were measured for the identified gully systems. Gully profiles were positioned at regular intervals of 20 m along the gully length, giving rise to gully sections that are portions of a gully between neighbouring profiles. Gully volume was computed for every gully section. Soil loss was estimated from the computed gully volumes using soil bulk density. It showed that soil loss per unit of gully surface is very high (1.7 tons/m 2 of gully surface over the 30 years time period) indicating the severity of gully erosion in the subcatchment. There was significant agreement between breakline measured and field measured gully depth and width for selected points. This is an indication of the potential of the method for reliable gully volume estimation and hence quantification of soil loss due to gully erosion. The possibility of predicting hill-slope areas susceptible to further gullying based on the topographic attributes such as specific contributing area and local slope from an interpolated detailed DEM with 2-m resolution was tested. It was found that wetness index alone or a combination of the wetness index and the compound topographic index at the threshold values established in literature would give better results as viewed from better connectivity of flow lines and higher conformity of the predicted susceptible areas to natural gully headcut and expansion in a landscape (reality).

Soil loss predictions with three erosion simulation models

Quantification of soil loss is one of the greatest challenges in natural resources and environmental planning. Computer simulation models are becoming increasingly popular in predicting soil loss for various land use and management practices. In this study, three soil erosion prediction models — the Water Erosion Prediction Project (WEPP), the Erosion Productivity Impact Calculator (EPIC), and the Areal Nonpoint Source Watershed Environment Response Simulation (ANSWERS) were used for simulating soil loss and testing the capability of the models in predicting soil losses for three different tillage systems (ridge-till, chisel-plow, and no-till). For each model, the most sensitive model parameters were calibrated using measured soil erosion data. After calibration, models were run and predicted soil loss values were compared with the measured soil loss values. The measured soil erosion data were collected from an erosion experiment field of Kansas State University at Ottawa (Kansas), USA. Field experiments were conducted from 1995 to 1997 on small plots to measure runoff and soil losses under all three tillage systems. All three models were evaluated on the basis of individual event, total yearly, and mean event-based soil loss predictions. Results showed that all the three models performed reasonably well and the predicted soil looses were within the range of measured values. For ridge-till and chisel-plow systems, WEPP and ANSWERS gave better predictions than those by EPIC model. For no-till system, WEPP and EPIC predictions were better than those by ANSWERS. The overall results indicate that WEPP predictions were better than those by the other two models in most of the cases, and it can be used with reasonable degree of confidence for soil loss quantification for all the three tillage systems.

Assessment of tillage erosion rates on steep slopes in northern Thailand

The switch from shifting cultivation to more permanent highland cropping systems in northern Thailand. led to an increase in soil tillage intensity. In order to quantify soil losses by tillage erosion, a tillage experiment was set up and an on-farm survey was conducted. Soil fluxes due to manual tillage on five slopes (32–82%) were measured by monitoring tracers, by measuring tillage step characteristics and by collecting soil material in a trench. The trench method yielded soil flux. values that were significantly smaller than those obtained by the tracer or the step method. Soil fluxes resulting from one manual tillage pass ranged between 39 and 87 kg/m on the tested slopes. On slopes up to 60%, there were no significant differences in soil fluxes. However, on slopes steeper than 70%, soil fluxes increased significantly because the angle of repose for soil clods was exceeded. The soil fluxes are used to construct a nomogram for estimating soil loss rates resulting from manual tillage erosion as a function of slope and plot length. Rates on a typical upland field (slope 30%–50%, slope length 30–50 m) range from 8 to 18 t/ha · tillage pass, so tillage erosion is a significant contributor to the total soil loss. It dominates on short fields and fields with buffer-strips, whereas water erosion is the more important form of soil loss on middle size and long fields. Increasing land pressure will result in increasing tillage erosion rates, and these need to be considered when assessing soil degradation rates or when studying hillslope evolution.

Processus de dégradation des terres et désertification dans les pays d'El Aioun-Tanecherfi, Maroc oriental

In this work, the question is to know if this region heads discreetly to desertification. Diachronic analysis of erosion forms and direct quantification of soil loss were caryed out. Those results show that in the steppes, the anthropic action can attenuate certain linear forms of erosion by triggering off and generalization of areolar processes. This substitution of process led to selective erosion, putting an end to soil fertility. According to soil occupation, the rains and erosive periodes have been determinated, so as to contribute to choose the actions to undertake for reduce the desertification risk.