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Soil crusts and surface roughness are properties which are highly dynamic in both space and time that change in response to biotic processes, meteorological conditions and farming operations. These factors, however, are difficult to quantify and are usually described using simplified expert-based classes. This hampers a clear identification of the controlling factors and their relation to soil erosion and sediment generation processes. The availability of new small portable multispectral cameras offers the potential to study soil surface dynamics at a high spatial and temporal resolution. The objective of this study was to analyse the relationship between soil crusting, represented by cumulative rainfall kinetic energy, and soil surface reflectance, as derived from vis-NIR multispectral imaging. We designed a series of rainfall-soil surface experiments to disentangle the effects of soil crusting on spectral reflectance factors from those related to surface micro-scale roughness. Partial least squared regression (PLSR) models were developed to predict both kinetic energy and roughness from multispectral images. We evaluated different roughness removal methods which were based on the transformation of reflectance through standard normal variate (SNV) and roughness thresholding using high resolution digital elevation models. Furthermore, we assigned the light scattering effect related to roughness in the multispectral spatial domain by calculating the inter-quantile range of the reflectance values in a kernel. Our experiments and workflow demonstrate that it is possible to model crust development, using rainfall kinetic energy as a proxy, from vis-NIR based multispectral imaging.

International audience; Tropical montane areas of Southeast Asia are exposed to high-intensity rainfall during the monsoon period. This is particularly problematic in areas where soils on steep slopes are cultivated as it can lead to heavy runoff, high soil erosion, and water pollution. The objective of this paper is to analyse the effect of the impact of raindrops on the dynamics of runoff on such steep fields. Experiments under simulated rainfall were performed at the plot scale (1 m2) to quantify water export from the surface of upland agricultural soils during overland flow events. Four 1 m2 plots were divided in duplicated treatment groups: (a) control with no amendments, and (b) amended with pig manure. Each plot was divided into two 0.5 m2 rectangular subplots. One subplot was designated as a rain splash treatment; the other sub-plot was covered with a 2 mm grid size wire screen that was located 12 cm above the soil surface. The purpose of the screen was to break the raindrops into fine droplets and to reduce fall height in order to drastically reduce their kinetic energy. Runoff was measured for each sub-plot. The results show that raindrop impact drastically enhances runoff generation on both bare soils and on manure amended soils. When the impact of raindrops was limited by screening, runoff was higher on amended soils than on bare soils.The temporal evolution of runoff was correctly modelled using a soil hydraulic conductivity that exponentially decreases over time of exposure to rainfall. Both experimental and modelling results showed that droplet energy induces a rapid evolution of the hydraulic properties of the soil surface due to crusting, resulting in a reduction of hydraulic conductivity and a concomitant increase in runoff rate.

Tradução de leaflet do projeto AGFORWARD - O sucesso da instalação das árvores depende de se minimizar a competição com outra vegetação pela água e nutrientes. Uma das práticas mais benéficas que um agricultor pode utilizar para obter um bom crescimento inicial das árvores é a aplicação de mulch.

La hausse des écoulements observée au Sahel pendant les années de sécheresse a amené de nombreux chercheurs à s’intéresser aux propriétés hydrodynamiques superficielles des sols et à leur impact sur les volumes ruisselés. Ce travail porte sur les processus responsables de la production du ruissellement dans deux bassins versants expérimentaux situés sur le socle cristallin de l’ouest du Niger. Il vise à caractériser le fonctionnement hydrodynamique à trois échelles spatiales emboitées. À l’échelle ponctuelle (50 cm²), l’étude porte sur l’infiltrabilité des couches supérieures (0-10 cm) du sol pour plusieurs états de surface. Sa valeur asymptotique est ici déterminée grâce aux mesures de conductivité hydraulique à saturation (Ks) effectuées in situ. À l’échelle de la surface élémentaire, les ruissellements produits sont mesurés sur des parcelles expérimentales de 10 m² tandis qu’à l’échelle du bassin versant (5 ha), les écoulements concentrés sont contrôlés par des stations hydrométriques installées à l’exutoire de ces bassins. Les mesures effectuées montrent que les conductivités hydrauliques sont faibles et varient de 10 mm.h-1 (valeur minimale, mesurée sur la croûte d’érosion ERO) à 40 mm.h-1 (valeur maximale mesurée sur la surface cultivée en moyenne sur la saison). Les coefficients de ruissellement sont en ordre inverse, de 0,60 sur ERO à 0,25 sur la surface cultivée. Les coefficients d’écoulement des bassins sont de 0,41 et 0,28, respectivement pour les bassins non cultivé et cultivé (culture pluviale). Les résultats obtenus à ces trois échelles sont cohérents et montrent la forte capacité de production de ruissellement des bassins étudiés qui découle de la faible infiltrabilité des états de surface et de la faible ré-infiltration, dans le réseau hydrographique, des écoulements produits sur les versants. Cette dernière ne représente ainsi que 4 % de la pluie annuelle alors qu'elle peut dépasser 50 % pour des bassins de même taille en zone sédimentaire. En conséquence, une pluie de 2 à 4 mm tombant sur ces bassins suffit pour générer de l’écoulement en raison de la faible épaisseur (moins de 20 cm) du dépôt sableux couvrant les lits des ravines. À l’échelle des surfaces élémentaires et des bassins étudiés, on note la quasi indépendance du fonctionnement hydrodynamique à l’état hydrique initial. The observed increase of the catchments flow values in the Sahel during the drought years has exacerbated the interest for the studies on the superficial hydrodynamic properties of soils and their impact onto runoff volumes. This work focuses on the processes responsible for the flow production within two experimental catchments situated in the granitic basement of Western Niger. It aims at characterizing the hydrodynamical functioning at three nested spatial scales. At point scale (50 cm²), soil surface infiltrability (0-10 cm) is determined for different surface features through hydraulic conductivity (Ks) measurements carried out in situ. At the plot scale, runoff is measured from 10 m² experimental plots while at catchment scale (5 ha), stream flow is controlled by stream gauges at the exutories. Measurements show low values of hydraulic conductivity ranging from 10 mm.h-1 (minimal value measured on erosion crusts ERO) to 40 mm.h-1 (maximal value measured on cultivated sites in seasonal average). Runoff coefficients are inversely ranked, from 0.60 on ERO to 0.25 on cultivated surface. Catchment scale runoff coefficients are 0.41 and 0.28 for the not cultivated and the cultivated basins, respectively. Results obtained at these three scales are consistent and reveal the strong runoff production ability of these granitic catchments resulting from both low point scale infiltrability and small re-infiltration of runoff water within the stream network. This last component represents only 4% of annual rainfall while it may reach more than 50% for sedimentary catchments of similar size. Within these catchments, thus, a 2-4 mm rainfall is sufficient to generate a flow at the outlet due to rather small (less than 20 cm) sand deposits covering the stream beds. Finally, both at the plot and at the catchment scales, hydrodynamical functioning was found quasi independent on the initial water content.

Article en open access; Surface crusts, formed by raindrop impact, degrade the soil surface structure causing changes in porosity. An experiment was conducted with the objective of evaluating the influence of the formation of a crusting layer on the porosity (percentage of area, shape and size) of a Haplic Acrisol under three tillage systems, and simulated rainfall. The tillage systems were: conventional tillage (CT), reduced tillage (RT) and no-tillage (NT). Each tillage system was submitted to different levels of simulated rainfall (0, 27, 54 and 80 mm) at an intensity of 80 mm·h−1. Undisturbed soil samples were collected and resin impregnated for image analysis in two layers: layer 1 (0–1 cm) and layer 2 (1–2 cm). Image analysis was used to obtain the pore area percentage, pore shape and size. The degradation of the soil surface and change in porosity, caused by rainfall, occurred differently in the tillage systems. In the CT and RT systems, the most pronounced pore changes caused by rainfall occurred in layer 1, but in the NT system, this change occurred in layer 2. The rainfall caused change of pore area percentage in the CT and RT systems, with reduction of complex and an increase of rounded pores. The NT system showed greater occurrence of the rounded pores (vesicles), originated by processes of wetting below the residue cover, and by alternating periods of wetting and drying. In this study, the changes in porosity were attributed to two main factors: (1) to the effect of the raindrop directly on the soil surface (for CT and RT tillage systems) and (2) water transfer processes in the soil surface (for NT systems).

Soil and carbon redistribution on arable land and the associated impacts on carbon sequestration and mineralisation may play an important role in the global carbon cycle. While our insight in the process-chain of erosion, transport and deposition has significantly grown over recent years, there are still major gaps in understanding making it difficult to make an overall assessment of erosion processes on carbon exchange between the soil and the atmosphere. One issue is the potential effect soil degradation and erosion processes may have on CO2 effluxes at eroding sites. The major goal of this study was therefore to analyse and understand the effects of interrill erosion, soil crusting and soil aggregate breakdown on in situ CO2 effluxes. Therefore a set of rainfall simulations were carried out on bare loess-burden soil with different antecedent soil moisture content. All treatments were compared with controls protected from rain drop impact using a fine-meshed geotextile. As expected, runoff and sediment delivery was significantly larger on bare compared to covered soils, while surface runoff and sediment delivery increased (in most cases) with rising antecedent soil moisture as well as rainfall duration. Crust thickness increased with antecedent soil moisture and rainfall intensity and was in general smaller for the controls. However, variations in crust thickness did not result in significant differences in in situ measured CO2 effluxes. Also the destruction of the soil crust after six to seven days of measurements did not have a significant effect. This leads to the conclusion that crusting and interrill erosion has no or only a minor effect on in situ CO2 effluxes. Nevertheless, it should be recognised that topsoil carbon is preferentially removed due to interrill erosion which may result in additional CO2 release at depositional sites or in stream and/or standing water bodies. � 2012 Elsevier B.V.

The outlined methodology in this article allowed correlating soil sealing and soil physical properties as structural stability, infiltration velocity, soil loss erosion, run–off, normal and torsion resistance and aggregate distribution. The output showed that improved pastures after eight years of sowing produced a relatively positive response to soil erosion and mechanical strengths. But periodic farming and fertilization that are required for corn production although resulted in good seed bed preparation and created a good environment to store moisture, it allowed an increase of soil erosion and decrease of soil structure stability.La metodología presentada en este artículo permitió correlacionar el sellamiento del suelo y propiedades físicas como estabilidad estructural, velocidad de infiltración, suelo erosionado, escorrentía, resistencia normal y torsional, y distribución de agregados. Los resultados mostraron que los pastos mejorados después de ocho años de sembrados produjeron una respuesta relativamente positiva del suelo a la erosión y a los esfuerzos mecánicos. Pero la labranza y la fertilización periódicas que demandó la producción de maíz, aunque prepararon relativamente bien la cama de semillas y propiciaron un buen ambiente para el almacenamiento de humedad, permitieron un incremento de la erosión hídrica y un decrecimiento de la estabilidad de la estructura.

La metodología presentada en este artículo permitió correlacionar el sellamiento del suelo y propiedades físicas como estabilidad estructural, velocidad de infiltración, suelo erosionado, escorrentía, resistencia normal y torsional, y distribución de agregados. Los resultados mostraron que los pastos mejorados después de ocho años de sembrados produjeron una respuesta relativamente positiva del suelo a la erosión y a los esfuerzos mecánicos. Pero la labranza y la fertilización periódicas que demandó la producción de maíz, aunque prepararon relativamente bien la cama de semillas y propiciaron un buen ambiente para el almacenamiento de humedad, permitieron un incremento de la erosión hídrica y un decrecimiento de la estabilidad de la estructura.The outlined methodology in this article allowed correlating soil sealing and soil physical properties as structural stability, infiltration velocity, soil loss erosion, run-off, normal and torsion resistance and aggregate distribution. The output showed that improved pastures after eight years of sowing produced a relatively positive response to soil erosion and mechanical strengths. But periodic farming and fertilization that are required for corn production although resulted in good seed bed preparation and created a good environment to store moisture, it allowed an increase of soil erosion and decrease of soil structure stability.

Soil erosion, along with the contributing factors of soil crusting and sealing, have received minimal scientific attention to date in Latin America. This study was conducted in an Andean hillside environment to determine how the local organic manuring and tillage practices influence the development of soil crusting and sealing, and the extent to which these practices influence soil water infiltration. The aim of this study was to identify treatments that prevented superficial soil structural constraints, i.e. treatments which maintain infiltration and therefore reduce potential soil erosion and run-off. Treatment results were measured with a pocket penetrometer and a mini-rain simulator on nine different cropping systems, mainly based on cassava (Manihot esculenta Crantz), from February to November 2000 and 2001. The cropping systems were laid out on a Ferrallic Cambisol, an acid, vulcanically influenced soil of the Andean region. In both cropping cycles, treatments with chicken manure application developed superficial soil crusts during the dry season. For a treatment manured with 8 t ha � 1 chicken manure, this crust meant an increase in penetration resistance from 2.3 kg cm � 2 in April 2000 to 16.2 kg cm � 2 in July 2000. The change in superficial soil structure created a notable reduction in final infiltration from 92 to 42.2 mm h � 1 . A minimum tillage treatment which displayed the highest penetration resistance during the dry periods of up to 46.4 kg cm � 2 presented no restricting effects on soil water intake (76.2 mm h � 1 final infiltration in 2000) due to an optimal aggregate development during 10 years of consecutive conservation practice. Measurements of penetration resistance and infiltration showed that soil conserving treatments, such as minimum tillage and crop rotations, improved the physical soil status and prevented soil crusting developing along with its negative effects on infiltration. These methods can therefore be strongly recommended to farmers. # 2004 Elsevier B.V. All rights reserved.

International audience; Changes in the hydrological regime of Sahelian Rivers are considered based upon the example of the Middle Niger River and its exceptional flood in 2010 near the city of Niamey. It is shown that rainfall in 2010 was only average with respect to the long term record, with neither the monthly rainfall distribution in terms of the amount of rainfall nor the distribution of rainy events changing significantly in the last few decades. Particularly , no increase in the number of extreme rainfall events is observed. In spite of this, the Niger River's right bank tributaries have shown a sharp increase in runoff since the 1970s, which is still ongoing, and has resulted in a modification of the Niger River's regime from a single hydrograph to a two flood hydrograph, the local flood, occurring during the rainy season being the more pronounced one. This modification is likely due to an increase of bare soils and crusted soil areas as a consequence of human pressure, resulting mostly from the spatial extension of crop areas and the shortening of fallow periods. Changes in connectivity of the river networks on both banks of the Niger such as endorheism bursting events also caused an increase in the contributing basin area. Policy makers should be alerted to the effects of intensive cropping, land clearing and overgrazing in some areas, on the hydrological regimes of Sahelian Rivers.

International audience; In northern Laos, small agricultural catchments are under high pressure because of the past decades of land management policies. Although the land degradation is well documented, no clear relationships have been established between land use and event scale runoff and erosion. In this study, we use a simple yet realistic model to obtain a per land use characterisation of small plot runoff production at the event scale. Runoff produced during 7 years (2003-2009) from 1 m(2) plots under different land uses is analyzed with the spatially variable infiltration (SVI) model. This model has been recently used by different authors to study infiltration at the plot scale under monsoon climate. It depends on rainfall intensity and a parameter to be calibrated that we called infiltrability. This parameter is an average infiltration rate during a rainfall event for given soil characteristics and a given land use. The analysis and calibration of runoff production with the SVI model led to a data set of almost 3000 infiltrability values. The statistical analysis of this data set with soil parameters, like surface features and slope, land use and the antecedent precipitation index (API), which is an indicator of soil moisture content, shows that infiltrability is strongly correlated with the percentage of crust and land use type. Except for certain land use like rice, the correlation with API is not well established and it is clear that minima of infiltration are reached in the rainy season after a close sequence of rainfall events and infiltrability is generally highest at the end of the dry season. Since we could not establish clear relationships between infiltrability, soil and meteorological parameters, we propose to consider infiltrability as a random function in space and time described, for each land use, by a log normal probability density function.

O processo de infiltração é influenciado pelas condições da superfície do solo e pela precipitação pluvial. O objetivo deste trabalho foi avaliar a influência de aplicações sucessivas de precipitações pluviais com diferentes perfis na formação do encrostamento superficial e, consequentemente, na taxa de infiltração de água em solo sem cobertura e com cobertura. Foram aplicadas três precipitações pluviais sucessivas para cada perfil de precipitação pluvial, em intervalos de 24 h, sendo usados os perfis de precipitação pluvial constante, exponencial decrescente, duplo exponencial adiantado e atrasado, com uma lâmina média de 55 mm por aplicação. Buscou-se ajustar um fator de decaimento da taxa de infiltração (Ti) em função da energia cinética acumulada da chuva, denominado de fator f, dado pela razão entre a taxa de infiltração estável (Tie) com efeito da chuva e a Tie sem o efeito da chuva. Foram avaliadas duas condições de cobertura do solo, solo sem cobertura e solo coberto com palhada, sendo os ensaios de infiltração realizados com um simulador de chuvas, em parcelas experimentais de dimensões de 1,0 x 0,7 m. Os resultados foram avaliados por meio de análises gráficas, análise de variância e teste de média. Verificou-se que os diferentes perfis de precipitação pluvial não influenciaram a infiltração de água no solo, tanto para o solo sem cobertura como para o solo com cobertura, sendo esta influenciada somente pelas aplicações sucessivas, com menores valores da taxa de infiltração obtidos na segunda e terceira aplicações. O decréscimo na Ti em solo sem cobertura foi devido à formação de encrostamento superficial, o que ocorreu logo na primeira aplicação. A Tie em solo sem cobertura teve decréscimo de 75 % se comparada à Tie em solo com cobertura. A lâmina infiltrada não foi influenciado pelos quatro perfis de precipitação pluvial. O fator f foi descrito com uso de uma equação do tipo exponencial. The purpose of this study was to evaluate the influence of successively repeated precipitation patterns applied to bare and to covered soil, to quantify their effect on the formation of a soil crust and, consequently, on the soil water infiltration rate (Ti). Three rainfall events were simulated in 24 h intervals, with constant, decreasing exponential, early and late double exponential precipitation patterns, at a rainwater amount corresponding to 55 mm. The decrease factor of the infiltration rate (Ti) was adjusted as a function of the accumulated kinetic energy of the rainwater, called factor f, given by the ratio between the stable infiltration rate (Tie) affected by rain, and Tie with no effect of rain. Two situations of soil cover were evaluated: bare soil and soil covered with crop residues. A rainfall simulator was used in the infiltration experiments, on 1.0 x 0.7 m plots. The results were evaluated by graphical analysis, analysis of variance and means testing. It was found that the different precipitation profiles did not affect Ti of bare or covered soil. The Ti values were only influenced by successive rainfall, and the water infiltration rate was lowest after the 2nd and 3rd application. The decrease in Ti of bare soil was due to a crust on the soil surface, which was first formed after the 1st application. The stable infiltration rate (Tie) on bare soil decreased by 75 % when compared to Tie of covered soil. The infiltration depth was not influenced by any of the tested rainfall patterns. The factor f was described by an exponential equation.

Contact: pare@supagro.inra.fr; International audience; Soil surface characteristics (SSC) influence strongly hydrological processes and are known to vary largely in space and time according to soil characteristics and soil management. Because tillage is a main source of variation, the goal of this study was to present and evaluate a prediction model of the temporal variation of the SSC after tillage at the catchment level. The study focused on bare soils prevailing in spring and summer. A logistic regression approach was used to predict the evolution along three stages, starting from the fresh tillage stage to the crusted soil stage. This method provides the probabilities of occurrence of each stage. The predictor candidates tested were a rainfall characteristic, namely cumulative rainfall depth or cumulative kinetic energy, basic soil properties and tillage features. The results showed that a model based on cumulative kinetic energy since tillage and soil stoniness accurately predicts the dynamics of SSC: the rate of well classified SSC was 91%. However, no significant difference in the prediction performance was found using as predictor either cumulative kinetic energy or cumulative rainfall amount since tillage. In the prediction model, the rainfall characteristic was the most significant predictor for the SSC evolution and the only one during the first stages of crust development since tillage. Stoniness was also shown to influence SSC evolution but only during the last stages of crust development: high stone cover speeds up soil surface evolution. The same approach using logistic regression can be applied elsewhere but will require a re-examination of the most relevant predicting variables. Finally, to be able to predict the soil surface characteristic evolution on an annual scale, weed growth characteristics must be considered in the list of predictor candidates.

International audience; In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).

International audience; In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).; Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas-fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an.

28 páginas, 4 figuras, 5 tablas., [ES] Las costras físicas (CFS) y biológicas (CBS) del suelo ocupan una gran extensión en zonas áridas y semiáridas de todo el mundo. En estos medios, el encostramiento del suelo tiene una gran influencia sobre los procesos hidrológicos y erosivos. Los objetivos que se persiguen en este trabajo son: analizar las características de las costras que influyen en la infiltración y en la erosión e identificar cuáles de estas características tiene una mayor influencia sobre estos procesos. En dos áreas semiáridas representativas en la provincia de Almería se identificaron los principales tipos de costras físicas y biológicas. Para cada tipo de costra, se analizaron las características físicas y químicas de la propia costra y del material subyacente, así como su rugosidad, hidrofobia, resistencia a la penetración y cobertura de diferentes comunidades de organismos. Para analizar la influencia de las costras sobre la infiltración y la erosión, se llevaron a cabo simulaciones de lluvia en parcelas con la costra intacta y para examinar los efectos de las características del material subyacente a la costra, los experimentos de simulación de lluvia se realizaron tras retirar la costra. La respuesta hidrológica de las áreas encostradas estudiadas se ve afectada no solo por las características de la costra, sino también por las características del suelo sobre el que se desarrollan estas costras y especialmente por la pendiente que aparece como una variable altamente predictiva para la infiltración y la erosión. Entre las características del material subyacente, resalta la influencia de la textura, el contenido en carbono orgánico y la conductividad eléctrica, y entre las características de la costra, la cobertura y rugosidad son las variables más predictivas que explican las diferencias en las tasas de infiltración y erosión entre ambos sitios., [EN] Physical soil crusts and biological soil crusts occupy a wide extension in arid and semiarid areas all over the world. In these regions, soil crusting has a very strong influence on local hydrologic regimes and erosion. The main purposes of this paper are: 1) to examine how crust characteristics influence infiltration and erosion and 2) to identify the most influential crust characteristics on runoff and erosion processes. Two semiarid areas in the province of Almería were chosen and the most representative physical and biological soil crusts were identified at both sites. For each crust type, physical and chemical characteristics of the crust and the soil underneath the crust were analysed, as well as other crust properties like roughness, hydrophobicity, resistance to penetration and cover. To analyse the influence of the crust on infiltration and erosion, rainfall simulations were conducted on plots with the intact crust, and in order to examine the influence of the soil underlying the crust, rainfall simulations were conducted on plots after removing the crust. The hydrological response of the studied encrusted areas is affected, not only by the characteristics of the crust itself, but also by the characteristics of the soil in where the crust is developed and specially by the slope gradient that appears like a highly predictive variable for infiltration and erosion. Among the properties of the soil material below crusts, it is remarkable the influence of soil texture, organic carbon content and electrical conductivity, and among the properties of the crust, cover and roughness are the most predictive variables explaining the differences in infiltration and erosion between sites., Este trabajo ha recibido el apoyo financiero de diferentes proyectos de investigación: PROBASE (Ref.: CGL2006-11619/HID), PREVEA (Ref.: CGL2007-63258/BOS), financiados por el Plan Nacional de I+D y COSTRAS (Ref. RNM 3614), financiado por la Junta de Andalucía y EC-DG RTD- 6th Framework Research Programme (1.1.6.3)-Research on Desertificationproject DESIRE (037046).

Physical soil crusts and biological soil crusts occupy a wide extension in arid and semiarid areas all over the world. In these regions, soil crusting has a very strong influence on local hydrologic regimes and erosion. The main purposes of this paper are: 1) to examine how crust characteristics influence infiltration and erosion and 2) to identify the most influential crust characteristics on runoff and erosion processes. Two semiarid areas in the province of Almería were chosen and the most representative physical and biological soil crusts were identified at both sites. For each crust type, physical and chemical characteristics of the crust and the soil underneath the crust were analysed, as well as other crust properties like roughness, hydrophobicity, resistance to penetration and cover. To analyse the influence of the crust on infiltration and erosion, rainfall simulations were conducted on plots with the intact crust, and in order to examine the influence of the soil underlying the crust, rainfall simulations were conducted on plots after removing the crust. The hydrological response of the studied encrusted areas is affected, not only by the characteristics of the crust itself, but also by the characteristics of the soil in where the crust is developed and specially by the slope gradient that appears like a highly predictive variable for infiltration and erosion. Among the properties of the soil material below crusts, it is remarkable the influence of soil texture, organic carbon content and electrical conductivity, and among the properties of the crust, cover and roughness are the most predictive variables explaining the differences in infiltration and erosion between sites., Las costras físicas (CFS) y biológicas (CBS) del suelo ocupan una gran extensión en zonas áridas y semiáridas de todo el mundo. En estos medios, el encostramiento del suelo tiene una gran influencia sobre los procesos hidrológicos y erosivos. Los objetivos que se persiguen en este trabajo son: analizar las características de las costras que influyen en la infiltración y en la erosión e identificar cuáles de estas características tiene una mayor influencia sobre estos procesos. En dos áreas semiáridas representativas en la provincia de Almería se identificaron los principales tipos de costras físicas y biológicas. Para cada tipo de costra, se analizaron las características físicas y químicas de la propia costra y del material subyacente, así como su rugosidad, hidrofobia, resistencia a la penetración y cobertura de diferentes comunidades de organismos. Para analizar la influencia de las costras sobre la infiltración y la erosión, se llevaron a cabo simulaciones de lluvia en parcelas con la costra intacta y para examinar los efectos de las características del material subyacente a la costra, los experimentos de simulación de lluvia se realizaron tras retirar la costra. La respuesta hidrológica de las áreas encostradas estudiadas se ve afectada no solo por las características de la costra, sino también por las características del suelo sobre el que se desarrollan estas costras y especialmente por la pendiente que aparece como una variable altamente predictiva para la infiltración y la erosión. Entre las características del material subyacente, resalta la influencia de la textura, el contenido en carbono orgánico y la conductividad eléctrica, y entre las características de la costra, la cobertura y rugosidad son las variables más predictivas que explican las diferencias en las tasas de infiltración y erosión entre ambos sitios.

International audience; In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).; Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an.

In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year)., Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an.

International audience; Au Sahel, l'explosion démographique de ces dernières décennies et les variations climatiques ont provoqué d'importants changements environnementaux. L'objectif de ce travail est de mesurer les impacts de la pression anthropique sur les écosystèmes dans la région de Niamey au cours des six dernières décennies. L'étude est fondée sur une cartographie diachronique d'une aire de 100 km² située près de Niamey au moyen de photographies aériennes (1950 et 1975) et relevés au GPS (2009). Il est apparu ainsi qu'entre 1950 et 2009, la végétation de la brousse tigrée a été complètement déboisée. Dans les vallées sableuses, les surfaces cultivées ont connu une extension passant de 20,7 % à 69,4 % entre 1950 et 1975. Ceci a favorisé l'emprise des érosions hydrique et éolienne qui ont abouti à une dégradation des terres par encroûtement des sols. Celui-ci est à l'origine de la baisse des surfaces cultivées entre 1975 et 2009 (de 69,4 % à 54,4 %). Dans les bas- fonds, la tendance est au comblement du fait d'un taux de sédimentation de plus de 4 cm par an. In the Sahel, the rapid increase of the population during the last decades and the climate variation lead to an important environmental degradation. This work aims to measure the impacts of the human pressure on ecosystem during the six last decades. A diachronic cartography of a 100 km² area close to Niamey was done with aerial photographs (1950 and 1975) and GPS measurements (2009). Results showed that the tiger bush vegetation was completely cleared between 1950 and 2009 while the fallow decreases from 7 % to 1 %. In the sandy valley, the increase of cultivated fields from 20,7 % (1950) to 69,4 % (1975) favoured wind and water erosions which allowed surface soil crusting. Between 1975 and 2009, the bare crusted soil dramatically developed at the expense of the cultivated area which represents only 54,4 % of the studied area. The valleys are going to be overloaded because of high sedimentation rate (> 4 cm per year).

La metodología presentada en este artículo permitió correlacionar el sellamiento del suelo y propiedades físicas como estabilidad estructural, velocidad de infiltración, suelo erosionado, escorrentía, resistencia normal y torsional, y distribución de agregados. Los resultados mostraron que los pastos mejorados después de ocho años de sembrados produjeron una respuesta relativamente positiva del suelo a la erosión y a los esfuerzos mecánicos. Pero la labranza y la fertilización periódicas que demandó la producción de maíz, aunque prepararon relativamente bien la cama de semillas y propiciaron un buen ambiente para el almacenamiento de humedad, permitieron un incremento de la erosión hídrica y un decrecimiento de la estabilidad de la estructura. The outlined methodology in this article allowed correlating soil sealing and soil physical properties as structural stability, infiltration velocity, soil loss erosion, run–off, normal and torsion resistance and aggregate distribution. The output showed that improved pastures after eight years of sowing produced a relatively positive response to soil erosion and mechanical strengths. But periodic farming and fertilization that are required for corn production although resulted in good seed bed preparation and created a good environment to store moisture, it allowed an increase of soil erosion and decrease of soil structure stability.

Thesis (PhDEnvironmentalManagement)--University of South Australia, 2007. This thesis considers various aspects of the use of ground-based methods and remote sensing of Biological Soil Crusts (BSC). They are mostly distributed in winter rainfall dominated areas such as those at Middleback Field Centre (MFC) in South Australia. They can be used potentially as an indicator of rangeland condition by estimating grazing pressure (trampling). Two BSC based indicators for rangeland condition assessment are species composition and cover. While there is strong agreement that BSC composition is a good indicator, there is less agreement that BSC cover alone is a good indicator. Although BSC have been included in previous remotely-sensed studies, their spectral characteristics, and hence their contributions to remotely-sensed spectral signatures, are not well known. Data collection methods were refined for suitable method selection, stratification and site characterization, and morphological/ functional group classification. Cover data of BSC were collected using a 100 m line-intercept method on the stratified land units and statistical analyses were based on the cover variance analyses. Spectra of BSC groups were collected and characterized for different remote sensing indices. Five grazing gradient models based on collected spectra were developed for the evaluation of BSC effect on remotely-sensed data. Both existing and newly developed remote sensing indices were examined for BSC detection. Sampling for cover of BSC in the field showed that there is indeed a detectable change with distance from water, suggesting that BSC cover can be used as an indicator of rangeland condition, provided that appropriate stratification of the study sites is carried out prior to sampling, and spectral differences in morphological and functional groups are taken into account. Spectral analysis of BSC components showed that different classes of organisms in the crusts have different spectral characteristics, and in particular, that the (commonly-used) perpendicular vegetation index (PD54) is not suitable for detecting BSC. On the other hand, ground-level spectral modelling showed that the Normalized Difference Vegetation Index (NDVI) and Soil Stability Index (SSI) did show a distinguishable contribution from BSC. A procedure for detecting cover of BSC was developed for image taken during the period after an effective rain, in contrast to the normal practice of selecting images of dry surfaces for interpretation. The most suitable intervals appears to be 2-4 days after rain in late autumn, winter and early spring. Of the existing indices, the SSI is the best for estimating cover of BSC from Landsat images. However, eight new indices, specifically designed for detection of BSC were developed during the cource of this work. The best results were obtained for indices using using the middle-infrared bands. These results are promising for application to rangeland monitoring and suggest that BSC cover is an important indicator of rangeland condition if appropriate stratification, classification and data-collection methods are used. The effects of BSC cover on a remotely-sensed method are considerable, and thus they can not be neglected during image interpretation. There are different phenological patterns for BSC, annual and perennial elements, thus there is the possibility for the selection of imagery based on each phenological stage to detect these elements. Application of certain indices such as the PD54 may create mis-estimation of land covers. Although some of the existing and newly developed indices had significant results for BSC cover estimation, there is a requirement for a standalone remotely-sensed method to conclude the best index.

Correspondance: E-mail: Chaplot@bondy.ird.fr; International audience; In this study our main objective was to quantify water interrill erosion in the sloping lands of Southeast Asia, one of the most bio-geochemically active regions of the world. Investigations were performed on a typical hillslope of Northern Laos subjected to slash and burn agriculture practiced as shifting cultivation. Situations with different periods of the shifting cultivation cycle (secondary forest, upland rice cultivation following a four-year fallow period and three-year continuous upland rice cultivation) and soil orders (Ultisols, Alfisols, Inceptisols) were selected. One metre square micro-plots were installed to quantify the soil material removed by either detachment of entire soil aggregate or aggregate destruction, and the detached material transported by thin sheet flow, the main mechanisms of interrill erosion. In addition, laboratory tests were carried out to quantify the aggregate destruction in the process of water erosion by slaking, dispersion and mechanical breakdown. The average runoff coefficient (R) evaluated throughout the 2002 rainy season was 30•1 per cent and the interrill erosion was 1413 g m−2 yr−1 for sediments and 68 g C m−2 yr−1 for soil organic carbon, which was relatively high. Among the mechanisms of interrill water erosion, aggregate destruction was low and mostly caused by mechanical breakdown due to raindrops, thus leading to the conclusion that detachment and further transport by the shallow runoff of macro-aggregates predominates. R ranged from 23•1 to 35•8 per cent. It decreased with the proportion of mosses on the soil surface and soil surface coverage, and increased with increasing proportion of structural crust, thus confirming previous results. Water erosion varied from 621 to 2433 g m−2 yr−1 for sediments and from 31 to 146 g C m−2 yr−1 for soil organic carbon, and significantly increased with increasing clay content of the surface horizon, probably due to the formation of easily detachable and transportable sand-size aggregates, and proportion of macro-aggregates not embedded in the soil matrix and prone to transport. In addition, water erosion decreased with increasing proportion of structural crusts, probably due to their higher hardness, and when cultivation follows a fallow period rather than after a long period of cultivation due to the greater occurrence of algae on the soil surface, which affords physical protection and greater aggregate stability through binding and gluing. This study based on simultaneous field and laboratory investigations allowed successful identification and quantification of the main erosion mechanisms and controlling factors of interrill erosion, which will give arguments to further set up optimal strategies for sustainable use of the sloping lands of Southeast Asia.

International audience; Soil surface features strongly determine whether rain water will infiltrate or runoff. This results in a segregation between several kinds of hydrological functioning of hillslopes and catchments. Using a deterministic model, it is attempted to define the spatial and temporal boundaries of Hortonian (infiltration excess runoff) and Hewlettian (saturation excess overland flow) hydrological behaviour. The model allows to calculate the role of the antecedent precipitation index and the soil water holding capacity in the runoff yield. These factors depend on soil hydrodynamic properties. In Northern Mexico, data collected in four experimental networks are used: one in the sub-humid Western Sierra Madre, one in its semi-arid foothill, one in the centre of the endoreic Bolson de Mapimi (the southern part of Chihuahuan desert) and the last one on the southern edge of the latter, in a limestone range. There is a regional distribution of these parameters because of rainfall distribution and overall because of the whole ecological context. The value of α parameter (which determines the depletion time of soil water content) and the proportion of bare soils are the most important explaining factors of geographical segregation between Hortonian and Hewlettian contexts. This study determines that the Western Sierra Madre, with its temperate climate, is mostly characterized by a Hewlettian hydrology, despite an increase in Hortonian behaviour due particularly to land degradation. Inversely, as it is well known, Hortonian runoff dominates completely the semi-arid and arid areas; however in certain circumstances, saturation excess overland flow can appear due to landscape or local roughness, and local or temporal possibility to infiltrate a great proportion of rainwater, i.e. during low intensity–large duration events (hurricanes crossing the mountains, winter rainy events linked to El Niño Southern Oscillation configuration).

International audience; Crack patterns were observed on the seedbed surfaces of several crops. Photographs were taken with a digital camera from sowing to emergence. We analysed the photos obtained for the maximum shrinkage observed for each plot. Total crack length and plate area increased with the development of crusts. By contrast, the plates kept the same hexagonal shape. The image analysis tool formulated in this study will allow analysing other situations with different soils and the progression of surface crusting through time from sowing as a function of climatic conditions.

International audience; The main objective of the SoDA (Soil Degradation Assessment) project is to realize a simulator of soil surface degradation by rainfall at the meter scale and including visualization. Soil surface structure and morphology deeply influence a lot of processes of high agronomic and environmental relevance, such as mass and heat transfer through the soil-atmosphere interface, runoff and erosion, seed germination and seedling emergence. The soil surface structure of agricultural field is in continuous evolution: it is strongly affected by tillage, and in between tillage operations, erosion by rainfall and runoff causes a progressive degradation of the structure whose intensity and speed partly depend on the initial state associated to tillage modalities. A soil surface degradation model could allow one to predict this evolution of the soil surface structure, and even to help choosing adequate tillage practices and sowing dates. Erosion modeling has been addressed by soil scientists but also by computer graphic scientists in order to add realism to virtual landscapes. Mixing both of these points of view would be interesting to simulate and visualize the evolution of the soil surface of a cultivated soil. Based on a 3D cellular automata approach using the knowledge accumulated by soil scientists about the physical processes involved in erosion, the principles of our simulator and its first implementation are presented in this paper.

Field- and laboratory-scale rainfall simulation experiments were carried out in an investigation of the temporal variability of erosion processes on interrill areas, and the effects of such variation upon sediment size characteristics. Poorly aggregated sandy soils from the semiarid environment of Senegal, West Africa, were used on both a 40 m(2) field plot and a 0-25 m(2) laboratory plot; rainfall intensity for all experiments was 70 mm h(-1) with a duration of 1 to 2 hours. Time-series measurements were made of the quantity and the size distribution of eroded material: these permitted an estimate of the changing temporal balance between the main erosion processes (splash and wash). Results from both spatial scales showed a similar temporal pattern of runoff generation and sediment concentration. For both spatial scales, the dominant erosional process was detachment by raindrops; this resulted in a dynamic evolution of the soil surface under raindrop impact, with the rapid formation of a sieving crust followed by an erosion crust. However, a clear difference was observed between the two scales regarding the size of particles detached by both splash and wash. While all measured values were lower than the mean weight diameter (MWD) value of the original soil (mean 0(.)32 mm), demonstrating the size-selective nature of wash and splash processes, the MWD values of washed and splashed particles at the field scale ranged from 0(.)08 to 0(.)16 mm and from 0(.)12 to 0(.)30 mm respectively, whereas the MWD values of washed and splashed particles at the laboratory scale ranged from 0(.)13 to 0(.)29 mm and from 0(.)21 to 0(.)32 mm respectively. Thus only at the field scale were the soil particles detached by splash notably coarser than those transported by wash. This suggests a transport-limited erosion process at the field scale. Differences were also observed between the dynamics of the soil loss by wash at the two scales, since results showed wider scatter in the field compared to the laboratory experiments. This scatter is probably related to the change in soil surface characteristics due to the size-selectivity of the erosion processes at this spatial scale. Copyright (c) 2006 John Wiley and Sons, Ltd.

The SIMPLE model can predict the emergence of various types of crop. It includes an empirical module predicting seedling emergence from under a crust. That module is based on the observation of the emergence of sugar beet crops in a field network with silt loam soils. This module is basic, but effective, and the purpose of this work is to allow the extension of forecasts to other sowing conditions and other crops (seedbed types, soil types, different characteristics of the seeds), given the huge problems in crop emergence resulting from the formation of crusts on the surface of seedbeds for various crops. Field experiments were carried out to characterize the different tactics used by the seedlings to break through (penetration, rupture and passage through cracks), according to the different types of crust, in the case of spring (sugar beet, flax and bean) and autumn crops (wheat). On the same plots, we characterized the dynamics of crust formation for the various initial states of seedbeds (aggregate size, soil water content) according to the rains cumulated since sowing. We also analysed soil crusting data acquired on a long term experiment, thus complementing our two experimentation campaigns. The crusts were characterized by their stage, their thickness and their resistance to penetration. Various quantities of cumulated rains, from 11mm for seedbeds mainly made of thin dry earth to 27mm for coarse soil surfaces, were determined in order to obtain the formation of structural crusts, which are already enough to penalize emergence if they dry out. Complementary experiments were led in the laboratory in order to obtain under simulated rainfalls more advanced stages of crust unobserved during the two years of experimentation in the field, and characterize them. We quantified the evolution of the forming network of cracks according to the degree of degradation and the moistening/desiccation alternation. Those data made it possible to parameterise and test geometrical models for the appearance of cracks. The forces exerted by the seedlings and their changes in time, according to the variations in the seed mass, the varieties and the species studied, were measured with force sensors in the laboratory. All those elements made it possible to build a new module integrating factors which had not been taken into account before when predicting emergence through crusts. This module integrates initial states for various seedbeds (structure and water content) and different cumulated rain values which lead to successive stages in the formation of the crust. With each type of crust, a characteristic distribution of resistances is associated. The maximum force of every seedling that comes to the surface is drawn at random in a distribution, then decreases with the age of the seedling. That value is compared day after day with the resistance of the material via a coefficient that makes it possible to determine whether the seedling will emerge or not. That coefficient results from an adjustment to the data observed. It was based on sugar beet, a crop for which numerous and precise data were recorded. The first simulations carried out with this new module show the importance of the effects of the initial states of the seedbeds on emergence rates. The effect of force differences due to the mass of the seeds is more limited (about 10% on average) but it reaches about 20% in certain sowing conditions. We also tested the possibility of using this coefficient for other crops, the force of which had been measured. The extension to other species gives encouraging results but requires additional work. We finally discuss the possibility of using various parameters measured on soils in the laboratory in order to be able to adapt the proposed model to other soil types., Le modèle SIMPLE prévoit les levées de différentes cultures. Il comporte un module de prévision des levées sous croûte empirique, basé sur l'observation de levées de betterave sucrière dans un réseau de parcelles pour un type de sol de limons argileux. Ce module est donc assez sommaire bien qu'efficace et l'objectif de ce travail était de pouvoir étendre les prévisions à d'autres conditions de semis et pour d'autres cultures (types de lits de semences, types de sols, différentes caractéristiques des semences) étant donné l'importance des problèmes de levée liés au développement de croûtes à la surface des lits de semences de différentes cultures. Nous avons entrepris de caractériser les types de franchissement des plantules (pénétration, rupture et passage par fissure) au champ face à différents types de croûtes pour des cultures de printemps (betterave, lin et haricot) et d'automne (blé). Nous avons caractérisé sur ces mêmes parcelles la dynamique de formation des croûtes pour les différents états initiaux des lits de semences (granulométrie, humidité au semis) en fonction des pluies cumulées depuis le semis, en complétant les observations de nos deux campagnes d'expérimentation par celles acquises sur un essai de longue durée. Les croûtes ont été caractérisées par leur faciès, leur épaisseur et leur résistance à la pénétration. Des quantités de pluies cumulées, allant de 11 mm pour des lits de semences fins et secs à 27 mm pour des états de surface grossiers ont été mises en évidence pour obtenir la formation de croûtes structurales, qui suffisent déjà à pénaliser les levées si elles se dessèchent. Des expérimentations complémentaires ont été menées au laboratoire pour obtenir sous simulateur de pluie et caractériser des stades plus avancés de croûtes non observés au cours des deux années d'expérimentation au champ. Nous avons quantifié l'évolution du réseau de fissures se formant en fonction du degré de dégradation et des alternances humectation-dessication. Ces données ont permis de paramétrer et tester des modèles géométriques d'apparition de fissures. Les forces exercées par les plantules et leurs variations au cours du temps en fonction de la masse, des variétés et des espèces étudiées ont été mesurées avec des capteurs de force au laboratoire. Les différents éléments acquis par ces expérimentations ont permis d'élaborer un module intégrant des facteurs de variations non pris en compte jusqu'alors pour prévoir la formation d'une croûte et les levées. Ce module intègre des états initiaux de lits de semences variés (structure et teneur en eau), des cumuls de pluie différenciés aboutissant à des stades successifs de formation de la croûte. A chaque type de croûte est associé une distribution de résistances caractéristique. Pour chaque plantule arrivant à la surface, sa force maximum est tirée au sort dans une distribution puis diminue avec l'âge de la plantule. Cette valeur est confrontée jour après jour à la résistance du matériau via un coefficient qui permet d'établir si la plante passe ou reste bloquée. Ce coefficient résulte d'un ajustement aux données observées. Il a été établi sur la betterave, culture pour laquelle les données les plus nombreuses et précises étaient enregistrées. Les premières simulations réalisées à l'aide de ce nouveau module indiquent l'importance des effets des états du lit de semences initiaux sur les taux de levées. L'effet de différences de forces dues aux masses des semences est plus limité mais non négligeable dans certaines situations. On a ensuite testé la possibilité de l'utiliser pour prévoir la levée d'autres cultures, dont la force avait été mesurée. L'extension à d'autres espèces donne des résultats encourageants mais nécessite un travail supplémentaire. La possibilité d'utiliser différents paramètres mesurés au laboratoire sur le sol pour pouvoir adapter le modèle proposé à d'autres types de sol est discutée.

The SIMPLE model can predict the emergence of various types of crop. It includes an empirical module predicting seedling emergence from under a crust. That module is based on the observation of the emergence of sugar beet crops in a field network with silt loam soils. This module is basic, but effective, and the purpose of this work is to allow the extension of forecasts to other sowing conditions and other crops (seedbed types, soil types, different characteristics of the seeds), given the huge problems in crop emergence resulting from the formation of crusts on the surface of seedbeds for various crops. Field experiments were carried out to characterize the different tactics used by the seedlings to break through (penetration, rupture and passage through cracks), according to the different types of crust, in the case of spring (sugar beet, flax and bean) and autumn crops (wheat). On the same plots, we characterized the dynamics of crust formation for the various initial states of seedbeds (aggregate size, soil water content) according to the rains cumulated since sowing. We also analysed soil crusting data acquired on a long term experiment, thus complementing our two experimentation campaigns. The crusts were characterized by their stage, their thickness and their resistance to penetration. Various quantities of cumulated rains, from 11mm for seedbeds mainly made of thin dry earth to 27mm for coarse soil surfaces, were determined in order to obtain the formation of structural crusts, which are already enough to penalize emergence if they dry out. Complementary experiments were led in the laboratory in order to obtain under simulated rainfalls more advanced stages of crust unobserved during the two years of experimentation in the field, and characterize them. We quantified the evolution of the forming network of cracks according to the degree of degradation and the moistening/desiccation alternation. Those data made it possible to parameterise and test geometrical models for the appearance of cracks. The forces exerted by the seedlings and their changes in time, according to the variations in the seed mass, the varieties and the species studied, were measured with force sensors in the laboratory. All those elements made it possible to build a new module integrating factors which had not been taken into account before when predicting emergence through crusts. This module integrates initial states for various seedbeds (structure and water content) and different cumulated rain values which lead to successive stages in the formation of the crust. With each type of crust, a characteristic distribution of resistances is associated. The maximum force of every seedling that comes to the surface is drawn at random in a distribution, then decreases with the age of the seedling. That value is compared day after day with the resistance of the material via a coefficient that makes it possible to determine whether the seedling will emerge or not. That coefficient results from an adjustment to the data observed. It was based on sugar beet, a crop for which numerous and precise data were recorded. The first simulations carried out with this new module show the importance of the effects of the initial states of the seedbeds on emergence rates. The effect of force differences due to the mass of the seeds is more limited (about 10% on average) but it reaches about 20% in certain sowing conditions. We also tested the possibility of using this coefficient for other crops, the force of which had been measured. The extension to other species gives encouraging results but requires additional work. We finally discuss the possibility of using various parameters measured on soils in the laboratory in order to be able to adapt the proposed model to other soil types., Le modèle SIMPLE prévoit les levées de différentes cultures. Il comporte un module de prévision des levées sous croûte empirique, basé sur l'observation de levées de betterave sucrière dans un réseau de parcelles pour un type de sol de limons argileux. Ce module est donc assez sommaire bien qu'efficace et l'objectif de ce travail était de pouvoir étendre les prévisions à d'autres conditions de semis et pour d'autres cultures (types de lits de semences, types de sols, différentes caractéristiques des semences) étant donné l'importance des problèmes de levée liés au développement de croûtes à la surface des lits de semences de différentes cultures. Nous avons entrepris de caractériser les types de franchissement des plantules (pénétration, rupture et passage par fissure) au champ face à différents types de croûtes pour des cultures de printemps (betterave, lin et haricot) et d'automne (blé). Nous avons caractérisé sur ces mêmes parcelles la dynamique de formation des croûtes pour les différents états initiaux des lits de semences (granulométrie, humidité au semis) en fonction des pluies cumulées depuis le semis, en complétant les observations de nos deux campagnes d'expérimentation par celles acquises sur un essai de longue durée. Les croûtes ont été caractérisées par leur faciès, leur épaisseur et leur résistance à la pénétration. Des quantités de pluies cumulées, allant de 11 mm pour des lits de semences fins et secs à 27 mm pour des états de surface grossiers ont été mises en évidence pour obtenir la formation de croûtes structurales, qui suffisent déjà à pénaliser les levées si elles se dessèchent. Des expérimentations complémentaires ont été menées au laboratoire pour obtenir sous simulateur de pluie et caractériser des stades plus avancés de croûtes non observés au cours des deux années d'expérimentation au champ. Nous avons quantifié l'évolution du réseau de fissures se formant en fonction du degré de dégradation et des alternances humectation-dessication. Ces données ont permis de paramétrer et tester des modèles géométriques d'apparition de fissures. Les forces exercées par les plantules et leurs variations au cours du temps en fonction de la masse, des variétés et des espèces étudiées ont été mesurées avec des capteurs de force au laboratoire. Les différents éléments acquis par ces expérimentations ont permis d'élaborer un module intégrant des facteurs de variations non pris en compte jusqu'alors pour prévoir la formation d'une croûte et les levées. Ce module intègre des états initiaux de lits de semences variés (structure et teneur en eau), des cumuls de pluie différenciés aboutissant à des stades successifs de formation de la croûte. A chaque type de croûte est associé une distribution de résistances caractéristique. Pour chaque plantule arrivant à la surface, sa force maximum est tirée au sort dans une distribution puis diminue avec l'âge de la plantule. Cette valeur est confrontée jour après jour à la résistance du matériau via un coefficient qui permet d'établir si la plante passe ou reste bloquée. Ce coefficient résulte d'un ajustement aux données observées. Il a été établi sur la betterave, culture pour laquelle les données les plus nombreuses et précises étaient enregistrées. Les premières simulations réalisées à l'aide de ce nouveau module indiquent l'importance des effets des états du lit de semences initiaux sur les taux de levées. L'effet de différences de forces dues aux masses des semences est plus limité mais non négligeable dans certaines situations. On a ensuite testé la possibilité de l'utiliser pour prévoir la levée d'autres cultures, dont la force avait été mesurée. L'extension à d'autres espèces donne des résultats encourageants mais nécessite un travail supplémentaire. La possibilité d'utiliser différents paramètres mesurés au laboratoire sur le sol pour pouvoir adapter le modèle proposé à d'autres types de sol est discutée.

The study was aimed at evaluating the effect of rainfall and tillage-induced soil surface characteristics on infiltration and runoff on a 2.8 ha catchment located in the central region of Senegal. This was done by simulating 30 min rain storms applied at a constant rate of about 70 mm h(-1), on 10 runoff microplots of 2 m(2), five being freshly harrowed perpendicularly to the slope and five along the slope (1%) of the catchment. Runoff was automatically recorded at the outlet of each plot. Hydraulic properties such as capillary sorptivity and hydraulic conductivity of the sandy loam soil close to saturation were determined by running 48 infiltration tests with a tension disc infiltrometer. That allowed the calculation of a mean characteristic pore size hydraulically active and a time to ponding. Superficial water storage capacity was estimated using data collected with an electronic relief meter. Because the soil was subject to surface crusting, crust-types as well as their spatial distribution within micro-plots and their evolution with time were identified and monitored by taking photographs at different times after tillage. The results showed that the surface crust-types as well as their tillage dependent dynamics greatly explain the decrease of hydraulic conductivity and sorptivity as the cumulative rainfall since tillage increases. The exponential decaying rates were found to be significantly greater for the soil harrowed along the slope (where the runoff crust-type covers more than 60% of the surface after 140 mm of rain) than across to the slope (where crusts are mainly of structural (60%) and erosion (40%) types). That makes ponding time smaller and runoff more important. Also it was shown that soil hydraulic properties after about 160 mm of rain were close to those of untilled plot not submitted to any rain. That indicates that the effects of tillage are short lived. (c) 2004 Elsevier B.V. All rights reserved.

Konya ovasında kaymak tabakası problemi bulunan alanlardan alınan toprak örneklerinin bazı özellikleri ile kırılma değerleri arasındaki ilişkiler incelenmiştir. Kay mak tabakası açısından en problemli olan toprağın bu bozuk özelliğini düzeltmek için on değişik ıslah maddesi denenmiş, etkili olan beş ıslah maddesi ile 100 günlük inkübasyon denemesi kurulmuştur. Kırılma değerini en fazla düşüren ve agregat stabi- litesini en fazla artıran iki ıslah maddesinin (çimento ve buğday şamam) buğdayın sürgün çıkışı ve toprağın penetrasyon direncine etkileri sera şartlarında incelenmiştir. En etkili bulunan ıslah maddesinin (çimento) oluşturduğu agregatlann donma- çözünmeden nasıl etkilendiği ortaya konmuştur. Ayrıca etkili ıslah maddesinin 30°C sıcaklıkta en fazla agregasyon artışı sağladığı saptanmıştır., In this study, from the stand point of soil crusting the most problematic soil in the research area among Konya-Çumra-Karapınar in Konya plain and the relationship between modulus of rupture and some soil properties was determined. Ten different ameliorative materials were added to the most problematic soil for decreasing modulus of rupture. A hundred days icubation study using the five effective ameliorative materi als was set up. This study was carried out under laboratory condition at about field ca pacity. It was found out that cement and wheat straw were the most effective amelio rative materials in reducing modulus of rupture and increasing aggregate stability. The influence of this two materials on wheat plant seedling emergence and penetration re sistance, under greenhouse condition were also studied. Finally the effects of freezing and thawing on the stability of the soil aggregate formed by cement were tested. In ad dition, the effects of the soil temperature on aggregation in the soil sample supplied with cement were tested at field capacity.

In Venezuela rainfall-induced crust is the main problem in obtaining adequate stands of sesame (Sesamwn indicum L.) The effectiveness of seven commercially available soil conditioners to prevent soil crusting and their influence on sesame seedling emergence was tested on Parlo silt loam. The chemicals were sprayed on the soil surface at different rates and dilutions . The rates were established to give about the same treatment cost. Two chemicals: a polyvinyl alcohol (Elvanol 71-30) and an anionic asphalt emulsion (Humofina B-2864) gave the greatest increase in sesame seedling emergence. Thereafter, the two chemicals were tested on two Utah soils (Parlo silt loam and Nibley silty clay loam) and four Venezuelan soils. Three of the Venezuelan soils (Turen silt loam, Nontilled Turen silt loam, and Agua Blanca silty clay loam) were sampled in areas actually being used for sesame production, but with problems of soil crusting. The fourth Venezuelan soil (Guanipa sand) was collected in an area where sesame has recently been introduced.