Geography

The influence of slope gradient on erosion rate differs for rill and interrill conditions. Rill erosion increases substantially more with increasing slope gradient than interrill erosion. Combining the two erosion processes into single data sets led to the development of regression equations Ž . e.g., USLE that overestimated the effect of slope gradient on erosion rate for low slope gradients and short slopes. This study investigated the change in interrill erosion rate with slope gradient Ž . and examined its relationship to runoff velocity. A sandy loam grey brown luvisol was packed in 100 = 40 = 10 cm 3 soil trays and subjected to simulated rainfall for a period of 75 min. Rain-impacted flow erosion and downslope splash were monitored, and runoff velocity measurements were made at three positions within the flume. Downslope splash erosion never accounted for more than 20% of the total erosion. Rain-impacted flow erosion peaked early in the simulation then decreased to a constant rate; erosion rate was therefore probably detachment-limited. For a constant runoff rate, rain-impacted flow erosion increased roughly with the square root of slope Ž . gradient, as did the runoff velocity. Soil loss was correlated 0.81 with runoff velocity under the experimental conditions. q

Mediterranean environments have been subject to major land cover change since the end of the second world war. Housing, agricultural activities, forests, green spaces and other land uses have shifted due to urbanisation and tourism. These changes influence runoff, and municipal authorities often cannot estimate the net impact of complex land cover transitions. During this period, elected representatives have become increasingly sensitive to the risks of flooding and have implemented a number of channel management strategies. The main objective of this case study was to analyse the impact of land cover change on total storm runoff between 1950 and 2003 in a Mediterranean catchment near St Tropez, France. A secondary objective was to compare these changes to the impacts of channel management on bankfull discharge. Aerial photographs were used to classify land cover in 3 urban categories, vineyards and bare soil, forests, and green spaces. Stream discharge was estimated using a distributed event based total runoff approach. After validating the model for a large winter event (114 mm) for 1982, runoff was calculated for the same event for 1950 and 2003. Land cover changes occurred mainly in the alluvial plain area. Total gauge catchment urban area increased from 30.1 ha to 393.8 between 1950 and 2003 at the expense mainly of agricultural land, but this was compensated in part by an increase in grassed area. Some of the loss in vineyards was replaced by clearing forested land on the first hills close to the plain. Bank stabilisation and channel maintenance since the 1980's reduced surface roughness and increased channel area, thereby greatly increasing bankfull discharge. While the impact of urbanisation on runoff was small, channel management effects increased bankfull discharge substantially. Flood damage from extreme events was not studied here.

Soil surface crusting has a major impact on water infiltration and erosion in many soils. Considerable progress has been made in describing crusting processes and in modelling the impact of crusting on infiltration. Most studies, however, have neglected the high spatial variability in crust characteristics observed in the field. The objective of this experiment was to determine the influence of runoff depth on infiltration rate in the presence of a surface seal varying in hydraulic characteristics with microtopography. The Blosseville silt loam has a low aggregate stability and forms crusts readily. The Villamblain silty clay loam has a greater aggregate stability due to its greater clay and organic matter contents, and it is more resistant to aggregate breakdown processes under rainfall. Samples of the soils were sieved to retain aggregates less than 2.0 cm and packed in 50 = 50 = 15 cm soil trays. The trays were surrounded by a 10 cm soil border to compensate for splash loss. After molding the surface into a mound and depression microtopography, the samples were subjected to simulated rainfall at an intensity of 22.8 mm h y1 . Hourly measurements of surface roughness showed that the original roughness was smoothed out due to the infilling of depressions by sediments detached from the mounds. For the final hour, runon was added to the top of the soil tray to increase the runoff rate and depth. For both soils, infiltration rate increased more than could be attributed to the increased ponding pressure head. The change in infiltration rate was particularly great for Villamblain. The measurements of hydraulic resistance showed that structural crusts had a lower hydraulic resistance than sedimentary crusts. They also ) Corresponding author. 0341-8162r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.

Major forest fires occur periodically in the Mediterranean environment. The impact of forest fires on soil erosion and runoff processes has been studied extensively, and it has been demonstrated that the first months after a fire represent a high risk period. Erosion control strategies must be established in the weeks following a forest fire to reduce fire impacts during the critical phase. Both dead and live vegetation techniques can be used to either reduce soil loss from slopes or trap sediments in streams before they enter the main channel: these include log erosion barriers, log debris dams and grass filter strips. The major challenge is defining where to position the measures quickly in a catchment where several thousand hectares have been burned. The method proposed here is based, firstly, on a simple soil erosion model that identifies critical erosion zones, and secondly, on a more detailed analysis of high risk slopes according to their potential impact on the main river channel, the value of the soil resource to be protected, the availability of pine logs, and accessibility to the site. Measurements of suspended sediment load in the main channel and tributaries confirmed that the erosion risk map was suitable for the purposes of the study. Many of the recommendations suggested were implemented and a study is underway to evaluate their effectiveness in reducing erosion and trapping sediments.

Soil crusting decreases infiltration, increases erosion, and impedes vegetation establishment, so reducing the impact of crusting is of major importance in combating desertification. Although surface crusting has been the subject of considerable research over the past 50 years or more, the practical management of soil crusts remains a challenge for many dryland communities. Crusting occurs in two steps, an initial aggregate breakdown period that occurs under rainfall and a subsequent hardening phase during drying. Several factors influence crust development, but the single most important one is soil aggregate stability. Strategies to reduce crusting can be based either on protecting the surface from raindrop impact or improving aggregate stability, or a combination of both. However, crust control is labor and/or capital intensive and must be thought out clearly in terms of the benefits to be achieved.

Effective erosion control requires both a sound understanding of the physical context and sufficient motivation on the part of farmers to act, and in Europe this almost inevitably depends on financial subsidies. In south-west France, intense spring and summer storms lead to large scale sediment deposition on roadways, and cleanup costs are a major concern for local authorities. Current erosion control subsidy rates are not sufficient to encourage farmers to limit erosion and reduce cleanup costs. The objective of this study was to define the topographic characteristics which control sediment deposition on roadways in order to compare the cost of planting grass strips along these roads to current cleanup costs. Topographic conditions were characterised by midslope inclination (with a range of from 6 to 8%) and distance of this slope to the road (range of 50-200 m), and these were defined by a combination of field visits, air photos, and GIS techniques. The effect of grass buffer strips on sediment deposition was estimated using the revised universal soil loss equation (RUSLE). The financial benefits of planting grass strips were based on the estimated efficiency of the buffer strips and the length of roads affected according to the topographic conditions. More than 70% of roads requiring regular cleanup occur within 200 m of a midlsope inclination of 6%. Errors in predicting sediment deposition sites according to topography occur mainly when small scale changes in topography are beneath the resolution of the Digital Elevation Model (DEM) or when sediment deposition occurs due to the intersection of talwegs with roads. Modelling the effects of grass strips on soil erosion rates suggests that buffer strips of 12 or 24 m wide reduce sediments leaving the field by 32 and 54%, respectively. Planting grass buffer strips along predicted problem road sections can reduce cleanup costs associated with sediment deposition. Savings in the first year of planting are in the order of about 2% of current estimated cleanup costs (100,000 ) for 12 m wide strips, and this amount increases to almost 35% in subsequent years for 24 m strips. Savings in cleanup costs can be used to increase erosion control subsidies, thereby ensuring a higher level of farmer participation. In addition to reducing sediment deposition on roadways, the grass buffer strips serve as demonstration plots for the farming community. The strategy does not directly eliminate soil degradation upslope, but it is a first step towards better erosion control.

Forest fire risk is the result of two distinct but interrelated factors. The first depends on meteorological conditions and fuel characteristics. These conditions form what is called the ''dynamic danger'' due to their evolutionary nature. The second factor is related to the probability of fire ignition. This can be either natural, as in the case of lightning, or human. Approximately 90% of fire ignitions between 1991 and 2003 in the Alpes-Maritimes were of human origin. It is therefore reasonable to attempt to estimate the probability of fire ignition as a function of location with respect to human settlements since it is in proximity to human activities that most fires find their origin.

Forest fires dramatically alter vegetation cover, soil properties and hydraulic characteristics of a catchment. These properties are of significant importance for runoff and erosion processes. Catchment response time therefore tends to be shorter for burned areas, and peak runoff and erosion rates are both greater than for pre-fire conditions. As a consequence, there is a greater risk of flooding downstream, river sediment loads are greater, and soils are degraded by the loss of sediments and attached nutrients.

Micromorphological techniques have frequently been used to describe processes leading to surface crusting. Image analysis can be helpful in quantifying changes in porosity necessary for estimating hydraulic parameters in crust infiltration models. The objective of this study was to quantify changes in pore area and frequency for three pore shapes (round, irregular, and elongate) and four size classes and >500 Am). Porosity decreased exponentially as the surface was approached from a depth of about 10 mm. The number of pores was roughly inversely proportional to pore area as larger pores were fragmented into numerous small pores. Intermediate-sized (350 -500 Am) elongate pores were the most sensitive to crusting processes, and these were divided into small (50 -200 Am) and small intermediate (200 -350 Am) round pores. Changes in pore shape and size distribution make estimating crust hydraulic conductivity and thickness extremely difficult, but a thickness of 5 mm appears to be a reasonable estimate for modelling purposes. D

Forest fires are common in Mediterranean environments and may become increasingly more frequent as the climate changes. Destruction of the forest cover and litter layer leads to greater overland flow and increased erosion rates. The greatest risk occurs during the first rainstorms following a major fire, so local authorities must act quickly to put erosion control methods in place in order to avoid excessive post-fire sediment loads in river channels. Deciding on which methods to use requires accurate knowledge of their impact on sediment load and an estimate of their cost efficiency. The objective of this study was to evaluate the efficiency of Log Debris Dams (LDDs) and a sedimentation basin for their effectiveness in trapping sediments. Paired sub-catchments were studied to quantify the amount of sediments trapped in stream channels by a series of LDDs and a sedimentation basin. Cost efficiency was evaluated for each of the measures as a function of the cost per unit volume of sediments trapped. In addition, grain size analyses were performed to characterise the nature of the sediments trapped. A third sediment trapping method, Log Erosion Barriers (LEBs) was evaluated more superficially than the first two and conclusions regarding this method are tentative. LDDs trapped a mean volume of 1.57 m 3 per unit (median ¼ 1.28 m 3 ); mean LDD height was 105.4 cm (std. dev. ¼ 21.9 cm), and mean height of trapped sediments was only 50.0 cm (std. dev. ¼ 22.9 cm), showing that the traps were only half filled. Sediment height was limited by the presence of gaps between logs or branches that allowed runoff to flow through. Comparison of the textural characteristics of slope and trapped sediments showed distinct sorting: particles greater than 20 mm were not mobilised from the slopes during the study period, sediments in the medium to coarse sand size fractions were trapped preferentially by the LDDs, and sediments in the sedimentation basin were enriched by clay and silt sized (<0.050 mm) particles as coarser sediments were trapped upstream by the LDDs. Cost efficiency of LDDs was estimated at about 143 V m À3 for the LDDs and 217 V m À3 for the sedimentation basin at the time of sampling. The LDDs are therefore a cost effective method of trapping sediments, but they can only be used when pine trees or straight-trunked trees are locally available. In this case, they should be combined with LEBs, which had a cost efficiency estimated at about 250 V m À3 . Installation of the LEBs had not been optimised and they have the advantage of trapping sediments on the slopes where they can continue to play an ecological role, so this method can give better results with more care. Sedimentation basins can be emptied if necessary and are useful in areas where pine trees are not available and where the site can be secured.

Soil conservation is a major concern for catchments affected by forest fires. The lack of vegetation cover and the development of soil water repellency increase the risk of topsoil erosion. This paper evaluates two soil conditioners (a wetting agent and a polyvinyl acetate) for limiting overland flow and erosion in inter-rill areas. Unburned and burned soil samples were treated with one or both soil conditioners. The effects of these treatments on run-off, water retention, erosion and plant growth were assessed using laboratory rainfall simulations. Polyvinyl acetate had little impact on water retention but was effective in reducing soil loss. The surfactant had little impact on water run-off or soil loss but substantially improved water retention and plant biomass production. Application of soil conditioners on post-fire areas could be a valuable technique in a soil conservation strategy. To maximize their benefits, soil conditioners could be applied with seeding using hydroseeding techniques and be limited to particular areas, such as paths and roadsides. Laboratory results indicate that field testing should also be carried out.

In South-East France, soil erosion is common in vineyards and a suitable strategy to combat erosion depends on accurate knowledge of its spatial distribution within a catchment. Soil erosion risk was mapped by measuring rill erosion within a reference zone and the relationships identified were then extrapolated to all the vineyards in the catchment.

Upscaling from a catchment scale to a regional scale is generally rendered difficult by the lack of relevant and precise data at the larger scale. In this case, a winter rill erosion hazard map was produced for the Nord-Pasde-Calais region using a linear regression erosion model originally designed for the catchment scale. Upscaling entailed adapting and applying the model at the county scale for all of the counties within the region. In upscaling the model, the difficulties associated with the nature of the data were dealt with in three stages: (1) the modification of the model for the county scale as a function of the nature of the data available, (2) an analysis of the influence of the spatial distribution of the data, (3) an analysis of the effect of the loss in precision of the data on the model output. Reference areas were used to verify the accuracy of the upscaling process before applying it to all of the counties in the region. In this case, the most significant limitation was the spatial coverage of the data: the basic administrative unit for which data is collected is the county, and it does not correspond to the erosion process scale which is the catchment. Defining erosion risk in terms of hazard categories rather than estimated erosion rates overcomes this difficulty to some extent. The use of reference areas provides several advantages in an upscaling procedure: these are mainly related to minimizing data collection and obtaining a reliable estimate of the accuracy of the predicted output.

A soil erosion risk map is an essential tool for soil conservation planning. It provides a general view of erosion problems and makes it possible to identify sectors requiring emergency action. The erosion risk map produced for this study was achieved by spatialising the Revised Universal Soil Loss Equation (RUSLE). The aim of this article is to demonstrate the steps involved in developing a soil erosion risk map.

Forest fires are a recurrent phenomenon in Mediterranean environments around the world. Each year, hundreds of thousands of hectares are burned, leaving the soil bare and unprotected against rainfall. Runoff and soil erosion risks are therefore greatly increased during the first year after a fire. This study used a partially distributed hydrological model to examine the potential effect of forest fire on runoff in a Mediterranean Catchment in SE France. The Hydrological Engineering Center’s Hydrological Modeling System (HEC-HMS) was used for this purpose. Daily discharge and rainfall data from 1975 to 1991 were available. Different short events were selected and two representative storms were modelled with and without a fire scar. The model was calibrated and validated with the observed data and was employed to simulate the after fire scenario. The model revealed that, after forest fire, the peak discharge in the stream can rise from 10% to 50 % while the general pattern of discharg...

Les mesures de pertes en eau et en terre, réalisées dans un contexte de région limoneuse intensivement cultivée du Nord-Ouest de la France à l&#39;échelle de 1 m2, sont présentées et commentées. L&#39;érosion hydrique est l&#39;un des processus majeurs dans la redistribution des particules solides du sol au sein des paysages. Aussi, afin d&#39;estimer dans un premier temps les zones sensibles et ensuite de lutter contre les exportations de terre par des mesures agro-environnementales, il est important de pouvoir apprécier l&#39;effet de l&#39;inclinaison de la pente. Malgré de nombreuses études sur le sujet, l&#39;influence de l&#39;angle de la pente sur le niveau d&#39;érosion inter-rigole n&#39;est pas clairement établi. L&#39;objectif de l&#39;étude est de déterminer au champ, sur une surface uniformément battue, l&#39;influence de l&#39;angle de la pente (2, 4, 8%) sur les pertes en terre. L&#39;expérimentation, menée à la fois sous pluies naturelles et simulées, nous a permis d...

Une carte du risque érosif est un outil indispensable dans une démarche de lutte contre l&#39;érosion. Elle permet en effet d&#39;avoir une vue d&#39;ensemble des problèmes et donc de localiser les secteurs nécessitant une intervention prioritaire. La carte des risques réalisée dans cette étude résulte de la spatialisation de la « Revised Universal Soil Loss Equation » (RUSLE). Le but de cet article est de montrer les différentes étapes nécessaires à la spatialisation de ce modèle et à la réalisation de la carte.

Over the past few decades, the influence of surface sealing on infiltration has been the subject of numerous publications. However, very few of these experiments have considered the effect of spatial variability in seal hydraulic characteristics on infiltration. Field and laboratory observations have demonstrated that seal characteristics vary considerably within a range of a few cm, and the changes are often related to microtopography. The objective of this experiment was to investigate the influence on infiltration of a seal with varying hydraulic properties in a surface depression. Two soils of different aggregate stability were used in the experiment: Blosseville silt loam and Villamblain silty clay loam. Soil columns (23 cm diameter by 30 cm length) were packed with aggregates of <2.0 cm and a 6 cm depression was formed in the soil surface. A sedimentary crust was formed in the centre of the depression, and the sample was rained upon for 60 min at a rainfall intensity of 35 mm h−1. Subsequently, water outflow at the base of the column and pressure heads in the column were measured during two ponding periods corresponding to depths of 0.5 and 4.0 cm. Water outflow rates increased with ponding depth substantially more than could be attributed to the increases in ponding pressure head and submerged area for both soils. The experiment demonstrated that where seal hydraulic conductivity varies with microtopography, a significant increase in infiltration rate occurs as areas of greater hydraulic conductivity are submerged. The increases in ponded pressure head and submerged area are negligible compared to the influence of spatial variability in seal hydraulic conductivity.