Yves Le Bissonnais

Mapping erosion risk for cultivated soil in France

Abstract Surface runoff and soil erosion are major threats to sustainable agriculture and mapping regional erosion risk is increasingly needed by national and international environment agencies. Because erosion results from the interaction of several parameters which vary in space and time, no simple model can take into account all relevant factors, particularly in cultivated areas where human influences are predominant. The aim of this work is to develop a methodology based on present knowledge and available data for the evaluation of erosion risk at national scale. The various erosion factors have been graded for different geographical situations and erosion mechanisms have been expressed with the help of expert decision. The various erosion types observed in France had been previously classified. Soil crustability is considered as a key factor in runoff and erosion risk on cultivated soils. A geographical database has been created for French territory, and a model of erosion risk has been developed within a Geographical Information System (GIS). The model uses expert rules to combine data on land use (CORINE Land Cover database), soil crustability and soil erodibility (determined by pedotransfer rules from the French soil database), relief (Digital Elevation Model from the National Geographic Institute) and meteorological data from Meteo-France at the scale of 250×250 m pixels. Results are spatially aggregated using various administrative or environmental units. The main areas affected by erosion risk are the north, west and east of the Paris basin with intensive agriculture on crusting soils, the Rhone valley and southwest of France where vineyard or spring crops cover large areas. Other areas like Britany, the south of the Massif Central or the Mediterranean area are moderately affected. Areas with a permanent cover of woodland or grassland show a low erosion risk.

Runoff features for interrill erosion at different rainfall intensities, slope lengths, and gradients in an agricultural loessial hillslope

In agricultural landscapes, factors affecting V under steady-state conditions of infiltration are well docuVarious interactions, particularly those existing between the rainmented (Kinnell, 2000). The effect of slope angle on fall intensity, the slope gradient, the slope length, and the tillage runoff for interrill erosion has also been fully investisupposedly can affect the runoff features for interrill erosion. Despite gated. Runoff may increase at steeper slopes because numerous studies, their effect on runoff production and pathways and the resulting soil losses have seldom been analyzed. This is especially of a decrease of the ponds’ ability to retain water (Fox true under field and tillage conditions. This study investigated the et al., 1997). Several authors have confirmed the influeffect of rainfall intensity, slope length, and gradient on runoff amount ence of slope gradient on soil losses by interrill erosion and pathways for interrill erosion in tilled fields. Runoff features and (Huang, 1995; Fox and Bryan, 1999). The increase of soil losses were evaluated on bounded plots of 1and 5-m length detachment and transport of soil particles with higher located on 4 to 8% slope gradients, and under natural and simulated flow velocity has already been demonstrated by laborarainfalls with intensities ranging from 1.5 to 30 mm h 1. Runoff coeffitory experiments (Torri and Poesen, 1992; Fox and cients (R) ranged from 34 to 98% whereas sediment concentrations Bryan, 1999) and field investigations (e.g., Chaplot and (SC) varied from 2.9 to 49 g l 1. The runoff coefficient was affected Le Bissonnnais, 2000). Fox and Bryan (1999) argued: by all three factors: rainfall intensity (r 0.48; P 0.0001), slope gradient (r 0.51; P 0.0001) and slope length (r 0.29; P 0.02); “For a constant runoff rate rain impacted flow erosion whereas SC was correlated with only rainfall intensity (r 0.48; P increased roughly with the square root of the slope 0.0001) and slope length (r 0.44; P 0.0004). The runoff coefficient gradient (2 to 40%). Soil losses were correlated (r2 and SC ratios between 1and 5-m long plots were systematically 0.81) with runoff velocity.” However, Govers (1990) greater for the intermediate rainfall intensity. Runoff features mainly showed that slope gradient might have a significant negaffected by tillage implements may explain higher interrill erosion at ative effect on runoff and erosion because of differential longer and steeper slopes. Lower differences between 1and 5-m plots soil cracking. Moss and Green (1983) attributed a deat high rainfall intensity may reflect greater ponded runoff absorbing crease of interrill erosion at the steepest slopes to an raindrop kinetic energy and lowering detachment and transport proincrease in the water depth in ponds. When this depth cesses. Finally, the effects of rainfall intensity, slope length, and gradiexceeds the diameter of two drops it protects the soil ent and tillage are discussed in respect of possible erosion processes operating in the experiments. surface from drop impact (Kinnell and Cummings, 1993). It is also well known that R and SC increase with the increase of rainfall intensity (e.g., Wischmeier and W movement within landscapes is fundamental Smith, 1978; Fraser et al., 1999) because of: (i) the augfor the prediction of soil erosion and the conservamentation of runoff fraction of rainfall (Williams et al., tion of water (Mermut et al., 1997). Rainwater not only 2000); (ii) the augmentation of soil detachment with moves up and down through the soil profiles and saproincreasing drop detachment forces (Kinnell, 1990; Torri lites by percolation and evaporation, but also moves and Poesen, 1992; Mermut et al., 1997) and (iii) a better laterally on the surface and through the subsurface. Retransport and remobilization of particles by raindistribution of soil particles by surface flow remains the impacted flow (Hairsine and Rose, 1992; Chaplot and most important factor in tropical and intertropical areas Le Bissonnais, 2000). However, in some conditions, esbecause of extreme rain events (e.g., Puigdefabregas et pecially when a crust surface was formed quickly, no al., 1999). In temperate climates, overland flow predomsignificant relation has been found between soil loss and inates when the natural infiltration capacity of the soil rainfall parameters from 10 to 103 mm h 1 (Uson and surface is altered (Valentin and Bresson, 1992). Ramos, 2001). Soil loss rate (SL, mass per surface and time unit) Limited studies have been conducted to investigate may be defined as: the effect of slope length on runoff for interrill erosion, especially under field conditions. Since overland flow SL V SC [1] velocity is important for soil detachment and transport With V, the volume of surface water per time unit and capacity, the interrill erosion rate should also be strongly SC, the mass of sediment per unit volume of water. influenced because longer slope lengths allow higher runoff velocities. Using field surveys Horton (1945) was V.A.M. Chaplot, IRDAmbassade de France, BP 06. VIENTIANE, one of the first to quantify the effects of the slope steepLAOS PDR. Y. Le Bissonnais, INRA, Science du Sol, Avenue De ness and length. He demonstrated that erosion increases lab Pomme de Pin, B.P. 20619, Ardon, 45166 Olivet cedex, France. Received 28 June 2001. *Corresponding author (chaplotird@laopdr. com). Abbreviations: CEC, cation-exchange capacity; DEM, digital elevation model; SC, sediment concentration; SL, soil loss rate. Published in Soil Sci. Soc. Am. J. 67:844–851 (2003).

Importance of surface sealing in the erosion of some soils from a mediterranean climate

Abstract Poor aggregate stability and high erodibility of soils found in Mediterranean climates are often due to low organic matter content. The low organic matter content is sometimes compensated for by iron oxides that stabilize soil structure. Low water stability of aggregates often determines the propensity for soils to form surface crusts and seals (wet crusts). Seals reduce soil infiltration rate and may increase erosion by increasing runoff. We conducted rainfall simulation experiments on 30 cm×30 cm plots inclined at 9% slope in which we measured soil splash and wash erosion, runoff and splash volume for seventeen California soils. The soils ranged in clay content from 8 to 36%, ESP from

The implications of spatial variability in surface seal hydraulic resistance for infiltration in a mound and depression microtopography

Abstract 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−1. 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 showed that the crusts formed on Villamblain were of a lower hydraulic resistance than those of Blosseville. It appears that small changes in runoff depth can significantly increase infiltration rate when structural crusts of lower hydraulic resistance are inundated. The effect was less important in Blosseville which formed seals of relatively high hydraulic resistance everywhere. The results provide a suitable explanation for field observations of increasing infiltration rate with either increasing rainfall intensity or runoff rate. The results also have implications for the relationships between surface roughness, surface water storage, and infiltration.

Contribution of multi-temporal SPOT data to the mapping of a soil erosion index. The case of the loamy plateaux of northern France

Abstract Loamy soils of the northern European loess belt commonly are exposed to erosion caused by concentrated runoff. Such runoff generates mud flows that, when strong, may create major problems because of the damage they cause to infrastructure. Using multi-temporal SPOT data and GIS technologies, a method is proposed and tested for mapping surfaces affected by runoff, as well as for evaluating the effectiveness of such remotely sensed index to infer erosion. This work is part of a major research effort, jointly undertaken by BRGM and INRA, which develops a predictive approach for monitoring erosion at a regional scale. Results have shown that the method for estimating surfaces affected by runoff, is quite reliable for areas underlain by loamy soil. However, the correlation between such surfaces and effective erosion, i.e. soil loss quantitative measurements, remains low, even though it confirms the possibility of using statellite data rather than other sources of information. It became clear that the conditions of low erodibility during the period of our study were a handicap for this type of validation; another problem is caused by the choice of the optimum observation period, which can vary as a result of winter rainfall events.

Soil aggregate stability in Mediterranean and tropical agro-ecosystems: effect of plant roots and soil characteristics

AimsOur aim was to determine the effect of soil characteristics and root traits on soil aggregate stability at an inter- and intra-site scale in a range of agro-ecosystems. We also evaluated the effect of soil depth and the type of land use on aggregate stability.MethodsSoil aggregate stability, soil physicochemical properties and fine root traits were measured along land use gradients (from monocultures to agroforestry systems and forests), at two soil depths at four sites (Mediterranean and tropical climates) with contrasting soils (Andosol, Ferralsol, Leptosol and Fluvisol).ResultsAggregate stability was much lower in deep than in surface soil layers, likely linked to lower soil organic carbon (SOC) and lower root mass density (RMD). Locally, and consistently in all sites, land use intensification degrades soil aggregate stability, mainly in surface soil layers. Soil organic carbon, cation exchange capacity and root traits: water-soluble compounds, lignin and medium root length proportion were the most important drivers of aggregate stability at the inter-site level, whereas SOC, root mass and root length densities (RMD, RLD) were the main drivers at the intra-site level.ConclusionsOverall, the data suggest different controls on soil aggregate stability globally (soil) and locally (roots). Conversion from forests to agricultural land will likely lead to greater C losses through a loss of aggregate stability and increased soil erosion.

Evaluating the Impact of the Spatial Distribution of Land Management Practices on Water Erosion: Case Study of a Mediterranean Catchment

Abstract: The spatial distribution of land management practices (LMPs), such as the use of vegetated filters, may have a strong impact on their efficiency in trapping sediments and pollutants. Distributed water erosion models help managers, planners, and policymakers optimize the efficiency of these LMPs regarding their location relative to water and sediment pathways. In this work, the authors analyzed the impact of the spatial distribution of LMPs using an existing distributed model and sensitivity analysis procedures. The distributed model that was used is a distributed single-event physically based water erosion model developed to calculate erosion rates and sediment flow for small (less than 10 km2) agricultural catchments. To measure the impact of the spatial distribution of LMPs, the authors developed a stochastic model that generates LMP locations over the entire catchment. The stochastic model has three input parameters: the density of LMPs, their downslope/ upslope location probability, and the probability density function shape controller. Because of its ability to account for the cross effects between parameters, the variance-based Sobol method was used to calculate the sensitivity of the soil loss ratio of a typical Mediterranean agricultural catchment (Roujan, southern France) to the LMP location model parameters. Three measurement points (two subcatchment outlets and the main outlet) were used to examine the spatially distributed effects of the LMP locations. The simulation results indicated that 70% of the variation of the net erosion is explained by variations in LMP density for the main outlet catchment, making LMP density the most sensitive parameter. However, the total Sobol sensitivity indices indicate a strong interaction among the three parameters when the density values are low (few LMPs are applied). Thus, although the density of the LMPs is the most sensitive parameter, their location may influence their global trapping efficiency in (real) cases where few LMPs are applied.

Impact of Global changes on soil vulnerability in the Mediterranean basin

Hydric erosion is one of the major causes of soil degradation. In semi-arid areas, where the soil cover is already shallow, the consequences are often irreversible on a historical time scale. Global warming and the land use changes expected during the 21st century are going to influence the soils deterioration and the erosion processes. In order to protect the soil resource under the current bioclimatic context and prevent the future consequences, it is essential to apprehend the erosion risk. Many studies developed soil erosion risk modeling methodologies at various scales from regional to Continental scale. The MESOEROS project is the first which aims to understand the soil loss risk on the whole Mediterranean basin for the current climate context and also for the predicting climate changes expected for the 21st century. Two models are used: MESALES (expert rules model) and PESERA (physical based model). Both provide the soil erosion risk into five classes. Model inputs; soils properties (crusting and erodibility), climate data, DEM and land use data; come from homogenized regional datasets that cover the whole study area. After being calibrated with watersheds data and the PESERA modeling on Europe, the two modeling results are analyzed. MESALES estimates Italia, Andalusia, Catalan and Aragon regions, western part of Greece and Balkan region as threatened areas while PESERA models the arable region of Castellan y Leon, Near East and the high atlas range in Morocco as subjected to an erosion risk. The two methods model parts of northern Morocco, center and European part of Turkey, Lebanon and northern Portugal at risk while southern France, Libyan coasts and southern Greece are never threatened. Analyses of the parameter influences on the models and the modeling validation allow understanding the integration of climate change on modeling results. MESALES and PESERA point out an evolution of the soil erosion risk between the 20th and the 21st centuries around the Mediterranean basin. The two models assess a global augmentation of the soil loss risk at the Mediterranean scale. They both show an increase - in intensity and surface - of the soil erosion risk on areas already sensitive during the 20th century.