Tillage-induced spatial distribution of surface crusts on a sandy paleustult from Togo

The spatial distribution of crusts in coarse-textured soils and the processes affecting it are poorly documented, despite the potential impact of crusts on water infiltration. This study addresses the influence of tillage-induced microrelief on the morphology and spatial distribution of surface crusts in an oxic paleustult from southern Togo (West Africa). Replicate 1 m2 plots were exposed to 217 mm of natural rainfall during a 6-wk period, during which the surface topography was measured three times. Subsequently, 24 undisturbed crust samples were used for micromorphological analysis. The crusts exhibited a range of morphologies but were nevertheless adequately mapped and characterized according to two main types. Type 1 crusts (= runoff crusts) showed several superposed clay bands, 100 to 500 mm thick, buried within a micromass-depleted sand layer lesser or equal to 12 mm thick. Type 2 crusts (= erosion crusts) had an exposed clay band a few tenths of a millimeter thick. The spatial distribution of crusts at the time of sampling appeared better correlated with the initial than with the final microtopography of the plots. These findings suggest that crust distribution should be regarded as history dependent and that erosion and deposition processes largely governed the development of the crusts. This latter aspect is in agreement with the recent crust genesis model of Valentin and Bresson, as is the fact that clay bands in our plots were laterally continuous at all observational scales lesser or equal to 0.1 m. Other mechanisms proposed in the literature for the developement of clay bands did not seem able to account adequately for the observed pattern. (Résumé d'auteur

Integrating subgrid connectivity properties of the micro-topography in distributed runoff models, at the interrill scale

The spatial configuration of micro-topography affects the runoff connectivity at the interrill scale and, therefore, the shape of the hydrograph. In a previous study, we demonstrated the ability of the so-called Relative Surface Connection (RSC) function to capture, at the grid scale, the evolution of the contributing area as a function of the depression storage filling. However, this function neglects the effect of surface detention, which is proportional to the runoff rate and which must be taken into account if one wants to predict correctly the discharge dynamics. Therefore we tested two corrective procedures in association with the RSC function to integrate, at the grid scale, the effects of both depression storage and surface detention dynamics. The weighted-source corrective procedure consists in weighing the effective supply of water between depression storage and runoff using the RSC function. The weighted-surface corrective procedure consists in splitting a single grid into parallel independent strips whose sizes depend on the RSC function and which activate at various times and then participate to the global runoff production. Those methods allowed to mimic in a simple way and at the grid scale synthetical and experimental hydrographs for complex subgrid micro-topographies.The weighted-source and especially the weighted-surface corrective procedures improved the hydrograph prediction compared to the classical approach where runoff only starts when depression storage capacity is full. In a purely numerical framework with four runoff scenarios on highly contrasted micro-topographies, this improvement was reflected in a significant increase of the median Nash and Sutcliffe coefficients E-50 (E-50 = 0.29 for the classical approach, E-50 = 0.67 for the weighted-source procedure and E-50 = 0.94 for the weighted-surface procedure).For the depression storage filling, an alternative to the Linsley equation was found and allowed a better description of surface runoff before maximal depression storage was reached. This was reflected in an increase of the E-50 computed for 27 overland flow experiments under laboratory conditions and their equivalent model results(E-50 = 0.89 for the Linsley approach, E-50 = 0.94 with the proposed 'uniform' multiple-compartment conceptual approach, and E-50 = 0.85 for the classical approach where runoff only starts when depression storage capacity is full). (C) 2011 Elsevier B.V. All rights reserved

Targeting rural development interventions: Empirical agent-based modeling in Nigerien villages

The aim of this article is to analyze the impact of development interventions on the population of three Nigerien villages that differ in terms of their agro-ecological, social and economic characteristics. This is performed by simulating the behavior of individuals in an agent-based modeling framework which integrates the village characteristics as well as the family internal rules that condition access to economic and production activities. Villagers are differentiated according to the social and agro-ecological constraints they are subjected to. Two development project interventions are simulated, assuming no land scarcity: increasing the availability of inorganic fertilizers for farmers and an inventory credit technique based on millet grain. Two distinct approaches were used to model the rationale of farmers' decision making: gains or losses in economic value or gains or losses in within-village "reputation". Our results show that village populations do not respond en masse to development interventions. Reputation has little effect on the population behavior and should be considered more as a local proxy for wealth amongst villagers, suggesting the monetization of these societies. Populations involve themselves in the two simulated development interventions only at sites where savings are possible. Some level of household food security and investment capacity is actually required to take part in the development interventions, which are largely conditioned by family manpower and size. As long as uncultivated land remains available in the village territory, support for inorganic fertilizers has little impact in the absence of any intensification process. Inventory credit engages a maximum of 25% of the population at the site with medium agro-ecological conditions. Therefore, both interventions should be viewed as a potential support tool for a limited part of the population capable of going beyond the survival level, but not as a generic poverty-alleviation panacea.Farming systems Individual agent-based model Niger Sahel Development interventions

On-farm evaluation of field-scale soil losses by wind erosion under traditional management in the Sahel

In the West African Sahel, few direct estimates are currently available on the extent of soil losses by wind erosion in traditionally managed farmers fields. Field scale sediment balances in western Niger were derived from airborne sediment fluxes measured using BSNE sand traps. Results from a first study in 1997 indicated that sediment fluxes in a cultivated field increased linearly over distances = 80 m, irrespective of wind power. Sediment deposition in an adjacent fallow was well described by an exponential decay function with a near constant trapping efficiency coefficient of 0.1 m-1 for incoming sediment fluxes = 10 kg m-1. Mass balances up to -17.5 and + 10.5 Mg ha-1 were measured in a single storm in the field and fallow, respectively. Starting in 1998, a second study was setup in a newly cleared, 8 ha farmer's field equipped at 87 locations with BSNE sand traps. Whereas a net sediment balance of +5.4 Mg ha-1 was measured in 1998, the experimental field was subject to a net soil loss of -5.0 Mg ha-1 in 1999. This was attributed to changes in ground cover and differences in sediment influx from adjacent fields

Electrical resistivity in a loamy soil: Identification of the appropriate pedo-electrical model

Electrical resistance tomography (ERT) can be used for the noninvasive characterization of soil moisture and soil structural heterogeneity. Any attempt to relate electrical resistivity measurements to soil moisture content or soil bulk density, however, must rely on a "pedo-electrical" function, i.e., a conductivity model for soils. This study aimed to test five pedo-electrical models for their ability to reproduce electrical resistivity as measured by ERT in a silt loam soil sample across a range of moisture and bulk density values. The Waxman and Smits model, the Revil model, the volume-averaging (VA) model, the Rhoades model, and the Mojid model were inverted within a Bayesian framework, thereby identifying not only the optimal parameter set but also parameter uncertainty and its effect on model prediction. The VA model outperformed the other models in terms of both fit and parameter consistency with respect to independent estimates of surface conductivity obtained with published pedotransfer functions. Sensitivity of the electrical resistivity was then studied by means of the calibrated VA model, revealing an approximately 1.5 times higher sensitivity to soil moisture content than to soil bulk density. In addition, the sensitivity of electrical resistivity to soil moisture and soil bulk density was found to increase as soil moisture and bulk density decreased. The VA model calibrated on the basis of resistivity measurements appeared to simulate relatively well the measured soil moisture content for electrical resistivity values < 100 Omega m. As opposed to water content, the soil porosity was badly approximated by the model. It appears therefore that ERT is more suitable for detecting heterogeneity in soil water content than differences in soil bulk density

Estimation of air quality degradation due to Saharan dust at Nouakchott, Mauritania, from horizontal visibility data

It is now irrefutable that air pollution caused by large amounts of Total Suspended Particulates (TSP) and respiratory particulates or Particulate Matter less than 10 mu m in aerodynamic diameter (PM10) has numerous undesired consequences on human health. Air quality degradation far from the African continent, in the US and in Europe, caused by high concentrations of African dust, is seen as a major threat even though most of these countries are very distant from the Sahara. Surprisingly, no estimates of TSP or PM10 levels near the Saharan dust source are available. Based on horizontal visibility observations which are reduced by the presence of dust in the atmosphere, TSP and PM10 levels are estimated throughout the year 2000 at Nouakchott-Airport, Mauritania, using relations found in the literature. It appears that concentrations of particles are significant both in terms magnitude and frequency, as the 24-hour PM10 thresholds established by the US EPA National Ambient Air Quality Standards and the EU Limits Values for Air Quality were exceeded 86 and 137 times, respectively. The average annual concentration is far above air quality standards and estimated at 159 mu g m(-3) for TSP and 108 mu g m(-3) for PM10. These very high particulate levels are likely to represent an important public health hazard and should be considered as a major environmental risk

Processes of structural crust formation on coarse-textured soils

Summary In order to understand better the mechanisms of structural crust formation on coarse-textured soils, mixtures of 92.5% sand and 7.5% Ca2+-kaolinite are exposed to simulated rainfall in experiments- involving a range of sand particle sizes, drop diameters and drop kinetic energies. For each combination of these variables, clay illuviation is measured and the changes in surface structure are observed on thin sections. The intensity of clay illuviation is inversely proportional to sand particle size and is shown to be determined by both rainfall and drop kinetic energy. A washed-out layer forms at the surface of all samples exposed to drop impact. The lower boundary of this layer is generally marked by a relative concentration of clay micro-aggregates in the form of a band. These micro-aggregates are probably fragments derived from grain coatings initially present in the washed-out layer. They accumulate at a depth that increases, on average, with drop kinetic energy and with the particle size of the sand fraction. Based on similarities with natural crusts, it is proposed that clay-band formation in structural crusts formed on coarse-textured soils may be initiated by the displacement of micro-aggregates or other small particles from the washed-out layer, followed by their accumulation due to mechanical straining. The experiments also point to the potential importance of the particle size of the sand fraction on the morphology of structural crusts