Runoff may be reduced by temporal water storage in depressions at the soil surface. Depressional storage is difficult to measure and is usually estimated from some roughness index. The objective of this study was to compare the ability of selected roughness indices to describe maximum depressional storage (MDS). Height measurements were taken on 221 tilled soil surfaces across a range of roughnesses. Maximum depressional storage was determined from digital elevation models (DEMs). The MDS values ranged from 0 to 13 mm. Five roughness indices were calculated from transects across these DEMs: random roughness (RR), tortuosity (T), limiting elevation difference (LD) and slope (LS), and mean upslope depression (MUD). Regression analysis of MDS on each of five roughness indices showed that RR best described depressional storage Prediction of MDS in the field based on RR has an uncertainty of ± 3 mm (95% confidence interval). Variation was due to RR and its nonspatial nature. To improve predictions of MDS, the spatial configuration of the surface has to be taken into account.

Summary Tillage and traffic modify soil porosity and pore size distribution, leading to changes in the unsaturated hydraulic properties of the tilled layer. These changes are still difficult to characterize. We have investigated the effect of compaction on the change in the soil porosity and its consequences for water retention and hydraulic conductivity. A freshly tilled layer and a soil layer compacted by wheel tracks were created in a silty soil to obtain contrasting bulk densities (1.17 and 1.63 g cm−3, respectively). Soil porosity was analysed by mercury porosimetry, and scanning electron microscopy was used to distinguish between the textural pore space and the structural pore space. The laboratory method of Wind (direct evaporation) was used to measure the hydraulic properties in the tensiometric range. For water potentials < −20 kPa, the compacted layer retained more water than did the uncompacted layer, but the relation between the hydraulic conductivity and the water ratio (the volume of water per unit volume of solid phase) was not affected by the change in bulk density. Compaction did not affect the textural porosity (i.e. matrix porosity), but it created relict structural pores accessible only through the micropores of the matrix. These relict structural pores could be the reason for the change in the hydraulic properties due to compaction. They can be used as an indicator of the consequences of compaction on unsaturated hydraulic properties. The modification of the pore geometry during compaction results not only from a decrease in the volume of structural pores but also from a change in the relation between the textural pores and the remaining structural pores.

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The two-compartment model of Hénin-Dupuis (1945) was tested to track the medium-term (13 years) evolution of C reserves in the silty soils of the rolling pampas in Argentina for various crop rotations. The coefficient of annual mineralisation, theoretically constant in the model, depends in fact on the amount of organic residues returned. The model is thus open to question. A simple model with three carbon fractions (harvest residues, active fraction and stable fraction) is proposed. It enables the changes in the soil organic reserves to be well simulated. However, a simple statistical fit of the three parameters of the model gives an infinite number of solutions. Use of the 13 C natural abundance method allows evolution of the young and old soil carbon fractions to be separated and tracked, and to determine the three parameters unambiguously. The results show that the stable fraction represents most of the initial soil organic C reserve: 60-68 %. The humification parameter increases in proportion to the lignin concentration of the crop residues. Intensification of soil tillage accelerates the mineralisation of the soil organic matter. (© Inra/Elsevier, Paris.) soil carbon / model / 13 C abundance / rolling pampas / cropping Résumé-Modélisation des l'évolution des stocks de carbone des sols de la Pampa argentine pour différentes rotations culturales. Le modèle bicompartimental de Hénin-Dupuis (1945) a été évalué pour suivre l'évolution à moyen terme (13 ans) des stocks de C dans des sols limoneux de la Pampa ondulée en Argentine, pour diverses rotations de culture. Le coefficient de minéralisation annuel, théoriquement constant dans le modèle, dépend en fait du niveau des restitutions organiques. Le modèle est donc ainsi remis en cause. Un modèle simple à trois compartiments carbonés (résidus de récolte, fraction active et fraction stable) est proposé. Il permet de bien simuler l'évolution des stocks organiques du sol. Cependant, un simple ajustement statistique des trois paramètres du modèle donne une infinité de solutions. L'utilisation de la méthode d'abondance naturelle en 13 C permet de séparer et de suivre les évolutions de carbone jeune et du carbone ancien du sol, et de pouvoir déterminer les trois paramètres de façon univoque. Les résultats obtenus indiquent que la fraction stable représenterait la plus grande partie du stock de C organique initial: 60 à 68 %. Le rendement d'humification augmente proportionnellement à la concentration en lignine des résidus de récolte. L'intensification du travail du sol induit une accélération de la minéralisation de la matière organique du sol. (© Inra/Elsevier, Paris.

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