No abstract

No abstract

No abstract

No abstract

No abstract

Concentrated flow erosion is the dominant form of winter erosion in northern France. This study correlates the ephemeral rill and gully volumes measured in 20 cultivated catchments (4-95 ha) for three consecutive winters with the size of the potential runoff-contributing areas. These areas were identified by characterizing soil surface state through crust development stage, importance of surface wheel tracks and roughness grade. A single and significant relationship was found between the size of runoff-contributing areas, estimated by this criterion, and the rill and gully volumes. This identified the proportion of the catchment area occupied by fields with a degraded surface structure as the main factor controlling the variability of erosion in a context of concentrated flow erosion on cultivated land. The extension of degraded areas was shown to be controlled by dynamic interactions between weather, land occupation and soil physical properties. This criterion accounts for the uneven distribution of rainfall in space and time. Morphological factors, such as talweg length and slope, are believed to determine part of the residual variability.

The optimization of tillage practices requires the evaluation of the long-term effects of cropping on changes in soil structure. A model is presented that simulates changes in soil structure in the tilled horizons of cultivated ®elds. The indicator of soil structure that we have modelled is based on a morphological description of the structure and the assessment of the percentage of severely compacted zones and clods showing no visible porosity in the tilled layer of a soil pro®le. The model takes into account the spatial variation of the structure induced by traf®c and tillage. Severely compacted zones are created under wheel tracks, they are cut and transferred during ploughing, and they are fragmented into ®ne soil during secondary tillage. The model was evaluated over 7 years in a ®eld trial on loamy soil in three experimental plots, where the crop sequences and ®eld operations induced contrasted changes in the percentage of severely compacted zones and clods. The model satisfactorily reproduced the changes with time of the indicator, and the correspondence between simulated and observed values was fairly good (r = 0.82, model ef®ciency = 0.61). However, some variations from one year to the next were poorly predicted. The sensitivity analysis did not suggest modi®cation to the parameter values but showed that the performance of the model could be improved if the fragmentation (due to weather and tillage) were more precisely described in the model. Nevertheless the present version of the model and the corresponding simulation program (SISOL) can be used to compare the effects of different technical choices on the evolution of soil structure.Correspondence: J. Roger-Estrade.

No abstract