In order to regionalize an erosion model based on a soil loss equation, the type of agricultural landuse and the percentage of ground cover play a major role. A supervised classification of multidate SPOT imagery has been used for mapping the crops, and vegetation indices have been derived from spectral data of each crop class in order to evaluate the soil coverage associated with these classes, in a hilly environment of the Quebec Appalachians in Canada.The relation between ground coverage and vegetation indices for each crop has been obtained by photographic and radiometric measurements on the ground at ten days interval throughout the growing season. Similarity between ground and satellite observations is reasonably good. Results of the field campaign show that, generally, vegetation indices overestimate the ground coverage at the beginning of the growing season and underestimate it at the end, with the appearance of senescence. These data will be integrated into a GIS for spatial mapping and modelling of erosion.

Crop management practices influence the amount and quality of organic matter and the state of aggregation in soils. The objective of this study was to determine the early changes in organic and microbial-biomass C contents and in water-stable aggregates of two loamy soils (Typic Fragiaquept and Typic Haplaquept) following plowing of a meadow and subsequent planting of either corn (Zea mays L.) or barley (Hordeum vulgare L.). The effects of 1, 2, and 3 yr of cropping with these two crops were compared with an undisturbed and a recently plowed meadow. Plowing decreased the organic and microbial-biomass C contents of the surface soil (0-6 cm) by an average of 40 to 50%, but, when the whole surface horizon (0-24 cm) was considered, there was no effect. Further cropping with either corn or barley resulted in no net loss of organic matter and microbial biomass but only a redistribution in the surface soil profile. Microbial-biomass C was not preferentially affected by cropping relative to total organic C. The mean-weight diameter by water-stable aggregates decreased not only with plowing but also with length of cropping both in the surface soil (0-6 cm) and throughout the top 24 cm. There was little difference in the specific effects of the two crops on organic C, microbial-biomass C, or aggregation.

Two airborne surveys, simulating the spectral and spatial resolution of the Thematic Mapper sensor of Landsat-5 were made during the summer of 1982 (10 June and 18 Aug.). The Agriculture Canada Research Station at Lennoxville was surveyed and two common field crops in Quebec were studied. The digital image analysis revealed two parameters which influence the spectral signatures of the vegetation: the alfalfa plots presented spectral variations relative to cover structure which were associated with growth stage, and the corn plot analysis suggests a relation between the drainage conditions of the soil and growth characteristics.Key words: Remote sensing, airborne surveys, Thematic Mapper simulation, field crops, digital image analysis, spectral and multidate analysis