Paysages virtuels et analyse de scénarios pour évaluer les impacts ...

The life-cycle analysis survey conducted in 2003 with the farmers (Payraudeau et al., 2006) located the
farmsteads of the catchment, and depicted a typology of the farming activities composed of four farm-types:
- dairy (13 farmsteads), which concerned 33% of the total agricultural area;
- pig (6 farmsteads), 34%;
- dairy-pig (10 farmsteads), 19%;
- other (8 farmsteads), 14%, which artificially grouped (i) farmsteads of unknown activity, and (ii)
farmsteads with marginal farm activities.
Given the locations of the farmsteads (Payraudeau et al., 2006) and of their fields (Bordenave et al., 2005),
we associated a distance-to-farmstead to each field. The euclidean distance between the farmstead and the
centroid of the field polygons seemed a reasonable estimator of the accessibility of the fields because the
road network is rather dense and isotropic in this catchment. Fig. III.19.c shows the distance-to-farmstead of
the field in year 2000 and the road network of Naizin.
The nine original classes of soil waterlogging described in the soil map of the catchment (Walter and Curmi,
1998) were aggregated into the following classes (Fig. III.19.b):
- 1 (well-drained): no soil redoximorphic feature within the first 80 cm. This class represents 43% of
the total agricultural area;
- 2 (intermediate): weak soil redoximorphic features at less than 80 cm, 44%;
- 3 (waterlogged): intense redoximorphic features from the surface, 13%.
Each field was characterized by the dominant soil hydromorphy class represented within its limits. This
waterlogging attribute can be considered constant through time since most of the artificial drainage systems
were setup before 1993 in this catchment.
The topography of the catchment is relatively smooth (mean slope is below 5%, with maximal values of
14%), its highest point is at 136 m above the sea level while its outlet is at 60 m (Walter and Curmi, 1998).
The overall gentle slopes are not limiting for the preparation and cultivation of crops.
Agronomic constraints on the agricultural landuse of Naizin
Beside the crop succession itself, which is known to play a key role in the farm management of soil fertility
and nutrients dynamics (Thornton and Jones, 1998), we tested the relevance of four presumed spatial driving
factors of crop transition in Naizin:
- farm-type: farmstead sharing the same activity and main production (milk, pigs, etc.) are submitted
to the similar constraints (Thenail and Baudry, 2004). Moreover, the life-cycle analysis also
revealed that fields of some farmsteads of Naizin were outside the surveyed area, and similarly, that
some surveyed fields of Naizin belonged to farmsteads located just outside the catchment divides.
The existence of these sparse field patterns made it more relevant to study the agricultural driving
factors of crop transition at the farm-type scale instead of the farm scale;
III. Stochastree, un modèle de successions de cultures basé sur des arbres de décision stochastique – p. 76