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

Cropping system factor levels
Two levels of cropping systems were compared: “intensive” and “moderate”. They differ mainly by winter
landcover and nutrient management. The “intensive” and “moderate” levels correspond respectively to the
practices observed during the 1995-1997 and 2001-2003 periods on the Frémeur catchment. In the time span
between these two periods, significant efforts were done by farmers to reduce soil N inputs. For each period,
farm-type, and landcover class, a typology of CMP was established based on the origin of the fertilizers
involved (mineral, organic) and the spread amounts (Durand et al., 2006). Each landcover class (for a given
period and farm-type) was usually characterized by three CMP modalities (up to eight for maize) and their
relative proportions were used as probabilities to combine landcover and CMPs in order to produce the data
entries for TNT2. Considering fertilization, the “moderate” level differs mainly from the “intensive” level by
a reduction of spread mineral and organic N along with a diminution of the grazed surface and excreta inputs
to grazed grasslands. Organic P inputs were estimated by the soil P model presented previously. Mineral P
inputs for the intensive cropping system were estimated a survey done in 2003 (Payraudeau et al., 2006)
from mineral P loads by farm-type and landcover class. For the moderate level, mineral P inputs were
systematically omitted.
The agricultural landcover dynamics were simulated at the field scale in two steps:
– Stochastree (Sorel et al., submitted; see also chapter III of this thesis), a model based on stochastic
decision trees was used to simulate summer crop transitions by considering temporal (crop rotation)
and spatial driving factors (farm-type crop area objectives, spatial distribution of crops around the
farmsteads, and sensitivity of crops to soil waterlogging). Stochastree was trained on the 1993-1998
period and the simulation started on year 2000, which was the most exhaustively surveyed year;
– winter landcover was simulated with a set of deterministic and stochastic expert rules, which
involved previous and subsequent summer landcover, derived from the Frémeur catchment survey
(Salmon-Monviola et al., submitted). Allocating winter landcover mainly consisted in managing the
harvested maize fields. In the “intensive” level, the fields were left as bare soils generally, whereas in
the “moderate” level, they were used to grow catch crops like Italian ray-grass or mustard, which
traps N and are then buried by a plowing before the next summer crop sowing (Baggs et al., 2000).
The observed and simulated areas of each crop were compared by a Wilcoxon test (p