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

Fal
33%
TempP
71%
PermP
100%
Forages Cereals Others
SRG
58%
GrMz
32%
Wh
20%
SlMz
24%
Fig. III.22: Box and arrow diagram of the generic matrix illustrating crop transitions for Naizin catchment, 1993-
1998. For clarity, only transition probabilities >10% were included, except for crops generated only by a
marginal transition probabilities (SRG, Rape, Fal, and PermP). Values within boxes are self-replacement
probabilities whereas the arrows indicates transition probabilities. Landcover classes are: SRG=seed ray-grass,
Fal=fallow, TempP=temporary pasture, PermP=permanent pasture, Wh=wheat, GrMz=grain maize,
SlMz=silage maize, Pot=potato, Rape=rapeseed, Veg=vegetable.
The generic transition matrix is represented in Fig. III.22 as a box-and-arrow diagram (Coppedge et al.,
2007). Cereals, and particularly wheat, played a major role in crop transitions since they were pointed by the
highest transition probabilities (wheat was the only crop targeted by probabilities superior than 35%). The
dynamics of cultivated areas on Naizin (Fig. III.20) showed that fallow, potato, rapeseed and seed ray-grass
were marginal and intermittent crops. This was modeled with marginal or low probabilities pointing to them
(meaning that other crops would seldom turn into them), and high probabilities to turn into a different crop
the following year. Permanent pastures have a 100% self-replacement probability and a marginal probability
originated from vegetables pointing to it, which means that the area occupied permanent pastures can only
increase with time. Such a crop in a Markov chain is called an “absorbing state” (Usher, 1979).
The probabilities of specific transition matrices differed mainly for forages (targeted with higher
probabilities for the dairy farm-type, lower probabilities for the pig farm-type) and secondarily for vegetables
(targeted with higher probabilities for the pig farm-type).
III. Stochastree, un modèle de successions de cultures basé sur des arbres de décision stochastique – p. 86
Pot
0%
Rape
0%
Veg
11%
Transition probabilities
x%
>35% (high)
15%-35% (medium)
10%-15% (low)