Always: a model to simulate the evolution of a silvopastoral plot according to soil, climate and management practices
Always : un modèle pour simuler l'évolution d'une parcelle sylvopastorale en fonction du sol, du climat et de l'itinéraire technique
Résumé
ALWAYS (Alternative Land-use With AgroforestrY Systems) is a silvopastoral model resulting from a European contract (AIR3 CT-92-0134) joining Mediterranean and temperate countries (Auclair, 1996). It describes the biophysical functioning of a silvopastoral system at a plot and daily scale (Bergez et al., 1999). In its current version, it simulates the functioning of a "broadleaved tree - natural pasture - sheep" association. However it is possible to parametrize it for other associations if proper parameters are available. ALWAYS is designed in an object-oriented way and is written in C++. The object-oriented structure allows to take into account the interactions between the different components of the system in a relative easy way. Five objects are implemented: the climate (with radiation, rainfall, temperature, PET, wind), the tree (growth depending on the water availability), the grass (growth depending on the water, light, temperature and nitrogen availability), the animal (different age strata with different energy requirement) and the soil (water and nitrogen cycles). A main core program links the five objects and deals with management options. Each object is coded in a separate dynamic link library to enable easy updates. The model is designed to compare different management practices such as weeding, pruning or thinning the trees, fertilization of the sward, forage harvest, grazing management, composition of the flock, etc. Outputs from ALWAYS are of three types; i) production data such as tree growth, forage production and animal performance; ii) evolution of the interactions between the components of the association (water stress, rainfall interception, light reduction, etc.) and iii) economic information related for example to timber production, forage harvest and animal stocking density. The ALWAYS model is designed : i) to allow Windows graphic user interfaces and ii) to target different end-users - from scientists to agricultural advisers. It offers three ways of running it: (i) interactively by running the simulation year by year and checking results at the end of each year using graphs and output files; (ii) automatically by describing the management over the full tree rotation and (iii) in a sensitivity analysis mode to test parameters from the yearly or rotation outputs.
Le modèle ALWAYS résulte de la coopération entre plusieurs pays tempérés et méditéranéens dans le cadre d'un projet européen (AIR3 CT92-0134). Il simule l'évolution d'une parcelle sylvopastorale (arbre, herbe et animaux) de l'échelle journalière à l'échelle de la rotation, en fonction du climat, du sol et de l'itinéraire technique. Always est écrit en C++ avec une structure orientée objet qui permet de tenir compte des intéractions entre les différentes composantes du système. 5 objets sont implémentés: le climat, l'arbre, l'herbe, l'animal et le sol. Un programme central lie ces différents objets et permet l'échange d'information entre eux. Le modèle est en particulier construit pour comparer différents itinéraires techniques et en particulier le désherbage des arbres, la taille, les éclaircies, la fertilisation de l'herbe, le calendrier de pâturage, etc.