External factors External factors are influencing the functioning of the SA plot, but are NOT influenced by the system (no feedback, hence external). They may be considered as forcing variables (or external limit conditions). Two types of externalfactors may be distinguished with regard to their behaviour in time : Time-Variable externalfactors may vary throughout time during the run of the model (examples : timber prices drop; grants or subsidies are modified; average annual temperature increases…) Time-Fixed externalfactors are fixed over time (examples : soil quality, topography, latitude, elevation, light regime as influenced by latitude, slope and orientation, temperature regime as influenced by latitude, elevation, slope, orientation, local topography effects, susceptibility to pest attacks as a result of local conditions…) They will usually vary between plots. However, the simulated plot in HySAFE will be homogeneous for space variables (no variations inside the plot : the slope, the soil characteristics, the climate will be considered homogeneous over the plot). The limit between these two categories is not easy to draw. Some fixed factors may change in time with very specific actions by the farmer (example : soil drainage may change the water-logging regime) The sensibility of the model to fixed externalfactors will be easy to study with different runs of the model. Time-Fixed externalfactors are not useful for defining scenarii. They are useful for defining the best targets for SA systems. On the contrary, variable externalfactors should be defined to allow scenarii. We will therefore concentrate on time-variable externalfactors. The Land tenure regime will not be considered as an external factor. The model should allow to simulate both farmer=owner and farmer landowner cases. But the land tenure system will be considered as fixed in a given run of the model. So far, only 4 families of externalfactors have been identified and will be considered in the HySAFE model : grant and subsidies, arable and timber revenues, discount rate, long term climate change. Grant and subsidies regimes for crops, trees and crop-tree mixtures. This should include the rules for allocating the grants. Such rules very often depend on criteria at higher scales (farm, region, state). Special rules should be considered for very small farms areas such as in Greece (Vasilios Papanastasis suggested a 0.4 ha minimum area for the plot to be planted). Arable and timber revenue
As a consequence of the market prices, the labour cost, the input cost, the land taxation regime, the cost for renting the land. Long term climate variability Mean annual temperature – How does agroforestry respond to a global warming Mean annual rainfall How does agroforestry react to a change in the average rainfall Short term climate variability (day to day and year to year) is not considered as an external factor in this approach. How to format externalfactors for use in the model The level of details to which we should describe the externalfactors has to be decided. For example, Arbustra used only a crop gross margin, where Popmod and Wanulcas detail every cost associated with the crop (fertiliser, fuel, seeds…). Do we want to study the impact of say … oil cost on agroforestry, or simply of crop revenues on agroforestry Progressive changes or one-off change in externalfactors Popmod allows both possibilities to be studied. We probably should do the same in HySAFE.