ZonAgri is a modelling framework to simulate agricultural production activities at the scale of a region, following this principle : a region is designated as an aggregation of farms, and each farm is considered as an aggregation of agricultural activities distributed at several sites or sub-spaces of the region. This nested representation is based on a conceptual entity representing one production activity.

The “production activity” (PA) entity designates a simple or composite activity that can be associated with a spatial location. A simple PA consumes inputs (e.g. m3 of water, days of labor, tons of fertilizers) and produces outputs (e.g. tons of wheat, hectares of stubble for cattle, kg of nitrate in soil); the quantities of inputs and outputs are defined by a production function associated with the entity. A composite PA is an aggregation of PA entities; its production function defines how to aggregate the sub-activities. Following the principle of simplicity, this function is a weighted
summation of sub-activities, each weight (wi) representing the contribution of the sub-activity to the overall PA. Relationships between sub-activities can be represented for example using a given weight as a function of the other weights (i.e. wi = f (wj)).

The PA entity is not specific to an agricultural sector. Inputs and outputs of PA can be different (e.g. employment, calories, mouths to feed).
This conceptual entity thus does not enable representation of the multiple relationships between activities outside a given PA entity. Such relationships consequently have to be “managed” outside the model. But the simple model enables changes in such relationships to be taken into consideration in collaboration with the stakeholders through changes in the farming systems: relationships between farms within the region or between production units on a given farm.