Hierarchical model of the movement of nutrients and artificial radionuclides in the soil-plant system

Resumen

A model is presented of the movement of nutrients and artificial radionuclides in the soil-plant system. The model has a hierarchical structure and is composed of modular elements (submodels) that integrate themselves additively at different levels, with each sub-model containing the submodels of the previous levels. This integration is achieved using the state and flow variables of each submodel as auxiliary variables of the higher level submodels. This hierarchical scheme is an operative solution that simplifies the construction of complex models with different and strongly related dynamic processes. The levels of the model are represented by the following processes: level I: plant biomass dynamics; level II: nutrient movement between the soil and the plant; and level III: deposition of radionuclides and movement between the soil and the plant. Each submodel is structured in different compartments an has its own boundaries. Definition of the compartments of a given submodel is not only dependent on the process that it represents but is also determined by its subsequent integration at higher levels. The plant biomass dynamics submodel is composed of four compartments: root + shoots, young leaves, old leaves and litter. The boundaries of this submodel are defined by the plants themselves. The nutrient dynamics submodel adds two new compartments representing the soil system and uses the differentiation between young and old leaves, established at the previous level according to the changes that take place in their chemical composition. The boundaries of this submodel are defined by the above ground parts of the plants and by the depth of the roots underground. The radionuclide dynamics submodel integrates the two previous submodels and incorporates some additional compartments for the surface of the leaves and the above ground parts of the root + shoots fraction. This submodel has the same boundaries as the previous one