## Conceptual model for the Kraalingen approach

In order to develop a conceptual diagram for the Kraalingen approach, let us take a closer look at the equations of this model. Equations 1, 2, and 3 (see Chapter 8, The Kraalingen Approach) represent the relationships between objects plant, soil, and weather. Equation 2 shows that soil water factor data are needed to calculate changes (delta) in the leaf area index of the plant. The same equation shows that the temperature-based limiting factor, calculated by Equation 1, is needed to calculate delta changes in the leaf area index. Equation 2 shows that temperature affects daily plant growth and that the amount of water in soil impacts plant growth as well. Other equations show that plant data are needed for calculations of processes occurring in soil and soil data are needed for calculations occurring in plant. As an example, soil data are needed to calculate daily net photosynthesis processes occurring in the plant. Plant data are needed for calculating potential évapotranspiration, potential soil evaporation, potential plant transpiration, and rate calculation. These are processes that occur in the soil.

Based on the equations expressing relationships between soil, plant, and weather, a diagram for the Kraalingen conceptual model can be presented as shown in Figure 8-9.

Figure 8-9. Conceptual diagram for the Kraalingen approach.

As shown in Figure 8-9, there is an association referred to as growsln that links Plant and Soil that reads that plant grows in the soil. This association is bidirectional, meaning that an object of type Soil can access data and behavior in an object of type Plant and vice versa; an object of type Plant can reach data and behavior in an object of type Soil.

Weather data are used for calculating soil processes such as potential évapotranspiration, runoff and infiltration. Therefore, the association usesWeatherData links Soil and Weather with navigation direction from Soil to Weather. Thus, an object of type Soil is able to reach data and behavior from an object of type Weather. Equation 1 states that weather data are needed to calculate the growth rate reduction factor in plant. Therefore, an association between Plant and Weather is needed with navigation direction from Plant to Weather. This association is referred to as usesWeatherData and allows an object of type Plant to access data and behavior from an object of type Weather. The conceptual diagram shows that while objects of type Plant and Soil should have knowledge of each other, object Weather does not have access to any of the objects of type Plant and Soil. This is because of the particular role the weather data play in the simulation; they are used by other objects to calculate processes occurring in these objects. There are no processes occurring in object Weather.; therefore, object Weather does not need to access data and behavior from objects Soil and/or Plant.

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