Figure 10-6 contains a diagram that represents the Irrigation Scheduling Model (ISM) developed by [GSR00]. As in the Ritchie model, the only methods assigned to class Weather are for calculation of reference ET. However, ISM provides several methods for its calculation, such as the Penman-Monteith, Blaney Criddle, or Priestley-Taylor methods. The model also allows for user input of already-calculated reference ET. The ISM Weather class has been provided with a larger number of attributes, compared to that of the Ritchie model. These extra attributes are needed to store the data values required by each of the methods of calculating reference ET. Not all the input data need to be supplied to the model - only those required by the selected calculation method. Class Plant in ISM has a richer set of attributes than Ritchie's model. This illustrates a difference between the two models. Whereas Ritchie's model requires the input of leaf area index (either from the user or from a linked model), ISM includes processes that simulate aspects of crop growth. As a result, much more information about the plant is required by ISM. Choice of a model may be, in part, determined by data availability. For example, use of ISM may be preferable if the leaf area index required by Ritchie's model cannot be readily obtained. In this case, corresponding data inputs required by ISM may have to be obtained to allow the model to make predictions about the condition of the crop.
Class SoilPlantAtmosphere is provided with data and behavior for determining water losses through evaporation and transpiration. The ISM model predicts évapotranspiration by combining the effects of both mechanisms to determine overall water loss from the plant-soil system. ISM converts reference ET values, provided by class Weather, into actual évapotranspiration based on soil moisture availability, provided by class SoilLayer, and the crop coefficient, provided by class Plant. The need for data from Weather, Plant, and SoilLayer justifies the associations class SoilPlantAtmosphere has with Weather, Plant, and Cell. Although there is no direct association between SoilPlantAtmosphere and SoilLayer, the communication between these two classes takes place via Cell.
Classes Cell, SoilProfile, SoilLayer, and Groundwater are modeled to function together as a component and could be considered as a Soil component. The main class in this component is class Cell, which plays the role of the gatekeeper. One may suggest that a direct link from class SoilPlantAtmosphere to class SoilLayer, bypassing class Cell, would shorten the communication between these two classes. The problem with this solution is that by shortening the path, we will increase the interdependency between classes of the system. In the case that some other Soil component providing the same behavior can be found, it will be difficult to use it, as our system will not allow the substitution of one group of classes with another one. Having class Cell controlling the dialog of the soil related classes with the rest of the system makes it easy to define an interface that provides the services the component can offer. An interface allows substitutability between different components implementing the same interface.
Layer-specific data and behavior are assigned to class SoilLayer. Unlike the Ritchie's model, ISM does not partition the soil into layers; it simply considers the soil profile as a single layer that extends to the bottom of the root zone. The association composedOf between classes SoilProfile and SoilLayer is one-to-one-or-many; therefore, the single layer approach of the ISM model is taken into consideration. Class SoilLayer is provided with a smaller number of attributes than the same class in the Ritchie's model, because the single layer approach requires fewer parameters for calculation of the water movement. Processes and attributes that apply to the soil profile as a whole are assigned to class SoilProfile. ISM model calculates deep percolation out of the root zone; thus, class SoilProfile is provided with a method referred to as calculateDeepPereolation.
Class Cell is provided with data and behavior for calculating the amount of water that enters the soil surface. The ISM model describes this as effective rainfall and uses the SCS method to determine this amount. The method calculateEffectiveRainfallSCS implements the SCS method for this calculation. Alternatively, a fixed percent of actual rainfall can be specified. ISM includes a number of irrigation-scheduling functions in class IrrigationManagement. The user can input either a desired irrigation interval or an allowable depletion value and the model calculates an irrigation requirement to guide the management. The ISM model does not take into consideration the groundwater; therefore, the Groundwater class does not provide any data or behavior.
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