Model Elements 

1.  Levels
From the causal diagram, we can identify the criteria that represent accumulations. In this model the following levels are: Population, Present Nitrate, Present Business Area, Present Domestic Area, Irrigated Area, Non irrigated Area, and Groundwater. For all of them, we have identified the initial values, considering 1975 as the start time of our model.

2.  Rates
The rates are the inflows and outflows that came into or from the existing levels in our model. The rates are always changing in time. We have identified three types of rates:

  • Rate= coefficient* level: That is the case of Death rate in where the coefficient is variable and the level in the total population.
  • Rate=(Variable – Level)/ Constant: Households Growth rate is an example in where the following equation is used (Demand_of_household_area-Present_Domestic_Area)/5
  • Rate= Normal Rate + effect or Rate: The Growth of the Irrigation Area is an example of this rate in where it depends on the difference between the current area and the available area for agriculture and the normal percent growth of it.

3.  Balance Equations
Once the model is been quantified, Powersim software is a useful tool for solving and having an understanding of the system.  In order to have the quantitative understanding of the system, some balance equations are been defined inside the model. In the case in where the equations indicates inflows and outflows of the levels, they are called rate equations.

4.  Auxiliaries
The auxiliaries are those equations that are needed on the model to represent a part of the system in where it doesn’t intervene any level or rate. In the case of the auxiliaries as in all the model, the units are very important.

5.  Table Functions
There are some information that cannot be identified by a simple equation. In this cases, in where the trends are known along the time but cannot be described easily, the use of the table functions are very important to introduce in the model this important aspects of the real world.

6.  Constants
In the model there are also some constant that must be introduced in order to define rate equations and auxiliaries.

The information is collected from different sources of Azag(Gaza) Strip reality (since 1975 to now), that could help in developing the model. PowerSim is used to develop the simulated model of the system, which would help to develop strategies and policies. Verification, Validation and Sensitivity analysis of the model is done before developing strategies.







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