Section 5

Section 7 


 Tradeoff Analysis: On-line course Kenya

Section 6 - Model simulation


The models are now ready to be used for simulation. The simulation includes a number of basic steps that are represented by buttons in the tradeoff shell:

Sample fields or farms

Simulation runs can be carried out for the survey fields or farms, or for a random sample of fields or farms drawn from the region according to criteria represented in the GIS data. The economic model for Machakos is a whole-farm model, so a sample of 150 farms has been created for the purpose of this course. This requires a large number of simulations and we will not repeat the sampling at this point in time. The file that has been created is called farmsamp.dbf. Locate the file and open it in the dBase viewer.

When fields are sampled the program does the following:

  1. Randomly draws a location within the study area (in other words X- and Y-coordinates are drawn from within the limits of the study area).
  2. Checks whether this particular location fulfills the spatial conditions as defined by the user. If not, a new location will have to be drawn. The spatial conditions allow the user to construct stratified random samples for simulation. The location is added to the file with a unique id and if requested the values of specific maps.
  3. Calculates the inherent productivities for all the activities that have been checked.


Exercise 8:  Review the file of sampled farms

  1. Locate the farmsamp.dbf file and open it in the dBase viewer.
  2. What is the variation in inherent productivity for the two activities?
  3. Are maize and bean productivities correlated?


Define Tradeoffs 

Tradeoff curves are constructed by varying model parameters. The model parameters that will be varied are defined under Setting - Variables - Tradeoff variables. Each row in the tradeoffs file corresponds with a tradeoff point. A change in the variables defined in the tradeoff definition file can be given. The base value is 100, indicating no change. A value of, for example, 80 indicates a 20% decrease, whereas a value of 120 indicates a 20% increase. 


Exercise 9: Tradeoffs

  1. Go to Setting - Variables - Tradeoff variables. What are the tradeoff variables?
  2. Go to the define tradeoff button in the tradeoff analysis system and load the tradeoff file called: TRADEP1.dbf. How many tradeoff points are defined?
  3. What is the range of maize price changes defined by this file?


Define Scenario 

The definition of scenarios is based on the parameters defined in the variable definition. The user already indicated whether the variable is changed in absolute terms or in relative (percent) terms. This is indicated in the last column by the unit (value versus %). It is essential that the user understands how to appropriately define these variables in the economic simulation model.


Exercise 10: Scenarios

  1. Go to Setting - Variables - Scenario variables. What are the scenario variables?
  2. List some scenarios that can be represented by changes in these variables.
  3. Go to the define scenario button and look at the basic scenario file: SCEN_BAS.dbf.
  4. One of the scenarios we are interested in is the increase in production of manure from adoption of zero-grazing units. How would you change the model parameters to represent the effect of this technology?


Run simulation 

Now the economic simulation model can be run. The names for sampled fields, tradeoff definition and scenario definition must be selected. The user needs to specify a code for the simulation run.  This is a five-character code that will be a part of the output file names. The simulation run will take place after pressing the run button.  The Machakos model operates at the level of individual fields (parcels) and at the farm level, so two output files are produced.  For example, if the scenario name is BAS then the parcel and farm output files will be BASoutp.dbf and BASoutf.dbf.  These files are written to the RESULTS directory.  


Exercise 11: Run simulation

  1. Go to the Run simulation button and check the names of the fields, tradeoff and scenario files
  2. Run the simulation model.
  3. Check the output files of the simulation run in the RESULTS directory.
  4. Can you explain the number of records in the output files?


Environmental impact 

The economic simulation model simulates crop choice and agricultural management for the different tradeoff points. The environmental impact button allows the user to evaluate management practices using an environmental impact model.


Exercise 12: Nutrient depletion

  1. Go to the settings-environmental fate models. What is the name of the environmental fate model?
  2. The model is based on the Nutmon model ( This model calculates a simple budget of nutrient inputs and outputs and adds the value of the nutrient balance to the output file. Run the Nutmon model now as follows: first, click the Calculation button; second, go to the Simulation Results window and select the parcel-level output file from your economic simulation.  Note you can select which nutrient outputs you want – they will be written to a new output file called.  If the original file from the economic simulation was BASoutp.dbf, the new output file created by NUTMON will be called BASoutpndep.dbf.  This file contains all of the original output variables plus the selected nutrient balances.  
  3. Check the newly created output file. What is the range in soil nutrient losses?


Section 5

Section 7