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Garden with Insight v1.0 Help: A cautionary note about simulations

One definition of a simulation is "the representation of certain features of the behavior of a physical or abstract system by the behavior of another system" (Ralston 1976). The most important words in this definition are certain features. A simulation model takes a complicated reality and simplifies it so that we can begin to understand it. Simplification requires deciding which aspects of the system we think are most important.

For example, consider something familiar like your own hand as it moves around. How would you make a simplified model of the movements of your hand? What aspects of that complicated system would you select to explore? The bones, which create the hand's structure? The muscles, which move the bones? The blood vessels, which supply the oxygen needed for movement? The nerves, which signal the muscles to move? The skin, which covers and protects the muscles? As you can see, there are many possible approaches.

Simulating some aspects of a complicated situation requires you to make some assumptions about the rest of the system. If you decide to simulate the movements of your hand by looking at only the bones and muscles, you will probably assume that the blood vessels, nerves and skin operate in a normal way. Your simulation will probably be reasonably correct in normal situations, but if for example the blood vessels of the hand are abnormally constricted, the simulation may not predict the correct result.

In the same way, Garden with Insight selects certain aspects of weather, soil and plant processes and assumes that all other processes are within normal bounds. These assumptions might not be correct in every situation. For example, soil salinity is not simulated in this version of Garden with Insight, and therefore it is assumed to have a minimal effect on the soil. If the soil in your area has a high salt content, the results may not hold.

As much as we have tried to make Garden with Insight as accurate as possible, the elements involved in gardening are extremely complicated and cannot be perfectly simulated. This program cannot predict the weather, and it cannot predict what will happen in any real garden. What it can do is to give you a look at an abstracted garden where issues that are important in most gardens are imitated in a simplified, selected way that presents important facts about these processes.

Specific limitations

The biggest limitation in this version of Garden with Insight is in the plant growth model and the plant parameters. Parameters for climates and soils are mostly reasonable, because those models are nearly the same as in the EPIC model from which Garden with Insight was mainly derived. The plant model, however, incorporates major changes from the EPIC model, mostly in reproductive growth, biomass allocation, and harvesting. Parameters for the plants included in this version are sketchy, and some are merely set at defaults because we have not collected this information.

Because of these problems, you might find strange or even ridiculous numbers coming out of the plant part of the simulation. For example, we recently grew an eight-pound carrot root using the simulation. The problem is that many parameters interact in producing this one number (seed weight, rate of biomass increase, allocation to the root, etc.), and it is difficult to trace back to the responsible parameter. Not only that, but the plant model has not been tested completely enough to be sure there isn't some problem in the model that makes the calculation wrong even if the parameters are correct.

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Updated: May 4, 1998. Questions/comments on site to
Copyright © 1998 Paul D. Fernhout & Cynthia F. Kurtz.