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11.07.2016 14:32Kalmar mesocosm 2016 (The Model Bacteria - BONUS BLUEPRINT) Christofer Karlsson

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06.07.2016 10:27Insights about modeling (BONUS BALTICAPP - Race Against Eutrophication Blog) Matti Sihvonen

During the last month I’ve learned a great deal about the fundamental aspects of modeling. I thought that work for the first paper was almost finished. So I started to seek information and write about the theoretical aspects of modeling. During the process I found such a good references that it actually helped me to figure out some totally new functional forms that I haven’t thought before. It didn’t take long until I got 20 functional forms for the model. I looked for reading which would help me to determine which functional forms to apply in the final model. I learned that there are at least three fundamental aspects that one needs to consider. First, one needs to think carefully what is the phenomenon that one is trying to model. That is, what assumptions one have to make about the properties of the model. Second, one takes a careful look at the data and examines it as deeply as possible. Third, one have to consider what for the model is to be built. That is, in what applications the model is to be used. The model have to work in those applications.

My supervisor Kari Hyytiäinen told me that model builders could basically be divided into process based and data based people. It seems that economist tend to be often process based whereas ecologist tend to be more data based. This takes us to the resent problems I’ve been struggling with. When we started this modeling process we made some assumptions about the properties of the model. This was before we even looked at any data. Then we contacted ecologists and got involved with the great data. We got into some arguments with the ecologists because they wouldn’t accept the assumptions we made because, although the assumptions were reasonable, the data didn’t support those.

So I jumped into the data based train for a moments; I removed the theoretical element related to the yield increasing effect of the soil phosphorus (STP) from the model. The ecologists were happy and I managed to build a model that delivered the aspects that were observable in the data. Then I started to focus on the application of the model: optimization and simulations. That’s where I run into trouble again. First I noticed that there was something suspicious going on with the optimization process: why the optimal soil phosphorus path didn’t go to zero if it was only bad in the model? I also noticed that absolute amount of the yield decreased as the STP increased when the model was simulated. This happened because STP was the bad guy in the model, but this was highly unintuitive and it wouldn’t make sense in the real life. I realized that the problems related to the missing model element describing the response to STP and the transition function describing the soil phosphorus dynamics.

At this point I had a meeting with a great environmental economist, Antti Iho, who knows a great deal about phosphorus modeling, among other things. He also noticed what was wrong. We realized that the transition function, which was suggested by the ecologists, was a failure. It only worked on a very limited data range. This is the problem with highly data based modeling; the produced model makes sense on a limited domain and thus it is not useful for any applications. I don’t think it’s a good approach. Anyway, Iho also said to me that we just couldn’t remove the increasing element of STP; the simulation and optimization result will be biased. This was true of course. It only confirmed my suspicions; the initial assumptions of us were correct. The problems won’t disappear if we just look the other way for a moment.

Hence, here I go again, in the conflict of the two modeling camps. But now I know, that the tree fundamental aspects of modelling must hold simultaneously. Otherwise the model just isn’t good. Currently I try to overcome this challenge, related to the mystery guy, STP, by applying additional data from the great report by Saarela (1995). Hope the model will be ready soon, before I completely loose my head with it.

 

Best regards: Matti Sihvonen

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