different numbers for the same tree and in the same location

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Joined: Fri Jul 09, 2021 4:23 pm

different numbers for the same tree and in the same location

Post by Basa »

I am trying to learn itree design because we would like to use it for a citizen science project in Edmonton, Canada. Using our elm tree I got different outputs on different days: On July 5 this elm saved us $ 11 in our no-air conditioned home (kWh conserved: 3.1, reduction of heating fuel 8.4 therms, CO2 reduction per year 143). Using the same inputs (tree species, diameter, house and tree placement) the next day (new project), July 6, this tree saved us $ -15 in our no-air conditioned home (kWh conserved: -7, reduction of heating fuel -18.6 therms, CO2 reduction per year -79). I expected some differences between two 'runs'/trials because the outline of the house and placing the tree on the map might not be exactly the same - but not a reversal of outputs (flippig from $ 11 savings to costs of $ 15 per year).
Next I pretended the Elm is a White Spruce, using itree design on July 5 that imaginary spruce saved me $ 25, using itree design on July 6 it cost me $15, same reversal pattern as for the elm.
So I keep trying more tree species and scenarios (with or without air conditioning), the results range widely (I am modelling each tree at least twice). What could be a reason for these wide ranging results?
In one case (but leaving the program between calculations) having a White Spruce and an Elm in the same location (southeast) gave me very similar results for these very different trees:
Results for both trees show savings of $ -15: Spruce conserves -6.3, the Elm - 7 kWh (OK, trees are the same size maybe shading is similar but crown shape between these trees would be very different), the White Spruce reduces heating by - 16.7 therms, the Elm reduces heating by -18.6 (so the Elm increases my heating bill even though it shades in summer and in winter it has no leaves - so passive solar heating occurs?).
I would really appreciate some advice, context or explanation as to how to reduce variation and have replicable/consistent results
i-Tree Team
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Joined: Fri Dec 21, 2012 7:45 am

Re: different numbers for the same tree and in the same location

Post by Jason.Henning »

Hi Basa,
Thanks for the question.
Do you happen to have any of those projects saved (the .dsgnprj type files created by cliking on the "Save Progress" or "Save Results" buttons) or the report .pdfs showing the changing results? If you do have any of those files please send them along our support email, info@itreetools.org.
I talked to our development team and they indicated there haven't been any changes to i-Tree Design since June. This means that the exact same scenarios should produce the exact same results during that time period. It would be possible to see those types of swings in results if you placed the trees in different segments of the "benefits web" that is visible in i-Tree Design. The energy benefits model lumps trees into 8 directions around the building (N, NE, E, SE, S, SW, W, and NW) and three distances (0-20 ft, 20-40 ft, and 40-60ft). This means there are only 24 different energy benefits estimates for any give tree. Moving between any of those 24 zones shown in the benefits web will cause different energy estimates.
The energy models also may not be considering all of the exact same factors you expect them to. You can find out more about how the energy models work by reviewing the "Energy" section starting on page 40 of this document, https://www.itreetools.org/documents/65 ... nrs200.pdf.
Again, please feel free to send along those files showing one of the differences you mentions and we will take a closer look.
A member of the i-Tree Team
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