Substantial Monetary Benefit Differences Between Models

Post here questions, comments, and suggestions about the general i-Tree project. To avoid duplication, please be sure to check the i-Tree website and the FAQs before posting. Relevant topics include background, purpose, requirements, hardware, installation, training and events.

Moderators: mbinkley, i-Tree Team

Post Reply
ggprevost
Posts: 2
Joined: Wed Mar 28, 2018 10:44 am

Substantial Monetary Benefit Differences Between Models

Post by ggprevost » Tue Apr 03, 2018 11:42 am

Hello,

I am trying to estimate the benefits of a group of trees as they age. The models I have been using are giving me substantially different monetary benefits and I don’t know why. I have been using i-Tree Eco (Eco), i-Tree Design (Design), and the National Tree Benefit Calculator (NTBC). I know that the models have different inputs, underlying models, and that there will be some differences depending on location-specific costs (ex. energy), but the differences I am getting seem too large to be attributable to these differences. I have also run Eco using different values (where I can) and still do not get results that agree. The results are below. If we assume that Eco is the most accurate because I could input the most site-specific information, the benefits from Design are between 73% to 157% higher and from NTBC are 1,080% higher than Eco. I would have expected that Eco would have estimated higher benefits because energy costs are likely higher in Ontario (my project location) than the assumed costs in the other models. Overall, I would also have expected that the annual benefits would be much higher than $30 (Eco) for such a large tree. The only thing I can think of is that Design and NTBC include the incremental annual property value increases, whereas Eco keeps this separate from the annual benefits, but even this doesn't seem to explain it all. Can anyone help shed some light on the differences and how I could move ahead to accurately estimate the benefits?

The tree is a 76 cm (30 in) DBH, healthy northern red oak (Quercus rubra) located in a residential area of Cambridge, ON.

i-Tree Eco annual benefits: ~$30 CAD (~$23 USD)
i-Tree Design annual benefits: ~$52 to $77 CAD (~$40 to $60 USD), depending on placement, assuming output of model is USD.
National Tree Benefit Calculator annual benefits: ~$355 CAD (~$275 USD) assuming Northeast climate zone and output of model is USD

Thanks - Glen
Jason.Henning
i-Tree Team
Posts: 138
Joined: Fri Dec 21, 2012 7:45 am

Re: Substantial Monetary Benefit Differences Between Models

Post by Jason.Henning » Wed Apr 04, 2018 9:21 am

Hi Glen,
Thanks for your question.
There are a few things going on here that cause the differences between the estimates.
1. They are indeed using different models. You are correct about this and it is one of the biggest factors in the differences here. The National Tree Benefits Calculator (NTBC) is based on the science in i-Tree Streets which was built on the regional Community Tree Guides available in our archives, http://www.itreetools.org/resources/archives.php, under the "i-Tree Streets Reference City Community Tree Guides" heading. The model used is often referred to as an empirical model and it essentially maps your entered tree to a tree that was measured in the reference city for the corresponding regional tree guide. It then assumes your tree provides similar benefits to a tree in that city. The methods and calculations in the NTBC haven't been updated since it was created and both the science and the values are now 10-20 years old as you can see from the dates on the Community Tree Guide publications. i-Tree Eco on the other hand is more of a process-based model and it is estimating the leaf surface area of your tree and how that surface area interacts with actual weather and pollution to provide benefits for your specific location. Additionally, Eco is constantly updated with the latest science and monetary values.
2. They are estimating different things. You are correct on this point too. i-Tree Eco and Design do not include property value. Additionally, i-Tree Eco estimates avoided runoff where the stormwater benefits in i-Tree Design and NTBC are using rainfall interception. As mentioned above Eco is also using actual rainfall data while both NTBC and Design are using data collected in that reference city. The big difference though is that avoided runoff is a more conservative measure of exactly how much the tree is contributing to keeping rainfall out of the storm drain while rainfall interception in a more generous measure how much rainfall is slowed before it reaches the ground. We are in the process of bringing our more modern tools Design, Planting, MyTree, and Landscape into alignment with i-Tree Eco. One way this has happened recently is that Eco and Design are using the same Egrid modeling numbers for pollutant avoided, see http://design.itreetools.org/resources ... ethods.pdf.
3. Location matters. When using the NTBC you will see that the benefits change significantly based on what climate zone you select. If you select the Northeast, that is based on a Community Tree Guide reference city of the borough of Queens, in New York City. As such it is using home prices from queens in the early 2000's to estimate the property value increase for your location. Similarly, it is using historical natural gas prices which have dropped significantly in the last decade.

i-Tree Eco will be the flagship software that includes the latest science and valuation. Legacy tools like the NTBC and i-Tree Streets will continue to exist because certain users rely on them, but the underlying values and science will remain static and continue to become more outdated. The NTBC is definitely intended to be an entry level tool that starts the conversation about tree benefits. However, we do have newer tools like MyTree and i-Tree Design that are using much more up to data science and valuations. If you want the most scientifically rigorous results than i-Tree Eco provides much more nuance, localization, and user customization, including the ability to enter actual energy prices.

In short if you are looking for the most up-to-date numbers then i-Tree Eco is the way to go. If you are looking for something quicker that is more accessible to the general public then a tool like i-Tree Design or MyTree will be more accurate than the static NTBC.

Thanks,
Jason
ggprevost
Posts: 2
Joined: Wed Mar 28, 2018 10:44 am

Re: Substantial Monetary Benefit Differences Between Models

Post by ggprevost » Wed Apr 04, 2018 1:58 pm

Hi Jason,

This very detailed answer is much appreciated and very helpful. I will be able to move forward with confidence now.

Thanks - Glen
Post Reply