You've probably seen the sleek marketing materials for i-Tree. They make it sound like you just click a button and suddenly you have a perfect map of how your city's oak trees are stopping a flood. But honestly? If you’re diving into the Research Suite’s crown jewel, i-Tree Hydro+, you’re going to hit a wall fast if you don't respect the data.
Most people mistake this for a simple calculator. It isn't. It’s a process-based, peer-reviewed beast born from a collaboration between SUNY-ESF and the US Forest Service. Unlike the standard i-Tree Hydro, the "Plus" version is part of the Research Suite, which means it’s grittier, command-line driven, and demands high-fidelity inputs that would make a standard GIS tech sweat.
The i-Tree Hydro+ data requirements infographic basically serves as your survival map. Without it, you’re just guessing.
The Raw Truth About Topography
Topography is the skeleton of your model. If the skeleton is broken, the water doesn't flow right. For a long time, users relied on simple Digital Elevation Models (DEMs). But Hydro+ is pickier. It needs a Topographic Index (TI).
What's a TI? Think of it as a map that tells the model exactly where water is likely to saturate the soil based on the slope and the area draining into that point. If you’re in the US, you’re lucky—the i-Tree team has pre-loaded TI data for thousands of municipalities. If you’re working on a custom site or international project, you’re looking at manual GIS processing. You'll need to download a 10-meter or 30-meter DEM from the USGS and run it through a specific "Topographic Index" generator.
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Mistake a pit for a pond in your data, and your hydrograph will look like a mountain range instead of a steady flow.
Weather: More Than Just "Rainy"
You can't just tell the model "it rained an inch on Tuesday." Hydro+ works on an hourly scale. It needs a continuous stream of data: precipitation, temperature, wind speed, and solar radiation.
The i-Tree Hydro+ data requirements infographic highlights the "Weather Preprocessor." This is a separate utility that takes raw NCDC (National Climatic Data Center) files and scrubs them. Why? Because sensors break. Stations have gaps. The preprocessor fills those gaps so the model doesn't crash at 3:00 AM in your simulation because of a missing dew point reading.
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International users often face a "shadow" problem. Since the model is optimized for US phenology (when leaves grow and fall), users in the Southern Hemisphere have to "spoof" the location. You basically pick a spot in the US with a similar latitude and climate just to get the seasonality right. It feels kinda hacky, but it’s the only way to make the biology work.
Land Cover and the "Under-Canopy" Problem
This is where the i-Tree Hydro+ data requirements infographic really earns its keep. Most models look at tree cover as a flat green blob. Hydro+ looks at what’s under the blob.
To get a real result, you have to break your area down into specific percentages:
- Tree cover over impervious surfaces (like trees over a street).
- Tree cover over pervious surfaces (trees in a park).
- Shrub and herbaceous cover.
- Directly connected impervious areas.
That last one is the "killer variable." If a parking lot drains directly into a pipe, that water hits the river instantly. If it drains into a rain garden first, the model treats it differently. You can get these numbers from i-Tree Canopy, but don't just trust the defaults. A city with 30% canopy but 90% of it over concrete will have a vastly different flood profile than a city with 30% canopy over soil.
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The Soil and Streamflow Mystery
Calibration is where the pros separate themselves from the amateurs. You don't need streamflow data to run the model, but if you want to be accurate, you need it.
Hydro+ uses a "PEST" (Parameter Estimation) routine for auto-calibration. You feed it a year of real-world stream gauge data, and it runs the model hundreds of times, tweaking soil parameters—like hydraulic conductivity and soil depth—until the simulated line matches the real one.
- Soil Parameters: You’re looking for things like the "wetted front suction" and "effective porosity."
- The Infographic Hack: If you don't have a gauge, use the NRCS SSURGO database (for US users) to find your soil type and use the suggested defaults. It's better than nothing, but honestly, it’s a "best guess."
Actionable Steps for Your First Project
Don't just jump into the C++ code. Start with the "Test Cases" provided in the Hydro+ package. They include a HydroPlusConfig.xml file that is already filled out.
- Download the Weather Preprocessor: Before you touch the hydrology, make sure you can actually format a weather file. If you can't get the hourly data to play nice, the model won't even start.
- Audit Your Land Cover: Use i-Tree Canopy to do a 500-point sample of your area. Specifically look for that "Tree over Impervious" vs. "Tree over Pervious" distinction.
- Verify Your TI: If you are outside a standard US municipality, learn how to use the "TauDEM" tools or the i-Tree TI generator script.
- Check the XML: Since Hydro+ doesn't have a fancy "Start" button, you’ll be editing an XML file in a text editor like Notepad++ or VS Code. One missing bracket
< >and the whole thing breaks.
Ultimately, the model is only as good as the hourly weather and the soil depth you give it. If you feed it bad data, you’re just making expensive-looking noise. Get the TI right, clean your weather files, and the model will actually show you how those trees are saving your watershed.
Now that you understand the data layers, your next move should be downloading the HydroPlus C++ source code or the compiled executable from the official i-Tree Research Suite page to see how the XML config file handles these parameters in real-time.