i-Tree Hydro is a more detailed process-based hydrological model: the water table is updated & tracked dynamically at each time-step; impervious surfaces can be 'directly connected impervious area' or unconnected, which defines whether the runoff will continue to runoff on impervious surfaces or whether it will split off and run on to pervious surface and be processed there; water on pervious surfaces will infiltrate through routines that consider water table level, macro pore storage, etc. until either infiltration excess or saturation excess runoff occurs. Along the way over both impervious and pervious surfaces, processes such evaporation, ponding and depressions storage occur. In addition to these robust model processes, Hydro also has much more flexibility for defining hydrological & soil parameters, weather data and location, and land cover.
Aside from the significant differences in calculating runoff outlined above, the hydrological processes specific to plants (e.g. transpiration) are modeled the same way for both i-Tree Eco and i-Tree Hydro. Overall, i-Tree Hydro has a more robust capacity for estimating runoff than i-Tree Eco, but i-Tree Eco (and other i-Tree Tools) are more user friendly and may provide sufficient results.
Note that other i-Tree Tools use a method similar to Eco's (as of May 2020), described in document 3 below.
Additional resource for understanding i-Tree Eco's hydrology estimates as compared to other i-Tree Tools, as of early 2020:
- 1: i-Tree Eco Precipitation Interception Model Descriptions - A description of the precipitation interception model integrated in the i-Tree Eco version 5 application. (Updated 01/30/13)
- 2: i-Tree Streets/Design/Eco Rainfall Interception Model Comparisons - A document discussing the differences in methodologies between the rainfall interception model, used by i-Tree Streets and Design, and the avoided runoff model methodology used in the i-Tree Eco version 5 application.
- 3: i-Tree v6 Pollutant Removal, Biogenic Emissions and Hydrologic Processes Descriptions - A document (pdf) describing common i-Tree v6 ecosystem services methods and processes used by i-Tree Landscape, Canopy, Design and Forecast including international adaptations for Australia, Canada and the United Kingdom.