How do avoided runoff estimates differ in Eco and Hydro?

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How do avoided runoff estimates differ in Eco and Hydro?

Post by rcoville » Thu Jan 14, 2016 12:31 pm

i-Tree Eco estimates runoff using precipitation derived from the weather year & location set by users in Project Configuration, along with a simplified i-Tree Hydro simulation. In this simplification, other than tree cover, land cover is set as either pervious or impervious based on averages for all 50 United States (Nowak and Greenfield 2012). Eco assumes all precipitation on pervious surfaces will infiltrate (with no pervious runoff, which Hydro simulates), and all precipitation on impervious surfaces will turn into overland runoff (as if in Hydro, the parameter DCIA is set to 100%). To calculate Avoided Runoff, Eco runs this simplified Hydro simulation with current tree cover and with 0 tree cover, and the difference between the two results equals the Avoided Runoff due to trees. This is explained further in the Eco Avoided Runoff methods (the 1st document at the end of this post).

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:
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