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 the United States. Eco assumes all precipitation on pervious surfaces will infiltrate, and all precipitation on impervious surfaces will turn into overland runoff. 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: ... ptions.pdf

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 method of estimating runoff than i-Tree Eco.
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