Project area: watershed or non-watershed?

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Project area: watershed or non-watershed?

Post by rcoville » Fri Jul 31, 2015 10:13 am

i-Tree Hydro can simulate how tree and other vegetation cover effects runoff for watershed areas, such as all land upstream from and feeding into a river mouth, and non-watershed areas, such as all land within the city boundary. Synonyms of watershed area that may be more familiar: drainage basin; catchment basin; river basin.

To help determine which type your project is, consider if you're interested in:
- A project that is assessing the entire area which feeds water and nutrients/pollutants into a river at a specific point (its mouth or elsewhere along its course).
This would be a watershed area project.
For example: assessing how changes in land cover would affect water quality and quantity for the Delaware River Basin, an area defined by a watershed.
- A project that is assessing the water quantity and quality contribution of most parking lots, parcels of land, or political boundaries, and is probably looking at an area which does not feed all of its water into one specific point, nor does it capture the entirety of water that is feeding any of the outlets its associated with.
This project would be a non-watershed area.
For example: assessing how changes in land cover would affect water quality and quantity for the City of Philadelphia, an area not defined by a watershed.

The two examples are certainly related, especially as the city is within the watershed. The two projects may even have overlapping results (changes to the city cause changes to the watershed and potentially vice versa). The driving factor to determine what type of area is right for your project is primarily management focus and scale: Is the project intended to inform management of the city, or of the entire watershed?

What difference does simulation type make?
The main consideration about modeling a non-watershed is you won't be able to auto-calibrate the model or identify exactly where predicted flow will occur, because your project area is not associated with a specific, single drainage point. Auto-calibration finds the soil & hydrology parameters which best fit predicted streamflow to user-provided observed streamflow at a watershed's discharge point. Because there's no single discharge point for streamflow observations in non-watersheds, we can't auto-calibrate. In i-Tree Hydro Step 3 you'll see the suggested default parameter set. These parameters can provide a useful standard for comparison between land cover scenarios, although they are not as accurate and localized as a parameter set auto-calibrated to a study area. You can adjust the default set based on local knowledge so that your simulation is more specific to your site.
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