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is GSSHA(wms8.0) able to be applied to the case for irrigation region? In the face of relatively flat area and settled drainage, how and what should I do? Besides, can I set up smaller grid DEM than 10m? for example 1m,thanks!

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Yes, GSSHA can be used for irrigation regions, though really flat areas can be tricky to model. You can certainly use a finer grid than 10 meters, although it doesn't makes sense to unless the resolution of your elevation data is finer than 10m. We don't have a standard approach for modeling flat areas because each project has different considerations. One suggestion is to be careful when using CleanDam to remove digital dams. If water really does pond, you may not want to remove the digital dams.

Hope this helps,

Clark

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Is the process defined in gsshawiki or other sources, about how does overland flow process work if we do not remove digital dams assuming water really does pond there?

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Sushban,

I think the overland flow process is described on http://gsshawiki.com. You should be able to find all the information you need there. Also, the GSSHA-specific tutorials included with WMS demonstrate how results from models with many digital dams compare with the same model that has digital dams removed.

Let me know if I can help with anything else,

Chris

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Thanks Chris,

From what I read in tutorials in gsshawiki,

" digital dam exists as a result of a lack of grid's elevation resolution",

" the presence of digital dams creates problems with surface runoff as the water ponds in artificial depression", and

it explains how these digital dams could be removed and fixed using different manual and automatic techniques.

Clark Barlow, in this thread, replied that if water really does pond, you may not want to remove the digital dams. So, if we do not remove some of the digital dams, which are natural depressions, then I am interested to learn how GSSHA performs overland flow process. Does the water ponded in the digital dams route to downstream rivers when there is excess water to over-top neighboring cell elevation? Or it can just be infiltrated and evaporated?

Sushban

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Sushban,

If the water in the cell overtops the neighboring cell elevation, the water would extend to that adjacent cell. Evaporation or infiltration could be occurring in the cell at the same time if you have those processes turned on in your model (you can turn these on in WMS from the GSSHA Job Control dialog).

I think for most models, you do not want digital dams in your model unless you're specifically trying to model water ponding or wetlands. Digital dams are normally created because the elevations on your grid do not adequately represent your terrain. You need to remove most digital dams to get watershed depths and flows that represent reality.

Chris

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Chris,

The purpose is to use GSSHA for irrigation farmland with relatively flat terrain. I understand that high resolution elevation data will be required to represent flat areas. But if there are regions with local depression in the field, it will also create digital dams. Will it be a good idea to remove these digital dams and instead represent these using retention depth in the mapping table?

http://gsshawiki.com/Surface_Water_Routing:Overland_Flow_Routing

Section 5.2.3 Runoff Retention

It explains that the water held in the retention storage, never becomes direct runoff and can only be removed from the land surface as infiltration. Retention storage is input as a depth (mm) as a table of values related to index maps by using the mapping table.

Sushban

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Sushban,

I think your model will run faster if you remove the digital dams and use the retention depth option. You could also try both options and see which option works best for you. I think you should get similar results either way.

Chris

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