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Is there any term of 'sub-sub watershed'? I refer it to the sub-part of sub-watershed. I have searched for it but could not find any satisfying finding. Thank you.
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I believe catchment or catchment area is a subsection of a subwatershed.
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I found mention of subwatershed, but the classificaion system does not go down to any further detail.

http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1042207.pdf

Hydrologic Unit: Watersheds in the United States and the Caribbean were delineated by the U.S. Geological Survey using a national standard hierarchical system based on surface hydrologic features and are classified into four types of hydrologic units: first-field (region), second-field (sub-region), third-field (accounting unit), and fourth-field (cataloguing unit). A fifth-field of classification (watershed) and sixth-field (subwatershed) are currently under development.
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I see. Thank you. I just read an article in my native language. It reads "...... The C 'sub-sub watershed' is part of the B sub-watershed, which is included in the A watershed. Maybe this term 'sub-sub watershed' only applies in my place (in my native language). Thank you.
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Here are a couple of links where the terms are defined:
http://www.stormwatercenter.net/Library/Practice/28.pdf

Aha, 'catchment', you are right. I just read one of the links you gave me. Thank you Emotion: smile