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Homework Help: Unit clarification

  1. Feb 9, 2009 #1
    1. I am reviewing a paper and came across a unit that I am having a hard time understanding. Has anyone ever seen the unit [mm a-1] as used for a recesssion rate or trend?

    My assumption is that is could be mm per annum but then why wouldn't they just use mm yr-1?


  2. jcsd
  3. Feb 9, 2009 #2
    What is the context? What you're suggesting makes some sense, but I don't know if it fits the context at all.
  4. Feb 10, 2009 #3
    This unit is found in a hydrological context, specifically when expressing the rate of change in aquifer storage, i.e. when a shallow aquifer in a hillslope decline during periods of drought. This makes the mm per yr guess seem quite small considering aquifer water storage levels are fluctuating orders of magnitude more than that each year. Could the unit [a] be a unit of area? that doesn't make much sense to me either though...

    Thanks for the interest and response.
  5. Feb 10, 2009 #4
    Hmm, that is odd. Let me ask a civil engineering friend of mine. I'll get back to you soon. If you find out what it is, let me know.

    My initial inclination is that maybe mm doesn't refer to millimeters. Could be it be a typo for mM (milliMolar)? But I don't immediately see how that would be referenced in this context.

  6. Feb 10, 2009 #5


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    Staff: Mentor

    I did a google search on your term, and got a few hits. This paper/book uses the unit, starting on page 21:


    and this one, starting on page 32:


    the first one uses the term "isobases" in association with the units, and adding that term to the search, gives this (see page 317):


    It sure looks like a "mm per annum" kind of unit, which turns out to be pretty commonly used:


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