Why Do Some Researchers Use [mm a-1] Instead of [mm yr-1]?

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Discussion Overview

The discussion revolves around the use of the unit [mm a-1] in the context of recession rates or trends, particularly in hydrology. Participants explore the meaning and appropriateness of this unit compared to the more commonly used [mm yr-1].

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the use of [mm a-1], suggesting it might mean mm per annum, but wonders why [mm yr-1] isn't used instead.
  • Another participant asks for context to determine if the initial assumption fits, indicating uncertainty about the unit's application.
  • A third participant notes that [mm a-1] is used in hydrology, specifically regarding aquifer storage changes, and expresses skepticism about the smallness of the mm per year assumption given significant fluctuations in aquifer levels.
  • One participant speculates whether [a] could refer to area, but finds that interpretation unconvincing.
  • A different participant suggests that [mm] might not refer to millimeters, proposing it could be a typo for mM (milliMolar), though they also find this unlikely in the given context.
  • A participant shares links to external sources that use the unit [mm a-1], indicating that it is commonly used in literature, particularly in relation to "isobases."

Areas of Agreement / Disagreement

Participants express various interpretations of the unit [mm a-1] and its context, with no consensus reached on its meaning or appropriateness. Multiple competing views remain regarding its usage and implications.

Contextual Notes

There are limitations regarding the assumptions made about the unit's meaning, its application in specific contexts, and the potential for typographical errors. The discussion does not resolve these uncertainties.

travroth
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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?

Thanks.

T
 
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What is the context? What you're suggesting makes some sense, but I don't know if it fits the context at all.
 
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.
 
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.

Cheers.
 
travroth said:
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?

Thanks.

T


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

http://books.google.com/books?id=hu...=X&oi=book_result&resnum=9&ct=result#PPA21,M1

and this one, starting on page 32:

http://books.google.com/books?id=gB...=X&oi=book_result&resnum=6&ct=result#PPA32,M1

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

http://books.google.com/books?id=2KlSteO7tiUC&pg=PA317&dq="[mm+a-1]"++isobase

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

http://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGLL_enUS301US302&q="mm+per+annum"

.
 

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