I'm wondering if it's not actually a typo, but an incorrect conugation based on an incorrect translation: imagine if somebody thought 'hewn from solid rock' connoted 'extract'; then when you want to 'extract' water, you 'hewn' it!
It would probably be easier to tell if we had some other examples from your text?

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Note added at 44 minutes (2017-08-16 20:50:19 GMT)
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Presumably, by back-translating 'hewn' into Spanish, you might be able to see where it's coming from?

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Note added at 50 minutes (2017-08-16 20:56:11 GMT)
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Yes, you see:

http://diccionario.reverso.net/ingles-espanol/hewn

Scroll down a few of the example sentences, and you come to some doublets where 'hewn' has been translated as 'extraída':

That Formica was hewn from the hills above Florence.
Esa fórmica fue extraída de las colinas de Florencia.

Now this limestone was hewn from a quarry that is very near to where I'm standing now.
Esta piedra caliza fue extraída de una cantera muy cercana a donde estoy parado.



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Note added at 11 heures (2017-08-17 07:41:27 GMT)
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Well, Asker, that's already VITAL extra context! It helps a great deal to know that these are "specialist construction services".
But your question is still confusing: you say your are "translating a list of services from Spanish", yet your question is about an EN term and the only cntext you give is in EN — so where does the Spanish come in? You also say "I keep coming across this word 'hewning'" — yet you fail to tell us if you only ever come across it in the SAME expression, of if not, how it is found used differently?
I'm sure it all seems painfully obvious to YOU, which is why you see fit to shout at those trying to help you, but for the benefit of the rest of us, please clarify your question!

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Note added at 12 heures (2017-08-17 08:15:56 GMT)
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There is an inconsistency between the term used in the EN list you have linked to and the Spanish term in the ref. found by Mike, which lists "...uso o canalización de agua en madera"

Now I can imagine that 'canalización' might refer to the 'capture' of water from a spring — though I don't quite see where the 'en madera' comes from?
But this doesn't seem to bear any relation to the explanation given in your EN ref "The service of using water to cut structural constructions on properties owned by others"??! But is this reference reliable? Its wording in places seems to differ from the official UN text.

A mis-hearing of 'using' seems unlikely, given that there is already 'use of' in the same expression.