[Tagging] How to tag a cistern?

[S Volk]

Hi,I've been mapping on Africa in HOT tasks and around, I've seen some cisterns, also know that some humanitarian NGOs have made many cisterns  on Africa (also water_wells).
Question:How to tag a cistern (it seems to be not like what is shown for "landuse=reservoir"; rather a small "man made" reservoir, on ground level or escavated, for either natural or treated water)?
Or, in addition, since there are tags for "man_made=water_well, "man_made=water_tower", wouldn't it be worth to have a tag like "man_made=cistern"? 
Thank you in advance, regards,Sérgio (user:SergioAJV)svolk2 at hotmail.com
 		 	   		  
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