Bad Paper said:
I was just trying to price the item according to the guidelines in the DMG. Doesn't make sense.
I ask because a player wants to create a magical well/font/aquifer/whatever in a tower (he has claimed Nightfang Spire as his keep, ha ha) in the middle of the desert. I'm trying to figure creation requirements. The well doesn't have to be fancy, but must provide at a minimum for about twenty creatures per day, but ideally will provide more, including possibly the ability to irrigate surrounding land, provide water for passing guests/horses/camels/mules, etc. This is a weird thing to scale... 40 gallons per day up to maybe a thousand? I don't know much about agriculture. OK, I don't know anything about agriculture; I can't even keep a potted fern alive.
Well, conventional wisdom says that a human needs eight 8-ounce glasses of water a day (this figure is disputed as being too high, but we're talking about people living in a desert, probably toiling all day, so it works). That's 64 ounces a day, or half a gallon. Since even at the lowest setting the decanter pours out 1 gallon a round, you could get enough water for 20 creatures in one minute.
For livestock, it varies, but from what I've found in Google (admittedly a minimal-effort search), cattle need about 20 gallons a day, horses around 15 (assuming a high average temperature since it's a desert).
Agriculture is harder to answer, as it varies by soil type, crop type, etc, and in everything I could find online they don't really measure it in terms of "gallons per day". But it's safe to assume that you need a lot of water to grow staple crops. Regardless, as written the decanter can probably cover it with minimal effort.
As others have stated, magic items are priced according to their short-term, dungeon environment use, and little consideration is made to what extended effects they might have on a campaign world. FOr another fun discussion along those lines, ask about the Lyre of Building ("Hey, we've got a free weekend... let's build a palace!")