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It's mentioned many times in the Discworld novels that the oceans continually flow over the edge of the disc in great waterfalls, turning into mist. But where does the water go after that, and, more importantly, how does it return to the disc? (Or, if it doesn't return to the disc, what prevents Discworld from running out of water entirely?)

I found a few speculative discussions on this topic, but nothing concrete, so my question is whether Pratchett ever gave an in-universe explanation of how Discworld's water cycle operates, and if so, what that explanation is.

It's possible that no answer was ever given beyond "it's magic", and if that turns out to be the case I'll accept the answer that says so. But then again he might have gone into more detail at some point about where the water magically comes from, so I'm leaving this open for a while to find out.

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    Pretty sure there's a footnote in an earlier book about the water "making its own arrangements"
    – HorusKol
    Commented Jul 24, 2016 at 14:59
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    @HorusKol - "“Let it be a sphere,” said Didactylos. “No problem with a sphere. No doubt special arrangements are made for everything to stay on. And the Sun can be another larger sphere, a long way off. Would you like the Moon to orbit the world or the Sun? I advise the world. More hierarchical, and a splendid example to us all.”"
    – Valorum
    Commented Jul 24, 2016 at 18:24
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    Obviously the rain keeps the seas full. I mean, think about it in reverse. If the seas weren't running off the edge of the Disc, what would stop the rain from filling them beyond their shores and flooding everything?
    – Xavon_Wrentaile
    Commented Jul 25, 2016 at 1:57
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    @Xavon_Wrentaile but then my question becomes "where does the Discworld's rain come from?" It can't all come from evaporation, because then there wouldn't be enough to offset the water falling off the disc.
    – N. Virgo
    Commented Jul 25, 2016 at 11:26
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    @Nathaniel That's pretty much how my printer works.
    – Howard Miller
    Commented Oct 10, 2016 at 2:45

4 Answers 4

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Terry Pratchett addressed this precise issue in The Discworld Mapp (1997), simply describing it as one of the 'great unanswered questions of our time'.

Exactly how this molten state [which powers the volcanoes and allows the continental plates to move] is maintained, and how the water that pours ceaselessly over the rim from the Circle Sea is replaced, are but two of the unfathomable mysteries of the world.

In a later interview for the 10/31/2009 issue of New Scientist, he went further and stated cryptically that "arrangements are made" and that it somehow involved the rain cycle and what we can assume is some manner of teleportation.

NS: Either Discworld has an infinite supply of water, or all the water that gets lost over its edge is conserved through some miraculous mechanism. What methods do Discworld's inhabitants use to conserve their supply of water?

Pratchett: Arrangements are made. It goes over the edge and comes back as rain. I'm not quite certain how it gets back, but on the other hand, we're talking about a giant turtle flying through space.

Luckily, Unseen University's 'Head of Inadvisably Applied Magic', Ponder Stibbons is on hand to offer us a fully in-universe explanation in The Last Hero. You'll be astounded by its technical complexity and sheer elegance.

"Really? Well, we can do without magic for a couple of years, can't we?" said Mr Slant, managing to suggest that this would be a jolly good thing, too.

"With respect," said Ponder, without respect, "we cannot. The seas will run dry. The sun will burn out and crash. The elephants and the turtle may cease to exist altogether."

It's magic

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  • The seas will run out and dry not because the water source stops working but because the sun is burning out (=supernova), that is the temperature is rising and the water simply evaporates. (no I don't seriously suggest this is the correct interpretation, this is to illustrate why your quote is not the answer, only a hint pointing in one direction which doesn't need to be the right one)
    – Nobody
    Commented Jul 24, 2016 at 17:29
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    @nobody - I'm at a loss what you're trying to say. Ponder is pointing out that the Disc's sun (which is tiny) is only replenished by magic. Similarly, it's only held aloft (several hundred miles above the disc) by magic. He's not saying it would go supernova.
    – Valorum
    Commented Jul 24, 2016 at 17:47
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    @Nobody: it’s pretty darn explicit. Stripped of drollery, the quoted exchange is essentially: Slant — “What would happen without magic?” Stibbons — “The seas would run dry (among other things).” So Stibbons is explicitly saying “Without magic, the seas would run dry.” It’s certainly possible that the causation could be indirect, as you suggest — magic causes X, and X keeps the water flowing — but it is explicitly saying that magic is a necessary part of the process. And the fact that “…the seas would run dry” is Stibbons’ first item seems to suggest that the causation is fairly direct.
    – PLL
    Commented Jul 24, 2016 at 19:30
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    – Null ♦
    Commented Jul 25, 2016 at 14:57
  • Eye of newt, a dash of Sprite, Send to us, a big ol' knight! It's MAAAAGIC!
    – Zenzizenzizenzic
    Commented Oct 3, 2022 at 17:46
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I don't believe there is a canonical answer for Discworld, though I'd suggest that magic would be the obvious answer.

In Pratchett's earlier novel 'Strata' there was a similar situation with a proposed technological solution called a molecule sieve. The molecule sieve essentially teleported the rimfall water back into the oceans. Perhaps the Discworld simply has a magical equivalent to this sufficiently advanced technology.

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    They use a technological solution on the Strata Disc precisely because they can't hand-wave it away with magic.
    – Valorum
    Commented Jul 24, 2016 at 9:48
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    @Valorum, I consider teleportation pretty hand-wavey, actually. :) I've only read one science fiction book ever that actually posited a not-implausible scientific (fictional) theory that reasonably explained the mechanics of teleportation without contradicting modern knowledge of physics. It involved a clearer understanding of space itself — specifically hypothesized — than we currently have on Earth.
    – Wildcard
    Commented Jul 25, 2016 at 11:49
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    @Wildcard what was that book?
    – Jeremy French
    Commented Jul 27, 2016 at 15:11
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    @Valorum Agreed. FWIW I think Terry Pratchett wrote Strata before the Disk World series (the magic ones).
    – DarcyThomas
    Commented Jul 28, 2016 at 12:08
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    @Darcythomas - He did indeed, although I gather the idea of a magical disc-shaped world came to him even earlier
    – Valorum
    Commented Jul 28, 2016 at 13:35
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Pratchett himself answers this question, in one of the earlier books. I believe it was The Light Fantastic. He said: "arrangements are made."

It's magic.

Sure, that's not a very satisfactory answer.

“And that’s why I don’t like magic, Captain. ’cos it’s magic. You can’t ask questions, it’s magic. It doesn’t explain anything, it’s magic. You don’t know where it comes from, it’s magic! That’s what I don’t like about magic, it does everything by magic!”

— Sir Samuel Vimes, Thud

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    That quote doesn't exist in The Light Fantastic. The only place that the expression "arrangements are made" is in Small Gods and then in relation to why the water wouldn't fall off of a sphere.
    – Valorum
    Commented Jul 28, 2016 at 16:31
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    I've located that quote. It was actually from a "New Scientist" interview article about fantasy writing.
    – Valorum
    Commented Oct 10, 2016 at 1:12
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Lacking a canonical explanation, here is what I always thought. Yes, it considers the canonical pseudo-explanation "because magic".

Where does the water go when it falls?

And for that matter, why does it even fall? We must assume that the Discworld has gravity [citation needed], but where does it come from? From the balance of the Disc on top of 4 elefants on top of a turtle that travels through space I state that Great A'Tuin (the turtle) is the main source of gravity. So the direction to the turtle is "down". So when the water falls off the rim of the disc it does so to go towards the turtle. That makes sense. Chelonii and water are two things that just make sense together.

But the waterfall wouldn't stay as a waterfall for long. After leaving the pressure of the atmosphere, the water would separate in billions of very tiny rain drops. Because that's just what water does in low pressure [citation needed]. That's why the waterfalls seem to disappear. The water is just not visible anymore, but it's there, all around Great A'Tuin, who swims in the thinest sea ever conceived.

How does it come back to the Disc?

When the sun goes under the Disc, it heats up all that water around A'Tuin and the steam goes up again. "Up" is the opposite of down [citation needed], so it would go away from the turtle. Why back to the Disc, of all directions? Pseudo-theories are welcome in the comments, but I think it's probably because the magic of the disc attracts it (you know, because magic and steam/smoke are things that also make sense together, have you never seen a magic show?).

Once back in the atmosphere (atmosdisc??), it tends to go towards the Hub. Why? Again, because magic. The Hub is stupidly cold because the magic field is so thick that the light can barely touch it. Also, because of the ice giants (or is it the other way around? Cause and effect are funny stuff). On its way to the Hub it will meet cold opposing cold streams and geography that it will make it fall as rain again. Probably on Ramtop mountains. Definitely not in XXXX. And from there, the cycle starts anew.

Why can't we just say "It's magic"?

But we just did. What I just proposed would be consistent with Ponder Stibbons statement:

Without magic the seas would run dry.

Because:

a) Without magic, the water wouldn't come back to the Disc and would eventually run out.

b) Even if the water did come back, the Hub wouldn't be so cold. The water would stay as steam, heated up by an unrelenting sun. The temperature of the disc would skyrocket. Life would mostly go extinct. And yesh, the seas would (practically) run dry.

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    @xDaizu I wouldn't say "down" is wherever A'Tuin is, aka, A'Tuin is the main source of gravity because in light fantastic (or Colour of magic, I'm not sure) there is a character who has arrived to the Disk by falling of the edge of another disk. Now if disworlds worked like you assume (having a planet-like gravity), you couldn't leave them by falling of the rim... you would just "fall around" and land somewhere near the other rim. I assume gravity works like "'down" is the direction of the turtle seen from the disk.
    – mg30rg
    Commented Jul 27, 2016 at 14:22
  • @mg30rg you would fall to another Disc if another force greater that gravity cancels it out. That would be, of course, Narrative Causality :P
    – xDaizu
    Commented Jul 27, 2016 at 14:34
  • @xDaizu The water troll (I think it was a water troll) said he was falling for hundreds of years, so I guess it didn't just fell of an other turtle which was passing by A'tuin.
    – mg30rg
    Commented Jul 27, 2016 at 14:40
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    @Miral Nice explanation, but don't forget the small gravity pockets made of narrativium here and there to explain the voyage of the troll from one turtle to the other.
    – xDaizu
    Commented Jul 28, 2016 at 8:05
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    @xDaizu Such is not necessary, assuming that the path between the two Discs (or more correctly, where the original Disc was and where A'tuin's Disc will be) is straight. (Or mostly so, since there will be some perturbation from passing stars or other turtles, even at a distance).
    – Miral
    Commented Jul 28, 2016 at 8:13

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