Where does the water to transform Dune into a non-desert planet come from?

Question

In the first three Dune books, where does the water come from to terraform Dune into a non-desert planet? They have the qanats (water tanks/reservoirs), but where does the water come from? Also, aside from spice, what do the Fremen eat? (I doubt they could raise crops with so little water.)

Answer 1

Where did the water come from?

The Fremen were collecting water, under the guidance of Dr Pardot Keynes from the atmosphere into secret catch-basins using hidden wind traps.

Station tools began finding their way into the sietch warrens - especially cutterays which were used to dig underground catchbasins and hidden windtraps.

Water began collecting in the basins.

_{Dune - Appendix I - The Ecosystem of Dune}

And from various plants, cactus like in their retention of water:

There was a rare native root plant that grew above the 2,500 meter level in the northern temperate zone. A tuber two meters long yielded half a litre of water.

_{Dune - Appendix I - The Ecosystem of Dune}

The water extracted from the deathstills† would also be available to be added to the catch basins.

When it was done, Muriz clapped his hands once. Attendants came and removed the bodies, taking them to the deathstill where they could be rendered for their water.

_{The Children of Dune}

Sandtrout were also placed into deathstills to extract their water.

And She thought Sandtrout? Many times in this flesh and other had she played the childhood game, poling for sandtrout, teasing them into a thin glove membrane before taking them to the deathstill for their water.

_{The Children of Dune}

This method as well as getting water will also remove some of the Sandtrout that are part of the Spice>Sandtrout>Sandworm cycle. Though this cycle would not be able to be broken until the terraforming of Arrakis begins with vast quantities of water

Using all of these methods Pardot Kynes estimated that it would take:

In the manner of a teacher answering a child who has asked the sum of 2 plus 2, Kynes told them: "From three hundred to five hundred years."

_{Dune - Appendix I - The Ecosystem of Dune}

Following the establishment of a proto-ecosystem of various grasses and a few hardier plants in some protected areas of Arrakis the estimate was locked down a little further:

From the charts emerged a figure. Kynes reported it. Three per cent. If they could get three per cent of the green plant element on Arrakis involved in forming carbon compounds, they'd have their self-sustaining cycle.

"But how long?" the Fremen demanded.

"Oh, that: about three hundred and fifty years."

_{Dune - Appendix I - The Ecosystem of Dune}

Then came Muad'dib, who upon becoming emperor increased the pace of water collection to create the terraforming needed for the vision of the golden path he had, including the use of weather satellites to bring rain. This was taken up by Leto after his father could not continue on the golden path, until only a small personal desert, the Sareer, was left in God Emperor of Dune.

As for food.

There is the Muad'dib mouse that Paul took his Fremen name from, this could be eaten, as could anything that preys on it.

There are also carrion birds mentioned by Liet Keynes

There are carrion eater birds over me. Perhaps some of my Fremen will see them and come to investigate.

_Dune

These again could be eaten, and presupposes there is an active enough ecosystem on Arrakis to support them.

Plants are shown to be tended in the sietches using plastic sills to gather water and funnel them to the roots.

One could also guess that mushrooms would grow in the various caves that make up a sietch.

Following the establishment of the nascent ecosystem the following fauna and flora were introduced:

They turned then to the necessary animal life - burrowing creatures to open the soil and aerate it: kit fox, kangaroo mouse, desert hare, sand terrapin . . . and the predators to keep them in check: desert hawk, dwarf owl, eagle and desert owl; and insects to fill the niches these couldn't reach: scorpion, centipede, trapdoor spider, the biting wasp and the wormfly . . . and the desert bat to keep watch on these.

Now came the crucial test: date palms, cotton, melons, coffee, medicinals - more than 200 selected food plant types to test and adapt

_{Dune - Appendix I - The Ecosystem of Dune}

All of which would conceivably be used to supplement their existing diet.

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†_{a machine used to extract water from a body, usually dead, though the person can be alive when used as Fremen capital punishment}

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Answer 2

The sandtrout encapsulates free water, binding it. Once sandtrout starts dying in numbers that water gets released.

As for food: the Fremen trap water, mainly from the atmosphere, and presumably use that for irrigation or hydroponics.

Answer 3

I don’t think the sandworms and aquifers has much to do with the arrival of water on Arakis. At the end of the film when Paul overcomes the empire and thus becomes emperor, a quick glimpse of the waters of Caladan are shown to the viewer. I believe that Paul, after drinking the water of life, acquired the ability to bend space and thus transport water from Caladan to Arrakis with his mind. For a moment there is a shot of Paul looking into the distance as if meditating as the rain arrives.

Answer 4

Arrakis has enough water locked in the atmosphere, also underground in permafrost. Vast man-made underground cashes were replenished almost exclusively from atmosphere.

Station tools began finding their way into the sietch warrens - especially cutterays which were used to dig underground catchbasins and hidden windtraps. Water began collecting in the basins. (Dune - Appendix I - The Ecosystem of Dune)