How to stop water runoff on floating islands that lack an available surface

Question

I am building a world with islands floating in air, and I'm looking for solutions to loss of water from runoff/erosion (think rivers emptying off the side of the islands). The islands vary in size, with the smallest around the size of Manhattan, and the largest about half the size of Iceland.

A few limitations: sunlight does not reach the surface below the islands, and nothing comes from the surface to the islands; the islands may as well be floating in a void in terms of how the surface affects them.

Not all islands are inhabited, and those that are developed their science and technology while on those islands, so while a more efficient modern solution is possible, there has to be a reason life didn't die off before it was reached.

Given the limitations, is there any way to avoid the loss of water through runoff? Solutions involving science, magic, or both are equally welcome!

Answer 1

Option 1 the surface is mostly ocean

If the the surface be mostly ocean that supplies plenty of evaporation and would also explain why things that fall off are never seen again, they hit the ocean and sink. Evaporation is influenced by wind patterns as much as light from the sun, the ocean does not need to be well lit to evaporate. Coriolis effect alone will generate updrafts. Keep in mind you need a LOT of floating land to actually prevent light reaching what is below it.

Its impossible for nothing to be exchanged with the surface, air moves and there is no much you can do about that. A bigger problem is how are the floating islands still there, if they are being eroded all the sediment eroded away ends up on the surface. millions of tons of island is being lost every year. that means that, at least on a geologic timescale, new floating islands must be generated on the surface (volcanic?)

Option 2 below the islands is a very thick layer of warmer air and clouds, most falling water re-evaporates , falling water does not stay a coherent stream air resistance breaks it into droplets fairly quickly. this can work with or without your deeper ocean layer.

does this layer of clouds make a lot of sense, no, but the neither does floating rocks. You already have gigatons of handwavium so it only needs to sound vaguely plausible.

I can even see why way to get you cloud layer, much of the clouds are not clouds but dust. Presumably whatever makes the islands float is also being eroded, and is washed off the sides by the rivers. But the fine particles can only sink so far before the "magic" keeps them afloat creating a layer of very fine dust. Since the particles will be banged against each other until they are so fine and light even a light breeze can carry them easily. This dust would also encourage cloud formation the same way cloud seeding does. this again also helps keep the below areas mostly dark. Although with wind patterns it can never be completely dark all the time, hurricanes for instance would create windows downward in their eye. Of course humans will only see it if they are cray enough to stuck their head over the side of an island and look down in the middle of a hurricane, so most will never see below the clouds.

Answer 2

The bottom of your islands would be of basalt, granite or similarly impermeable rock. The edges have a lip, formed through natural processes (heck, it could be simple survivor bias - all islands that formed without such a lip end up as baren, completely eroded disks of pure rock floating around); or mystic/god-based intervention if you prefer.

Now, all your life-carrying islands have a circular basin of water on their edge, where all the run-off pools up over time. The processes keeping solid sediment from filling this up and leading to overspill could lead to interesting plot point; i.e., cleaning these channels regularly could be a vital task performed by a holy caste, etc.

For the time before intelligent life, there could have developed plants that somehow transport the sediment in those channels back towards the center through some evolutionary benefit. Endless possibilities. Unless you find a way to make this interesting, I would just handwave the issue before any kind of life existed. Heck, even solid matter could be infused by volcanic activities, or a great meteor impact.

On the inland, water is recovered in the usual way through rain. Again, seasons could be a nice story anchor - you could build really interesting weather patterns combined with streams in the ocean which let the island float into the watery regions yearly, or something like that.

Answer 3

Magic

If the island were floating in the air, and gravity was pulling it downwards, it would be simple to add a portal at the bottom of the waterfall and link it to the start of the river/stream. The water could continuously flow downwards and reappear in an endless cycle.

Pros

• Never-ending cycle
• No physical labor required (other than magic)
• Simple and easy

Cons

• Water might have passed through some questionable stuff going downwards to the lower portal
• Evaporation (depending on the amount of heat)
• Water droplets may be lost on the way down (accumulation will effect the amount of water in the future)

Unsure Stuff (for you to decide)

Does the magic have to be renewed?

Answer 4

The first thing before we are going to give the reasoning behind why the water is not running off the floating islands is that why the islands are actually floating in the first place.

Floating islands should have a special ability to defy the surface's planet's gravity. The islands may as well have something to cling to, something like its land mass.

Therefore, everything includes the water above the floating islands will attract to the land mass of the floating islands, which is acting like the gravity for each individual floating islands. This will prevent all sort of things falling off the islands.

Answer 5

what if the bottom part of the floating island has some kind of enchantment that kind of generate somekind of force field or something that keeps them afloat. now when the water tun off the edges instead of falling off they are attracted by the field, and with gravity of something that also keeps the floating island not going anywhere , the water runs down at the bottom part neatly to the center of the island where the force that keeping the island afloat is the strongest and at first it looks like a reversed sea / lake, and then erosion makes the water breach the core of the island and making a circular flow.

Answer 6

It's Hot as Hell Down There

The farther down you go, the hotter your world gets. This means that all the rivers pouring off of your islands spills into a hot abyss and boils the water turning it into clouds giving the sky below the appearance of an endless sea of clouds, but falling water creates weather patterns that push clouds back up above the islands where they cool and rain back down onto the islands creating a complete water cycle even without any land or oceans below.

Answer 7

Ignore it

This isn't just a flippant "you should hand-wave it" answer. In reality, we have to be taught in school where the clouds come from. In your reality, it's a little more nebulous, but it's just as invisible.

Let's rationalize the entire system to be a 100-mile thick layer of gas giant clouds, where the islands are lighter-than-the-lower-air chemical suds encrusted with rock. The water that runs off of them would fall into the lower cloud deck, and rain would form in the clouds above you. The entire system would be unfathomably huge, but the water from below would still turn into clouds above, which would rain water down on the floating rocks. The rocks would likely have basins where the water would collect in lakes, and maybe even seas.

The point being that the water cycle will sort itself out once you've answered the more important questions, like "what is the rock cycle" and "why do we have volcanos."

Answer 8

The soil in these islands is made of impermeable material that is also very resistant to wear from water flow.

Seabeds come to mind. If they were permeable the oceans would have penetrated the seabeds and Earth's surface would be dry. Also oceanic trenches are formed from plate tectonics and not from erosion.

It may be that the floating islands were formed from chunks of the marine bedrock that for whatever reason began to float.

Answer 9

You don't stop it completely, you replenish what's lost

You cannot stop soil loss, even if you managed to stop all the water runoff there is the dust that is carried away by the wind and the small runoff caused by the occasional flooding that cannot be contained, on the long term the island would erode away. All the land we live in is a dynamic environment and the coastal land is extremely dynamic. Either sand is deposited by the sea currents or it is carried by the rivers from inland and partially eroded by the sea currents. Strips of land covered by mangroves slow down the exchange, but do not stop it completely. See what is happening to all the river deltas around the world, the increase use of the river waters reduced the amount of silt brought from inland and all the coastal areas are eroding.

The only solution is, first to reduce as much as possible the loss with a ring of mangroves around the island, but on the long therm that would not be enough. Then if you assume nothing comes from the sea currents it must be brought by the wind. The islands might be in front of a desert land, like the islands of Cabo Verde. In this world dust storms will be very frequent and they will bring sand from the desert.

Alternative. The island are trapped between two sea current that created a small loop downwind from a hotspot. A group of active volcanoes, that just pop out of the surface, spew a lot of ash that often deposits on the islands.

Answer 10

I have built a fantasy world and solved it in this world.

In this world, down is a real direction. Things made of the element earth pull things above them down. The range this happens grows with the size of the object in question, and it extends to the sides to a certain limited extent.

Water shields this effect somewhat.

Thus, water is pulled down by the islands, and flows off the sides. When it does so it ends up in the void, which is dominated by air.

Elemental Water in an area dominated by air forms clouds. These clouds in turn migrate.

When the clouds end up above a large amount of elemental earth (an island), it pulls the water down out of them. And you get rain.

If enough water gathers in the domain of air, it will clump and form a major storm system, or rarely an ocean. Oceans are like islands, but made of water. They are universally surrounded by storms.

Bad things happen when an ocean and an island intersect. That results in a flood!

The bottom of these islands is going to have a limited amount of down-pull (aka gravity), which means you can build structures hanging off the bottom. It is, as mentioned, dark down there.

As gravity is a product of elemental earth, ships that travel between islands use specially treated and reinforced wood to produce a local gravity field. If you are cast adrift from such a ship you just float in the void.

A lighter weight solution are wings, where you use the lack of gravity and large surfaces to "swim" through the air.

Below a certain threshold, islands don't produce sufficient gravity. These tend to get slowly pulled towards the nearest island.

Above a certain threshold, islands actually repel each other; so they are less likely to merge.

Answer 11

Ice Ice Baby...

There is a layer of really cold dense air just below the level of the islands caused by a permanent thermal inversion in the atmosphere. Water vapor accumulates and freezes on the underside of the islands in this cold zone. Gradually this ice builds up until water that flows off the surface of the islands also encounters these ice layers and starts adding to them. Eventually every island ends up being surrounded and contained inside an ice-bowl. The water near the center island remains liquid because it is constantly replenished with warmer water from the island but water near the edge tends to freeze into a ice dam because of atmospheric wind blowing up from the cold zone.

Answer 12

Keep your water in solid form.

This won't entirely stop water flow, but it'll slow to a glacial pace.

Thanks to the adiabatic lapse rate, it's just a matter of altitude. Here on earth it's about -10C/km.

Keep going up until the snow falls; problem solved.

Answer 13

Water coming on to land: precipitation.

Water coming off of land: evaporation.

If evaporation>precipitation, there is no runoff.

If precipitation>evaporation, there is runoff.

Now this limits the biomes you could worldbuild on these islands because there's a word for places where evaporation>precipitation and that word is 'deserts'. Anywhere verdant has to have an abundance of water.

One workaround I can think of is an aquifer in the belly of the island, replenished by intermittent/seasonal heavy rains, which feeds tree roots and springs.