~ desalinate with <5% the energy & equipment ~

TL;DR — We need to stop droughts, especially as climate change makes them worse. Desalination is nice, capturing clean water from the sea, yet it requires expensive equipment and lots and lots of energy. We can, instead, accelerate evaporation OUT into the air above our heads! We won’t be ‘capturing’ that clean water — it just rains-down miles away. Perfect, because we don’t need pipes and pumps to transport it! Using this sort of design, we can end droughts and re-take the land captured by deserts in recent decades, cheaply and quickly.

The Concept

Imagine you have a bowl of salty sea water, with a tiny hole at the bottom. The sea water is slowly dribbling out of that hole, into another bowl beneath, to then get pumped back up to the top. At that dribbling spout, you hang a loose, fuzzy length of yarn, straight down. The water is held against that yarn by surface tension, scuttling down along its surface.

As the wind blows past your wet-yarn-wick, it will cause evaporation from that surface, adding humidity to the air, to come back down as rain further downwind. (Place the evaporators geographically so that the wind carries the water to the general spot you plan to use.) With wide troughs, thousands of yarns packed per square yard, stacked a dozen feet high in layers, you can pump vast amounts of water into the air.

The cost of pumping sea water up onto those troughs, only a few dozen meters above sea-level, is less than 1 Megajoule per ton of water. A Megajoule generated on-site from solar, only pumping when the sun shines, costs about 1 or 2 cents, for a ton of water to evaporate and return as rain on farms and snowpack in the mountains. For anyone in Ag, this translates to “$12 per acre-foot of irrigation, with no pumping or well-drilling, maintenance, or ditches… because it just rains.” That sounds darn good to me!

Compare that to the top-of-the-line desalination plants, spending 10 to 20 Megajoules per ton of fresh water produced! If you want to irrigate with that, you need to pump it to the site, with the associated barrier to entry and lag to scaling, maintenance and risk, while the water itself still costs you $240 per acre-foot! Some regions of California have seen peak prices at $2,000 an acre-foot, which is crippling. At $12 a pop, we can get green again — in flora and finances.

Getting into Details

Salt, first — as you evaporate and regurgitate the water in those troughs, it becomes brine, and you pump that out. It’ll only be a fraction of the original volume, and it’ll be chock-full of lithium, magnesium, and potassium, so you might as well make money selling those to make batteries and feed chemical industries. The sodium salt, unfortunately, must be gently dripped back across the wide ocean, OR we can just pile it up to make a giant salt pyramid somewhere nearby. I favor the latter option, because it’s much cheaper and it would still take ages to accumulate a pyramid that would be so large as to be an ‘obstruction’ — plus, the ocean really won’t notice the salt-loss at all, being miles deep. It’d also be rad to ski down and take four-wheelers up it — there are tourist revenue streams for such a unique oddity!

Coastline, next — you do need to pump the sea-water short distances, and up only short elevations, to make it cheap. So, coastlines would need tall, layered racks of troughs, stretching inland as far as elevation allows, to spread water deep to the continent. Another option, with a higher capital cost and maintenance, yet which would allow you to use this option in cases where the coastline is too steep or precious to locals: buoys floating just off-shore, where the winds are still strong!

Adding to the coastline problems: you must pick coasts where the winds pull from the sea into the dry target-region. California’s central valley is a prime example. So are Libya and Israel, able to water deep into the dry lands. Yet, the regions receiving rain from such coasts are often a different country! That’s the biggest barrier to development, and it will be difficult to get those countries to form a neighborly arrangement, until other countries have demonstrated evaporation’s efficacy and value. Once other successes pile-up, the incentive to stop squabbling becomes larger — in order to cash-in on the opportunity.

Power is an issue, too — the best way to power the evaporators is with solar concentrators along that same coastline. Solar can extract hundreds of watts per square meter, which means you only use a thin ribbon of solar to pump for a wide, many-layered swath of evaporator-troughs and yarns. It’ll be the most expensive capital, yet it’s also a small percentage of the total area and equipment. And, by positioning the solar *up-wind* from the evaporators, you are guaranteed many days of clear skies, without your own clouds disrupting power-supplies!

The Big Picture

If these evaporators desalinate and transport irrigation water dozens of times cheaper, hundreds of miles inland, that makes it worth-while — profitable! — to convert many disregarded dry regions into crop land and pasture. That takes pressure off deforestation, much of which happens for the purpose of grazing lands. The more we green the deserts, the more native land we can return to the biome.

Further, we’ve been losing agricultural land, accelerated especially by desertification. This would reverse that trend in many locations, preserving what we do use. Rains are also acceptable for municipal water supplies, precious in regions without steady fresh-water reserves for drinking. And, the rain’s humidity stabilizes the wild temperature-swings of deserts’ day and night, protecting plants and making the region more comfortable and desirable for people to live there. (aka ‘real estate values’)

There is also a social dividend to the work: being the pioneer of it, as well as sharing rain with your neighbors. It can even be a diplomatic shield: who would attack one so useful, especially when that same diligent rainmaker could turn the water off? Evaporators are worth much more than lettuce!

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I'm missing numbers here. How much water could you evaporate, compared to the amount of sea water that evaporates every day?

My guess is this would be (pun not intended) a drop in the ocean.

My read suggests that OP is probably less interested in increasing evaporation overall (though it would increase) than controlling where the water enters the atmosphere. There are places that are dry only because there happens to be a mountain in between them and the ocean, for example. Moving the water a long way is something we already know how to do (think oil pipelines, but containing salt water instead of hydrocarbon slurry). If it scales, this could make a substantial difference to such places.

Downside is that weather is the output of an insanely complex set of interconnecting natural systems and cycles. Making changes to the climate of a region this way will have unforeseeable side-effects over vast distances. Given the likely cost laying pipe over a mountain or whatever, I doubt many governments will be willing to take the risk of their big expensive weather-modification project provably messing up rainfall patterns or creating geologic instability or something in another state or country and having choose between paying enormous damages or eating their sunk construction costs. Most likely they would be unable to make that decision in a timely manner and default to doing both in the long run.

Then again, fracking, so I might be wrong about that.

I've gone into clarifications, as well as running numbers on example build-outs and yields, in the comments below. I just wanted to make a particular point here, though: the difference between "Unintended Consequences" and "Unforeseeable Side-Effects".

When I build a bridge to ease traffic, and it leads to suburban sprawl, that's both unintended and unforeseen. When my country's coal particulate drops, because of clean-air regulations, and this removes sulfur and dust that was helping to cool the Earth, we have an unintended consequence: increased global warming, somewhat. Yet! This consequence was not unforeseen. We can use our knowledge of science, along with careful simulation, modeling, prototypes, staged roll-out, user feedback, community engagement, ... to avoid the unforeseen. Even when our actions have a few side-effects we didn't intend.

And, in particular, the claim that consequences are unforeseeABLE is bold. That would require "weather is beyond our ken, forever." Instead, weather modeling has improved radically with artificial intelligence, and we are roughly accurate with hurricanes a week in advance, and google does 'now-casting', which has historically been the hardest part of forecasting weather, while long-term models tend to average-out any slight perturbations nicely. SO! Weather is complex, and our actions will always have side-effects which were not included in our hopes - hence, un-intended. Yet, we have the power to test, empirically, and study, to avoid the unforeseen. I wake in cold sweats for the Unforeseeable.

claim that consequences are unforeseeABLE is bold. That would require "weather is beyond our ken, forever."

Maniac Extreme type argument on a minor semantic point.

We can make some pretty good guesses, but right now we have no effective means to fully and accurately predict the long-term and long-distance meteorological, geological, and hydrological side effects of a project that results in a moderate-to-major change in the annual rainfall of a region. There will be consequences that we are unABLE to forsee. Some of those consequences could be large, some could be negative. Some could be both, maybe we don't get either.

Oh, my apologies - I am happy to concede that "currently unforeseeable" is a reasonable limitation in complex systems; I hadn't noticed that qualifier.

And, if you had asked me four years ago "Might our weather models miss some catastrophic downstream consequence, which negates the potential value of returning jungle (now pasture) back to jungle, and preventing California droughts?" I would have given it a decent chance, which would negate the more intrusive, all-or-nothing interventions.

Yet - weather modelling is improving rapidly, with neural networks. Google is able to do "now-casting", which forecasts local weather condition at small time scales. That sort of modelling was previously out-of-bounds, because it requires much smaller & more numerous voxels and turbulence could throw everything off due to local traffic conditions or a factory being shut down for maintenance. The fact that we have now-casting, among other steady improvements, lowers my assessment of a catastrophic blunder. Especially if we roll-out in a place like California, such that we return water to its state in the 1960s, which obviously would not be catastrophically disruptive.

So, it's true that science misses catastrophe some times, and weather is complex, while very recent improvements in modelling reduce the risk of catastrophic disruption, especially when returning water to climate-change-parched regions, recently wet.


Fracking has a clear connection between action and reward.

Oil company uses fracking -> it's super effective -> oil company pumps oil -> oil sells at a high price.  Then the oil company makes some political donations and the government is encouraged to allow it despite any damages to less politically connected people's property (contaminated groundwater, microearthquakes or even the possibility that fracking causes this.  This is why fracking is banned in certain states and many EU countries)

Desalination has similar action -> reward mapping, as long as the water can be produced for a price that it is profitable to sell the water, it's worth doing.

This proposal just generally increases rainfall in the area it's done in.  But a lot of the water will just fall on barren desert, and the rainfall is inconsistent, and it's hard to tell how much of the rain is from the seawater evaporation.  And it can't be excluded as a good - you can't deny water to people not paying the subscription fee.  

My guess would also be that if there is airflow over the ocean the air would hold most of the water you could hope for with such an approach.

Yes! Only 11,000 years ago, the solar intensity over the Sahara was 7% greater (Milankovitch, natch!) which caused about 10% more convection of air over land... from the humid Mediterranean! That's just a 10% boost, but it was enough to cross a 'threshold' of humidity, when clouds can actually form, and rain can actually fall. There's still moisture above the desert - it's just never dense enough to come back to Earth!

So, the Sahara got 10% more moisture, which let a few plants grow... and plants are darker than desert rock, so they created a steeper gradient of heating every morning - the green inland regions got hot and humid fast, air rising, which pulled MORE sea-breeze and humidity deep inland! It's a feedback loop, which was how the Sahara was green for thousands of years, covered in laurel forests and grasslands, in cycles stretching back tens of thousands of years.

The Point: if we add just a little bit more water, we can get a feedback loop, just like our geological past. We're on the threshold of rain-formation humidity.

About 2 million cubic meters of water evaporates from the dead sea every day, but the sea is surrounded by desert in every direction. The primary wind is to the east, yet to the east lies 500 km of Arabian desert. You would presumably need a lot more water than that to evaporate to change things.

Meanwhile the same amount of desalinated water would supply the domestic needs for 10 million people, or irrigate 400 square km of farmland.

Meanwhile to supply enough water to turn the Arabian desert to (still very parched) semi desert you would need to evaporate around a billion cubic metres every single day. You can't do that much better than this by focusing on a smaller region, since you can't decide where the rain will fall.

Sure if you move large enough quantities of water (for huge values of large) you might be able to make the desert bloom for cheaper than desalinating water. But that would still be many times more expensive than just supplying desalinated water to everyone who wants it.

I just don't really think this would work in practice.

Let's do numbers! The first consideration is: it doesn't matter how many threads of yarn, over what distance traveled or residency-time, as long as the air that passes into the yarn-array comes out of the yarn-array near saturation, say 80%+ humidity.

So, suppose we erect one of these arrays around the entire perimeter of the dead sea. That's 135km, but wind only comes from one side, so let's round down and assume we're only benefitting from the projection's length - roughly 85km. Now, it seems that the dead sea gets decent wind - often above 8km/hr, but let's round that down to 3km/hr to be safe. If we're pumping to trays that drip onto wet yarn in layers, with the top-most being 4m high, then we can measure the amount of air that passes through these arrays in a day, and estimate how much water they would transport! (Remember, the trays may extend hundreds of meters, before saturation of the air... 'depth' of the array is set locally, and doesn't change the total amount of water at saturation.)

So, 3km/hr - 0.83m/s, and the array is 3m tall, for 2.5m/s air-flow per 1 meter of shoreline. There are 86,400 seconds in a day - 216,000 m3 pass through that 1m wide section of the array, each day. And the projection along the shoreline is 85,000 meters long! That's 18.36 Billion m3 of air per day. How much water would that hold? Ah, night-time temperatures are too cold, so we would have to cut our rate in a third and just use the daytime rate, to be safe again... And, winter is similarly cold; knock the total down to 1/4th. Of 25C or more.

20C holds 17grams of water per m3, 30C holds 30grams. Higher temps absorb water supra-linearly. Let's call it 20 grams of water per m3. And, we decided to cut our volume to 1/4th, accounting for times when "the wind is blowing, but it's chilly", giving us 4.59 Billion m3. Multiply that by 0.02kg - for 91,800 tons of water per day. That's not seven million tons a day; seventy times less!

Yet, as I mentioned, you would want solar concentrators to power the pumps, which will convect and drive a draft, multiplying the quantity of air-flow per day, and thus the water transported. With dispersed convection, you can mix-in dry air to evaporate a second round, further from shore. And, I only used 3m tall trough-and-yarn stacks, when we could easily construct permanent scaffolds many times higher. That same mixing happens while transiting the open waters, so opposite shores do have a chance to add to total evaporate, as well.

More importantly, the dead sea is small - the whole salty coast just west of it is 640km, for an eightfold increase in air-front. Consider also that your "2 million cubic meters" sounds big... but it's only a yard deep, less than one square mile big. It's only about 3,250 acre-feet, or almost a million bucks in rain, here. And, the desert isn't a desert because "the Dead Sea isn't evaporating enough" - it's the downdraft of dry air resulting from the Hadley and Ferrel cells meeting. The Dead Sea is fighting a continual desiccation-front.

So, just like it has every ten-ish millennia, a shift in sea-breezes and humidity are able to overcome that barrier, leading to an accumulation of water - the Sahara has done this many times. Once you hop over the threshold of 'all those first 3 million m3 evaporated', then the rest accumulates. Wondrously, once vegetation arises, that increases the daily convection, driving more sea-breeze, more rain. Sahara, again, demonstrates this. The initial shift in humidity was small, yet still enough to feedback into cycles of greening.

Further, considering that we only pumped the water up a few meters, needing some 100kJ per ton, then our operation provided that water at 1/100th the going fuel-rate, and more importantly - less expensive capital equipment. If desalination is 1/100th the cost, then even if only 2% of the rain fell on your own land, it's still better than laying irrigation. Ask anyone in Ag!

So let's leave the dead sea alone and move on to the Persian gulf.

The Persian gulf is huge - some 250,000 square km, and extremely hot. It probably evaporates about a billion tons of water a day.

And yet it's completely surrounded by desert in every direction. Even Dubai, sandwich with sea to the west, north and east is as parched as it can get.

Despite all the water evaporating from the sea, there's still very little rain in the Arabian peninsula or in Iran. I don't know why that is precisely, but it seems to me that it breaks down the seemingly simple calculation: evaporate lots of water here, get rain further downwind.

It seems to me that simply evaporating lots of water would be very unlikely to achieve the changes you would actually want to.

So how much water would you need to evaporate per day, and from where to make the Arabian desert bloom?

Let's start at a more practical scale: make the Negev Bloom.

The Negev is 12,000 km2, which, if we want grasslands, needs some 300mm extra rain or more each year. That's 3.6 billion tons per year, or just 10Mt a day. With 20g/m3 humidity, we'll need passage of 500 billion m3 of air-flow each day. With convection driven by solar concentrators (those same which drive the pumps) to increase wind velocity during the day to 4m/s, across trays stacked 12.5m high, provides 50m3/sec, 4.32 million m3 per day across each meter of intake.

Next, we pump rows inland, as each humid layer rises, to capture drier air as they mix and move-past. Additional solar concentrators power these, and conveniently, the concentrators' intense heat pushes humid air higher than it would during gentle billowing convection, rising to cool & enter the cloud-cycle faster. We would only be prevented from extending more rows if the elevation rises too high, or we create so much humidity and cloud-cover that our solar concentrators cease. Let's just say we have four rows.

With 4.32 million m3 per meter of intake width, we'll need 116,000 meters... that's only 72 miles. With our four rows, that's a length of coast 18 miles long. The Gaza Strip is enough to water the Negev.

And, as I mentioned in an earlier response to you, the vast majority of the humidity released by the Persian Gulf, Dead Sea, Red Sea, Mediterranean, is being used to fight-against the immense downdraft of adiabatically-heated and ultra-dry upper atmosphere, which is descending because of the boundary between Hadley and Ferrel cells. So, yes, there are billions of tons of water evaporating, and no rain!

Yet, we know from geological records as recent as 9,000 bc, the Sahara was wet, with vast lakes - because of a slight increase in humidity above the threshold for accumulation. The deserts are not 'infinitely' dry, such that all water never results in rain. Rather, they are just below a 'threshold', with water added by evaporation in huge amounts, and a slightly huger amount being taken away by adiabatic downdraft. If we add just a portion of humidity, we are doing exactly what occurred across the Sahara repeatedly, and it led to accumulation, because it was enough to cross the desiccation threshold. Our own soil records prove that the desert can be green, with just a little more water than it currently evaporates.

But there's already tons of water evaporating from the sea alone the coast of Gaza. What would this make a difference?

Yes, as I mentioned twice now:

The reason there is a desert is because the Hadley and Ferrel Cells are meeting and descending along the Horse Latitudes. As that air falls from 10km up, it compresses, which causes adiabatic heating. And, at that initial altitude, it only contained 0.1g water per m3 to begin with - so, as that air reaches the ground, hot and dry, it sucks-up all the humidity, keeping the air below the threshold for making rain.

We know as a geological fact that the amount of water being sucked-up by that adiabatic heating is only a little bit more than the amount of water that the seas are evaporating. This is because, only a few thousand years ago (and in cycles repeatedly) the entire Sahara was green, due to a slight increase in evaporate and sea-breeze due to increased convection. It was only a SLIGHT increase which allowed the humidity to cross that threshold of rain-cloud formation, leading to an accumulation of moisture, plant life, driving more convection in a feedback loop that brought lakes to the Sahara. You seem to ignore the fact that the whole Sahara was green!

So, we only need to add enough additional humidity to cross the threshold of clouds, before all further evaporate becomes rain, in surplus. I've explained this a few times now. Are you noticing the details I point out, most of all "crossing a threshold of adiabatic desiccation"?

Ok, so that's the important detail I missed. A small amount of extra moisture would cause all the existing moisture to rain down.

Do we have any way of knowing how much extra, or where?

Erm, no, still a mistake. Let's walk through a different format:

You have a bank account, which is auto-billed $3million a day by adiabatic downdrafts, so you are nervous about making that payment every day - especially because you are only evaporating $2million a day to add humidity to your account! You're going $1million into debt, daily. That's a desert.

Yet, the Sahara got 10% more sea-breeze convection, 11,000 years ago, which was enough for it to have a budget surplus - that's like saying "Sahara's account is being billed $3mill a day, and they were previously only able to put-in $2.9mill a day, but now Sahara is earning 10% more sea-breeze, which puts daily earnings at $3.19mill, for a surplus of $190K a day - Sahara will start forming lakes, now!" And that plant life it gets will pull more sea-breeze in a feedback, to help-out.

So, the added moisture is NOT causing "all the existing moisture to rain down." It's about exceeding the threshold, to generate a surplus. And we know, from Sahara's green periods, that the desert is actually pretty close to that surplus - Sahara needed only 7% more sunlight, to drive 10% more wind!

Ok, got it now. Still, is there any way of knowing how much that is? 10% of all the water evaporating from all the seas near the Sahara is still a huge amount of water. And if we do add more water, is there any way of knowing where it will rain down, as opposed to being spread over the entirety of the Sahara and ending up basically useless?

Or is this the sort of thing where you would have to invest huge amounts into infrastructure to do this, before you can tell what affect it will have?

How quickly it rains down depends on a few factors, and we can tip those in our favor:

--> Humid Rise - humidity (just the h2o molecule) is only 18g/mol, while oxygen molecules are 32g/mol, so humid air is quite buoyant! Especially considering that water vapor reflects heat (infrared) back to the ground, creating a heat bulge beneath it. The result is that, once humidity begins to rise, it naturally pulls air in from all around it, along the ground. It begins to drive convection. Yet! That humid rise is normally billowy and easily dispersed by cross-breezes, which means that the humidity cannot rise high quickly; it mostly travels far overland, or stays in place. Your rain wanders to an unexpected location! We want to form rain clouds nearby, instead, so we need that humidity to rise really high, quickly, without being torn apart by cross-breezes. That's where the solar concentrators help, with their tall tower at 1200C and radiant, they blast infrared into all the water vapor around them, pummeling a plume high up, carrying that vapor. Up high enough, the air pressure drops, which is key for causing a rapid cooling, and the formation of nice heavy clouds. The faster we take air from the ground up to a few kilometers, the more water it'll still be holding. [[Only a fraction of one gram per m3 is needed for the thinnest clouds, but we could toss a few grams up and it'll come down soon. We want the water to rain, evaporate, and rain down again, in as many cycles as it can. That gives plants time to grab it, in numerous locations, as well as time for the ground to catch some.]] When we look at water-demand for plants in the wild vs. water-resilient greenhouses, we can drop water demand ten-fold because nine-tenths of the water was lost in the leaves to evapotranspiration! As a result, if that leaf-sweat keeps rising and falling as rain as it travels further South, then the same bucket of water ends up getting ten times the use (assuming ground water is eventually used, as well).

--> Albedo - the desert rock is pretty bright, so the addition of vegetation and especially any water-bodies (!) will multiply the solar absorption, which will drive that heat-bulge and evaporation for humidity-buoyancy, to help loft water vapor and form clouds. This is how the Amazon does it - most of her clouds are her armpit fog, caused by solar-to-thermal foliage!

--> Vortices - the solar concentrators themselves can be rigged with a few flanges, to nudge their inflowing convection as it quickens toward the center, to spin that up-draft, helping it stay coherent and push higher, for rains nearby. Any Youtube video on Rocket Stoves by Robert Murray-Smith is best for enjoying such a vortex!

--> Swales - I love swales. I've been preaching swales since 2010. I heard, almost immediately, when Sepp Holzer started pitching his "crater gardens" ... which were dug by an excavator, four feet deep. I was aghast - my favorite swales are micro-swales, a few inches deep, in flakey soils that rain seasonally, to catch it as it dribbles. That's what they're doing in the Sahel, south of Sahara, to stop the deserts. By halting the flow of water along the ground, keeping it for seep, roots, and another evaporation, you prolong the residence-time of each ton of water, leading to a greater equilibrium stock - that is, a high normal lake line, because each ton of water rarely ever leaves.

And, as to infrastructure before success - California could probably boost rains enough to help farmers and forests, here, without needing to conquer an entire desert the size of Europe!

Thank you for diving into the details with me, and continuing to ask probing questions!

The water brought-in by the Sahara doesn't depend upon the area of the source; it's the humidity times the m3 per second arriving. Humidity is low on arrival, reaching only 50% right now in Tunisia, their winter drizzles! The wind speed is roughly 2m/sec coming in from the sea, which is only 172,800m/day of drift. Yet! That sea-breeze is a wall of air a half kilometer high - that is why it can hold quite a bit.

If we need +10% of a 500m tall drift, that's 50m; if we can use solar concentrators to accelerate convection, we can get away with less. And, we're allowed to do an initial row that follows the shoreline closely, while a second row is a quarter kilometer inland, running parallel to the shore, where mixing of air lets you add another round of evaporate. So, we could have four rows across the northern edge of the Sahara, each row as thick as it needs to be to hit high humidity, and 10m tall, to send +10% moisture over the entire 9 million km2 of the Sahara.

How much water would we be pumping? The Sahara carries 172,800m/day flow per m2 intake surface x 500m tall x 4,000km coastline at 10g h2o per m3 = 3.5 billion tons per day, a thousand or so dead seas. (About 1.25 Trillion tons a year, enough to cover the 9 Million km2 with 139mm of rain, on average, if it had fallen instead of being sopped-up by adiabatic heat.)

We need 10% of that, or a hundred and eighty dead seas. It seems monstrous, but much of the coastline there is low for miles, so pumping 1 ton to the top of 10m at even just 20% efficiency costs 500kJ. If you want to pump that in a day, using solar, you'll need 1/4th of a square foot of solar. That 1 ton, if we cross the threshold and it becomes surplus rain, will water 3 square meters their annual budget... and the solar is paying for that amount of irrigation every day; 1,000 m2 of rains from a dinner plate of solar, each year. It's that energy efficiency, combined with dead simple capital expenditures, which would make something so insane potentially feasible. I'd pick California to try, first!

500kJ per ton, for 350Mil tons per day - that's 175TJ per day, or 2 GW. That's a nuclear power plant. To pump enough water, continuously, to irrigate 9 million km2, potentially feeding a billion people, once we dig swales! (Check out Africa's better-than-trees plan: "Demi-Lune" swales that catch sparse, seasonal rain, to seep into the ground, with minimal tools and labor!)

Just something I've accidentally look into Only regarding the extraction of lithium and other rare metal from concentrated sea water.

Paradoxically, increased concentration actually WORSEN the efficacy of our current technique to extract rare metals from sea water (adsorption) which was not very efficient in the first place. Presumably from higher concentration of sodium salt. For further information you can look up extraction of uranium from desalination output.

This is a cool idea. However it strikes me as obvious - as in, there are a lot of very smart people already looking for solutions like this so I expect it has been explored before. Did you search for existing literature? I did very briefly but didn't come up with anything useful.

Large scale evaporation is already done in lithium mining, for example. Perhaps some related studies have been done there.

I also expect the are potential issues besides the geopolitical ones highlighted here.

What effects would this have on the coastline ecosystem - for example, due to concentration of salt over a large area?

What materials could be used for the wicks? If it's a natural material I guess it would decompose into a salty mush.

Not to mention, complex environmental systems are... Complex! How hard is it to be reasonably sure an experiment like this would actually have the intended positive effects? Rather than causing localized flooding due to unexpected concentration of the rainfall, for example.

Final note - you state several figures related to cost (megajoules of energy for pumping, cost to generate a megajoule with solar etc.). I don't have a reason to doubt these but generally you should provide sources along with numbers so that people can independently verify them.

Evaporating into the 'wild' isn't profitable, so it's understandable that no one sought this route - I only expect a government to fund it, because they'd see returns in taxes regardless of which farm got rain. There's also a loooong history of simple solutions going unnoticed for decades; I mentioned a few when Julia Wise argued the same on the EA Forum cross-post.

Another example of "simple-and-ignored" to add to the mix: last year, a mathematician was in a class on Knot Theory, and the teacher mentioned that "the Conway 11-Knot is unsolved"... she took a look at it, thought about it intermittently for a few days, and came back to her teacher saying "What about doing it this way?" She was right - Quanta magazine wrote her up! It turned out, EVERYONE had missed a simple solution, after writing it up in journals, grad students struggling in vain hope, Conway himself miffed, for FIFTY years. Yup. Simple is invisible, sometimes.

Geopolitical issues are only certain regions. Others, like California, are begging for evaporators! Australia? It's all just one contiguous desert, and they were trying cludgier water pipeline projects decades ago. Southeast India, too. And if Libya only received a quarter of the rain they threw into the air, it'd still be good for them.

Lithium ponds are doing a separation; they are intentionally not evaporating as fast as possible as much as possible. A better area of industry to look at: spray & contact cooling. When machines or air-flows are hot, we create a lot of wet surface area, and we measure the evaporation from those surfaces. By having hundreds of yarns per square yard, hanging in parallel, you expose the air to hundreds of times more surface area than a pond does. And, the air-flow moves among the strands, while ponds create a layer of 100% humidity that buffers further evaporation. When industry wants to cool things, they use wet surface area, and a dense array of threads maximizes that surface area.

For salt, I already mentioned: you either have to drip it across the wide ocean, to avoid over-salting the water (the halocline is an invisible barrier to life)... OR, you pile it into a vast pyramid. That pyramid could be a tourist attraction, considering how many folks visit salt flats to ride kite buggies or burn art pieces while on drugs. To prevent salt leeching into the surrounding landscape, you first pick a large rock outcropping - granite is best. Then, surround with more rock to form a foundational retaining wall, gravity-style. Fill the basin with a layer of sand, two inches. Hire all of Elong Mush's workforce, to blast the sand-bed with THERMAL LANCES (which will need a welding torch to ignite ... wtf?!?), melting it into a contiguous glass bottom. Repeat a few times for good measure. Spew brine on top and let it evaporate into a salt-flat piling for decades. Bonus: because you hired all of Elong's bruhs, his empire collapses and the world wakes to sanity!

Flash floods are when a hot & humid 'atmospheric river' of some 100km wide, 2km thick, thousands of km long, bashes head-long into mountains or a cold front, like a long locomotive piling-up at a painted tunnel! Evaporating from rows of yarn into the air from the shoreline is a steady addition, which is accelerated only when the air is drier and warmer... the exact same times you don't have to worry about a flood. To give you a sense of how much 'water per area per day' we would be adding, for agricultural purposes: you can get good grasslands on 300mm/yr, or just 1mm per day... and evaporate would be spread mostly evenly, with hotter summer months seeing perhaps triple - 3mm, or an eight of an inch. It's a drizzle, but it would come in the hot, dry months, when the plants need it most.

For wicks, that's not really an engineering problem. Fiberglass is inert, and it wouldn't be disturbed; salt accumulation is just flushed with higher water levels => higher flow rates. And, desal megajoules are on wikipedia; though be wary, because the most energy-efficient method also has the most capital-expense, such that the final cost of water is far above the "1cent per Megajoule" cost of energy, above. "1cent per Megajoule" is near the good end of solar concentrators - Morocco's is about that much, on-site, if I recall. Being in the desert, I expect solar would be on the cheaper end. [[That translates to 3.6cents per kWh, btw, and by the time it gets transmitted with losses, with profit and transmission-capital expenses, folks usually pay triple.]]

I hope that helps clear-up some details!

Ah, thank you so much! That first Nature article is inspiring - I'll be twiddling my brain-thumbs tonight. (Sadly, it seems sea water's salt and particulates might gunk-up their evaporator-engine's pores; it would be excellent to deploy for the fresh water bodies that accumulate as a result of sea-water evap, regardless!) That article mentions generating a third of a terawatt, and saving a hundred billion tons of water, in the US alone, which would be spectacular - I hope that they can keep capital costs down, when they get devices out of the lab.

Desiccant cooling is sweet - you can also use cold desert nights to make ice in shaded ponds, (Persians were first, I think...) and if that gives you brine for salt, add the salt back to the ice for even lower temperatures! And, atmospheric water harvesting is lovely, especially the small-scale, low-budget stuff for people denied infrastructure and credit. You could definitely add dew-nets deeper in a large desert, to increase the residence-time of any moisture that traveled so far, multiplying the accumulation from yarn-evaporators!

Most of all, thank you for linking Metaphor Search! Your hope has been fulfilled - and I hope you were able to rest well!