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This is a childhood dream.

What would happen if pipes were installed (similar in principle to the Great Man-Made River, but for seawater) to continuously pump seawater into the Sahara desert.

Let's say that a precise desert location, with no known aquifer nor water table, was chosen to be an inner sea. And let's say that wildlife was negligible at that location. And that solar panels were installed to provide energy for the pumping of the water. And that the water was pumped at the surface of the Mediterranean sea and that precautions were taken not to greatly damage sea life.

How hard will it be to fill, say, a Nevada sized inner salty lake? And would that humidify the air enough that dew - falling to the ground each night - would somehow start a greening cycle?

What if a little mud was added to the water to precipitate into a bed that would retain the lake?

Is there any modification to this childish plan that would make it, at least, realizable?

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  • $\begingroup$ When thinking about a possible answer, I arrive at "under what conditions would the new salt lake actually lead to dew?" $\endgroup$ – mart Feb 22 '16 at 15:32
  • $\begingroup$ Day, Heat, water, vapour, night, cold, condensation... No? It should seem clear by now that I have no knowledge of meteorology... But that made sense to me... It doesn't? $\endgroup$ – ZakC Feb 22 '16 at 15:37
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    $\begingroup$ See: en.wikipedia.org/wiki/Qattara_Depression_Project $\endgroup$ – Siv Jan 1 '17 at 11:36
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    $\begingroup$ Very loosely related. what-if.xkcd.com/152 I think xkcd is probably right, without flow, it would just be a mess that would dry up and not bring much life and leave a stinky pile of salt when it's gone. $\endgroup$ – userLTK Jan 1 '17 at 15:29
  • $\begingroup$ See also some of the answers at: worldbuilding.stackexchange.com/questions/66895/… $\endgroup$ – JeopardyTempest Jan 6 '17 at 21:02
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This is an interesting question & I've been waiting to see what answers, if any, would be written.

One affect of creating such a lake would be a localized increase in humidity in the vicinity of the new lake, but nothing that would significantly increase rainfall.

By bringing in salty sea water there could be a risk of contaminating existing underground fresh water stores with salt, making such sources of water unusable.

Also, by being salty, the water could not be used irrigate crops. It may be possible to establish a fishery in the lake, bringing in salt water species of fish, but that would need to be carefully managed.

If for whatever reason, the supply of water was not maintained in the lake, eventually the lake would dry up leaving salt behind. This would then be blow around by the wind, contaminating the nearby land with salt and killing plants on that land and making the land unsuitable for any crops that may grow there now.

Also, if dew were to form, there is no control over how much dew would be created. Many factors come into play when dew forms: atmospheric pressure, wet and dry bulb temperatures, the amount of moisture in the air (humidity) and enthalpy of water vapour and liquid water.


Edit

Having thought about this some more, something similar to what you are proposing has already been done in Egypt, when the Suez Canal was completed and opened in 1869.

Within the canal system is Great Bitter Lake, which is a sea water lake. The presence of such a body of water for nearly 150 years has not increased the amount of moisture in the vicinity of the lake that would allow lush vegetation to grow.

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  • $\begingroup$ When you say "contaminating nearby land", what area are we talking about if the lake is the size of Nevada? Aren't there, other large dried up salt lakes, like Great Salt Lake Desert ? there seem to have some surviving vegetation... How about a field of multiple enormous greenhouse-like structures, where water evaporates during the day and condenses on the inner walls during the night, is assembled and sent to the lake? Or just used to irrigate? Solar power can melt sand to make glass for these structures...I am an eternal dreamer. Thanks for the answer anyway ;) $\endgroup$ – ZakC Feb 23 '16 at 19:04
  • $\begingroup$ That's right. Groundwater contamination would be a huge problem. A lot of people in the region rely on it, e.g., the Nubian aquifer $\endgroup$ – stali Feb 24 '16 at 0:24
  • $\begingroup$ @ZakariaChihani: there is vegetation around most salt lakes however, such vegetation is somewhat salt tolerant. The degree to which plants are tolerant to salt varies. Most crop plants are not tolerant of salt. Even salt tolerant plants can be killed by too much salt. $\endgroup$ – Fred Feb 24 '16 at 2:05
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Nevada is 286,367 km² in area. With a conservative average evaporation of 2500 mm/yr in your desert, a lake of that size would evaporate 22686 m3/s. That is a flow larger than the average river discharge of the Nile.

As you can see in the plot, the best price for pumping water to an elevation of only 50 m above the sea level is 0.131 \$/3.78 m3, so to pump 22k m3/s you need to pay 786 \$/s, or 24 billion \$ per year.

Cost of pumping waterThe power cost ($/1000 Gal) per thousand gallons pumped for a pump with a BEP efficiency of 85 percent driven by a motor with an efficiency of 93 percent.

Similarly, you can easily estimate that the power needed to pump (all parameters kept the same) is around 1.25 billion (1e9) Watts. With a 50% total efficiency, you would need around 200 last-generation aerogenerator monsters like these to do it.

The only way to do this efficiently is by bringing water from a higher natural source. Some deserts in Egypt and Algeria lay below sea level, so bringing marine water to them would be cheap.

There are many proposed scenarios to do very similar things: flooding the Siwa desert, desiccating the Mediterranean... Some have already been put in practice.

But the key underlying question here is: What for? The fact that something is big, cheap, and doable does not make it worthful. The environmental implications of these huge geoengineering projects are enormous and difficult to predict in detail.

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I see three issues to be considered and will address them in what I think is reverse order of importance:

3) Cost: you have considered this with solar power and by trying to use mud to reduce infiltration through the lake bed. Fair enough, for now let's assume the scheme could be cost effective.

2) Negative impacts on the environment. You will get infiltration of saline water to the ground, although you may be able to minimise it. Eventually that will recharge the groundwater or form an unconfined or perched aquifer on top of low-permeability sediments. My understanding is that the Nubian Aquifer is largely confined except in recharge areas far to the south. So the saline water may not affect it except over exceedingly long time frames. The Nubian Aquifer has extremely old water so even if it were to be contaminated with salt, it might take so long to reach irrigation areas that people wouldn't worry about it. After all, they are essentially mining the fresh water out and not trying to match sustainable yields. In some places the groundwater is saline and not usable already. But these are things that would have to be addressed.

I'm not sure that salt at the surface would be a significant problem. Much of Australia has ephemeral saline lakes at the surface that don't have major impacts (in my opinion) on the surrounding land when they are dry. But you are talking very large scale. I expect that the dry lake bed would stabilise with a sediment cover blowing in so salt dispersion would cease.

1) Would it work? I think it's a great idea but I'm skeptical. There are a lot of areas near the oceans where dew formation doesn't seem to produce significant amount of green vegetation. Well there are parts of Chile where dew is important in supporting the ecosystem, but that ecosystem is pretty unique and sparse. Not something that would support much human population.

If I understand your question you are suggesting that the green vegetation would produce a feedback loop that promotes further dew or precipitation and becomes self sustaining. I can't say whether that could occur in some regions. If you look at the Great Artesian Basin of Australia, Lake Eyre and other very large lakes will fill with fresh water periodically and then go saline or even dry out. However, they haven't developed self-supporting vegetation, to my knowledge. The terrestrial plants rely on periodic rainfall due to storm tracks, generally from the west or north. If nothing else, thinking about it will teach you a lot about climate and weather. Hope this helps.

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One addition to the excellent answers already listed is the health of the new lake would steadily deteriorate with time. Generallly speaking, a healthy body of water requires inflow and outflow to avoid stagnation. In this instance, your goal seems to be creating an evaporative pond. That evaporation would leave behind a huge salt pit (e. g. https://en.m.wikipedia.org/wiki/Salton_Sea), with very little benefits.

Also, please send me the link for a solar powered water pump!

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    $\begingroup$ One of the proposals for keeping the Salton Sea from drying out and becoming a massive salt flat has been to build pipelines to pump in water from the Gulf of Mexico. This Tetra Tech Inc. report on Salton Sea proposals (www2.usgs.gov/saltonsea/docs/SSA/…) deemed the plan "impractical" due to the projected $10 billion to $40 billion cost in the early 2000s. One challenge is that while the seawater is less salty than the Salton Sea, it's far brinier than the current inflow, so super-salty water would have to be pumped back to the ocean, as well. $\endgroup$ – jeffronicus Feb 2 '17 at 18:26
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    $\begingroup$ By which, of course, you mean the gulf of California. $\endgroup$ – user7341 Feb 2 '17 at 18:40
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    $\begingroup$ Yes, thank you for the correction! BTW, here's a recent story in The Desert Sun about a more recent push for pump-in, pump-out: desertsun.com/story/news/environment/2014/09/28/… Proponents argue that energy costs can be offset by incorporating hydro power into the pipeline; pump water uphill when prices are low, let it flow downhill and power dynamos when prices are high. A related issue is that once you start pumping, you really can't stop without environmental impacts, either in the Sahara or the Salton Sea. $\endgroup$ – jeffronicus Feb 2 '17 at 19:56

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