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Suppose some magical civilization on Earth-like planet can create portals. Any given portal entrances are spherical surfaces of the same radius. Any particle enters one sphere and exits another in the corresponding spots. Inside space of the portal is corridor of Plank length with no gravity field. I'm aware of implication for energy conservation and causality, just accept it. This civilization decides to use this property in order to irrigate local version of Sahara desert.

One portal entrance is placed deep under the sea surface covered with layers of semi-permeable membranes, while another one - in the Desert, so the high pressure of ocean depth pushes water through membranes performing desalination by reverse osmosis. This civilization built several large such portals that work as sources of designed major rivers and myriads of small ones to keep water levels constant in artificial lakes and groundwater. What are ecological implication of this system?

The world has essential the same geography as ours and the Desert in question is has roughly the same parameters (size, temperature regime, etc) as Sahara. Quantity of water continuously going through system comparable to discharge of Amazon river. Oceanic entrances of the portals are placed in the ocean current in order to minimize rise of water salinity in the vicinity. The long term ecological issues, that I concerned with:

  1. Heat pollution. Such system breaks the law of conservation of energy and all kinetic energy, that water get's for free, will eventually turn into heat. Is this a crucial problem in this system?
  2. How significant will be rise of water salinity?
  3. How drainage will affect marine life, most importantly plankton that can't resist suction? Is periodical scraping of membranes is enough?
  4. Will it significantly disrupt global climate with all this water evaporation?
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    $\begingroup$ 1) You mean circular, not spherical, I assume? 2)Where does the excess brine go? 3) What quantities of water are we talking about? The implications will be felt on both ends on the portal $\endgroup$
    – nzaman
    Jul 27, 2019 at 12:24
  • $\begingroup$ If the portals, membranes, reverse osmosis etc. are irrelevant to the question the you should really edit them out. You could simply say: "Assume that sources of fresh water producing so many cubic meters per second exist in the Adrar des Ifoghas, Ahaggar, and Tibesti ranges, thus transforming the Tilemsi, Tamanrasset, Bardagué, Enneri Yebige and Enneri Miski into permanent rivers." (List of course what specific intermittent wadis you want to make into permanent rivers; I have listed only the most prominent four.) $\endgroup$
    – AlexP
    Jul 27, 2019 at 13:04
  • $\begingroup$ Could you specify where the salt is? I mean is it in the river-sources on top of membranes needing to be regularly scraped off and transported elsewhere? Or is it left in the deep ocean... . It also seems rather broad as a question as timescale would be important to effect ecological transformations, nothing is instant. $\endgroup$
    – Escaped dental patient.
    Jul 27, 2019 at 15:14

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People seem intrigued by your desalination method. It is just reverse osmosis powered by a pressure gradient like it always is. Here is my old scheme to irrigate Death Valley using such a method which uses the below sea level altitude of Death Valley to provide the pressure gradient. http://www.halfbakery.com/idea/Lake_20Death_20Valley#1033923600

I would put your membranes on the outside of the sea side not the outside of the land side. Salt will diffuse away into the larger ocean on the sea side. If on the outside of your land side local salt concentrations might build up. Or maybe the two sides are the same thing via your dimensional magic.

But your question is what would happen to the desert. As regards the implications of irrigating a desert: I refer you to the Imperial Valley.

imperial valley https://en.wikipedia.org/wiki/Imperial_County,_California

It was desert before. You can see the desert in the distance. Now it is some of the most productive farmland in the world. Irrigating deserts works great to make farmland and that is what you would make. Farmland is an artificial ecosystem which will supplant that of the desert. Our species has been doing this for about 5000 years. If your water is salt free that will sidestep one big problem with desert irrigation which is salt buildup in the soil That is problematic now in the places like Egypt where they have been doing this for 5000 years because even fresh water usually has some salt.

Desert critters will retreat to places nearby that are still desert. They will probably do well too because of the proximity of your crops, because they will sneak in and eat them. A barrier of desert might be a good thing to exclude more serious agricultural pests that will not easily traverse the desert.

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    $\begingroup$ It's still a question of how much water is being taken per day. At sufficiently high volumes, the local salinity level will increase to the point of killing off marine life. Possibly a case for using multiple small portals spread out across the length of the Atlantic, rather than one large one. +1 for the answer, nonetheless $\endgroup$
    – nzaman
    Jul 28, 2019 at 12:38

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