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A nation wants to utilize a large, currently brackish & endoheric body of water for irrigation & drinking. Damming the rivers & moving the water from there isn't considered viable for the reasons that they want the inland sea to remain intact, & that the areas where the rivers enter the lake are on the exact opposite side of where the major cities that need the water & ports for exporting food are.

The plan id to take the water from the lake to the ocean at a similar rate as water is entering the lake, over time this outflow would lower the salinity as it would no longer be functioning as an endoheric lake. After long enough the water would have low enough salinity for drinking & irrigation from the provision of the outflow.

Ecological collapse isn't considered much of a concern as most of the life in the endoheric lake/inland sea in question is also found in the rivers draining into it. The body of water is a bit less saline than the Aral sea before it started to dry up at 8g of salt a litre (vs 35 grams of salt a litre for sea water) & is somewhat larger than the Aral sea was at around 71,000km^2. The technology they're trying to do this with is roughly that of the early cold war, think 1950s-1960.

Would turning a inland brackish sea into freshwater by artificially adding an outflow be possible?

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  • $\begingroup$ Perhaps you could have a giant balloon pipe (this would perhaps be very expensive but it is just an idea) that the river is directed into and other side next to city utilities, and the evaporation of this water is reduced since theres a balloon blocking sunlight, then you could add more outflow over time to keep same level then this balloon occupies the entire lake and you can pop it and now you have fresh water. The main problem with this (besides the practicality of making such a large balloon that lasts long enough) is that the brackish water being heavier makes it float, impeding ships. $\endgroup$
    – Lee Nagel
    Aug 23 at 19:19

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The lake will be significantly drained

It is not possible to quantify the result exactly without far more detailed information on the climate and geography, but a qualitative answer is possible. If the water level of the lake remains seasonally constant then it means that the amount of water coming in is, over time, equal to the amount of water evaporating from the 71,000 km^2 surface of the body of water - let's call this rate of input and evaporation X.

As soon as you add an outflow at a rate of, for example, 0.5 X, then suddenly there is an imbalance - water is coming in at the same rate, evaporation is continuing at the same rate, which means that the lake is losing water at a rate of 0.5X. The water level will drop until the surface area is only 35,500 km^2, at which point losses from evaporation will be 0.5 X - half the surface area equals half the rate of evaporation - and the equilibrium is restored. (Note that "half" is just an example, but whatever fraction of the incoming water is removed by the outflow is roughly the fraction by which the surface area of the lake will be reduced.)

Needless to say, shrinking the lake/sea to a fraction of its former size will have drastic ecological and economic effects on the area - wetlands will be drained, practically all docks will be left high and dry and so on even if the lake is only reduced in surface area by 20%. More realistically, it probably needs to have an outflow more on the order of 40-60%, which means that the majority of the lake is no longer a lake.

Whether it is economically plausible for such an engineering project to be undertaken depends on a number of factors not specified in the question - the outflow rate needed to reduce the brackishness, the length of the pipes/canal to take the outflow to the ocean, and, most importantly, the gradient between the outflow point and the ocean.

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It would take forever.

Even ignoring the balance problem KerrAvon2055 mentioned it seems impractical.

There is a metric for lakes, water "residence time" - the mean time that water spends in a particular lake. You can see it at many Wikipedia pages about lakes. Even for lakes with significant outflow it is huge, for lake Superior it is 191 years. For Caspian Sea, which is somewhat similar to your case, it is 250 years.

To bring salinity levels from 8 to ~0.5 g/l, 15 of 16 parts of water has to be replaced. The fastest way to do it would be to drain the lake and let the rivers fill it again. ~250 years. If it is done gradually it would be much slower. Initially fresh water comes in, saline water comes out. Then they mix. Salinity would get from 8 to 7 g/l much faster then from 7 to 6. The closer to the goal the slower it goes.

Exact numbers depend on lake depth, inflow and evaporation rates, water layers mixing, etc. But it would take centuries, potentially thousands of years.

Laying pipelines from the rivers would be a much more practical solution.

"the major cities that need the water & ports for exporting food are" - there are no major cities in places without fresh water and they don't really export food, they import it.

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    $\begingroup$ the cities that need the water do have some fresh water but have issues with it due to rising population caused by recent growth. The cities aren't making the food but their ports are where it's exported overseas. $\endgroup$
    – OT-64 SKOT
    Aug 23 at 10:55
  • $\begingroup$ It's a self solving problem. If recent growth made a city worse place to live people will move. Port doesn't really need a city to exist and it works just fine with any water. $\endgroup$
    – D'Monlord
    Aug 23 at 11:49
  • $\begingroup$ The cities mostly grew due to a mineral boom, the biggest problem they're having is water from all the people moving in due to it. $\endgroup$
    – OT-64 SKOT
    Aug 23 at 12:10
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    $\begingroup$ There actually was something like that. In 1958 Soviets built a town in the desert near Caspian sea, Shevchenko. Uranium and oil I think. They supplied it with water using desalinization plant powered by nuclear reactor. Current name is Aktau, population ~200k. The nuclear plant was shut down ~25 years ago, now kazakhs use gas for desalinization and power. Much more feasible options, either of them. $\endgroup$
    – D'Monlord
    Aug 23 at 13:26

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