[CliffordK]

I like the idea of using solar heat to vaporize salt water, and using the column of steam to transport it up to your pipe system (at altitude, air is naturally cooler, and will help with condensation).  As mentioned insulating, and perhaps even heating your stack will also help. 

Your condenser may also be able to be designed to pull a vacuum.  It will need a temperature differential, but might be able to reduce the heat of vaporization (and reduce parasitic condensation).

Your problem would lie in routing the pipe.  So, if you were pumping from sea level in California up to Colorado at about 1 mile high...  that would be an awfully tall aquaduct. 

And, of course, it would need to be really big.  A pipe a foot or two in diameter would be ok for drinking water for a small town, but would barely water a small pasture for agriculture.

So, if you're thinking of an agriculture aquaduct, perhaps a pipe 30 feet in diameter or 10 meters...  which would be a lot of water and energy for evaporation.  And even that much water could be used up pretty quickly.

Now, there are a few things you could do to help the problem.  So a coastal mountain range might help you gain elevation for both cooler air at elevation for condensation, as well as helping gain elevation/pressure for distribution.

[CliffordK]

One point that comes up with some of the previous comments is that one often thinks of a closed system employing pipes and canals. 

But, if one had predictable wind circulation, one could also do a completely open system. 

Evaporate the water at sea level, and let it naturally gain elevation and condense out as rain.  One might think of this as bad heating ocean water, but if one concentrates solar energy, it might have a net benefit of transporting energy into the upper atmosphere without a net increase in open sea water temperature.

Essentially just improving on Mother Nature.

The problem is that many deserts have a lack of rainfall for reasons that would be more difficult to overcome.  Thus we have the largest rainforests surrounded by deserts.

Here we talk about rainy Oregon, but that is primarily a single valley.  The Cascade Mountain Range has the effect of trapping and condensing moisture in the air and giving lush green farmland to the West and desert to the East.  The weather is also very seasonal here, and global warming could impact our "dry season".

It may be that there would be some coastal areas where one could build rain evaporators that would be less affected by winds out in the middle of the Pacific and Atlantic.

Nonetheless, to make a dent in rainfall, such a system would have to be HUGE.

[CliffordK]

There have been some studies indicating that trees, and in particular rain forests can help shape their environment.  Essentially transporting surface water into their leaves and needles to promote evaporation which then falls as rain elsewhere.  So, an attempt to help recover desert may involve selecting trees that might increase humidity.

I was considering salt water trees recently, and the most prominent salt water trees are in the Mangrove family which in theory could help desalinate water, and promote evaporation of salt water. 

Perhaps it is something that should be promoted along beaches and canals in desert regions with adequate temperature to keep the alive.  With, of course, the risk of them getting out of control.  But, they could bring in multiple environmental benefits.

[myuncle (OP)]

One point that comes up with some of the previous comments is that one often thinks of a closed system employing pipes and canals. 

But, if one had predictable wind circulation, one could also do a completely open system. 

Evaporate the water at sea level, and let it naturally gain elevation and condense out as rain.  One might think of this as bad heating ocean water, but if one concentrates solar energy, it might have a net benefit of transporting energy into the upper atmosphere without a net increase in open sea water temperature.

Essentially just improving on Mother Nature.

The problem is that many deserts have a lack of rainfall for reasons that would be more difficult to overcome.  Thus we have the largest rainforests surrounded by deserts.

Here we talk about rainy Oregon, but that is primarily a single valley.  The Cascade Mountain Range has the effect of trapping and condensing moisture in the air and giving lush green farmland to the West and desert to the East.  The weather is also very seasonal here, and global warming could impact our "dry season".

It may be that there would be some coastal areas where one could build rain evaporators that would be less affected by winds out in the middle of the Pacific and Atlantic.

Nonetheless, to make a dent in rainfall, such a system would have to be HUGE.

The good thing about desert greening, is that you need just a bit of fresh water to kickstart, you are not obviously going to plant thirsty almond trees. And once the forest is made, it will create its own climate keeping moisture and creating even rivers.

[wolfekeeper]

Maybe if you had some solar thermal collector on the sea, and then lead the water vapour up the west side of an Atlas Mountain with what was basically just a big black plastic bag tube. It would fill up from the water vapour, which is after all, lighter than air, kinda like a tethered hot air balloon, but you tether it to the side of the mountain and let it carry the water vapour over the mountain.

[alancalverd]

You may find this data on population density helpful

Population density is the one ecological variable over which we have absolute control. We should use that control, rather than muck about with the environment.