As others have mentioned fog/moisture capture and transport from other regions, but perhaps something to compliment... how to maximize the impact of what water you do have.
The design of spaces and usage of plants to create micro-climate effects to maximize water impact. Consideration of succulent type plants for pioneering/climate modification.
So, regarding plants/micro-climate. If you've produced an enclosed space and have grown trees these are beneficial because you moderate the exposure to the sun. If you look at that image of the qanat, this is indicative, the trees shade the ground and water, minimizing the evaporation of what is there. The more shade you have, the more you conserve what water you do have.
https://forestsnews.cifor.org/10316/make-it-rain-planting-forests-to-help-drought-stricken-regions?fnl=en https://en.wikipedia.org/wiki/Desert_greening
Furthermore, the trees will buffer wind, and can themselves capture a bit of moisture in the air, and of course slow down evaporation. On a larger scale, trees actually help to produce rain, as they do breathe off water themselves.
Consider this, if there is moisture in the air, but not enough to rain... as this moisture passes over a forest, where trees are breathing off moisture, the combined effect of the ambient air moisture + tree breathed moisture, combines to produce enough total moisture to cause precipitation (enough water combined will fall as rain).
So downwind somewhere you'd probably want to consider capture and some sort of piping/aqueduct, like the qanats. 591
And send that moisture back to your cultivated forests upwind. This could be an expanding system that, although fragile, could be something that would slowly scale.
More trees would shelter more land/water, and breathe more moisture, which would interact with the evaporated moisture from the sea, which could precipitate, leading to more water to pipe back to said trees, which could be expanded to shelter/breathe more moisture, interacting with more sea moisture, etc.etc.etc...
Anyhow, regarding succulents. There are various species of plants, like cactus, and these are of course species most adapted to low moisture levels. https://en.wikipedia.org/wiki/Succulent_plant
These could be interesting considering a systematic approach to the landscape/microclimate and if you used the tree/shade/moisture model, could be used as a notion of "pioneering" species. So before you plant trees, you have cacti and other succulents, helping to intercept moisture/attract animals and shade the ground.
Every bit of shade/water interception counts in this model.
On top of that, if you consider what "agriculture" means, there are succulents that are used in that regard. Consider agave: https://en.wikipedia.org/wiki/Agave
Agave can produce food, and of course is used in tequila production.
Furthermore, there are plant species that are highly salt tolerant. These could utilize some of the untreated water. They would again be able to provide shade/animal habitat, and would themselves contribute into the water feedback loop described above. https://www.gardeningknowhow.com/special/seaside/gardening-salt-water-soil.htm
Considering the availability of salty water, these salt tolerant plants + succulents could help form the "outer boundary" and pioneer the land, helping prepare for a slow adaptation of regular trees/plants to enter the space as water is available to irrigate.
So, in total, this is a multi-dimensional approach to the usage/protection and increase in moisture, rain production, capture, and forming a slowly scaling feedback loop to promote more and more rain production/interception and water presence in desert greening type of way.
If you combine this sort of "perma-culture" approach with the technological captures/transport described by others, you could have a more holistic approach to water production/capture/conservation.
Hopefully that is inspiring, and not just in your fiction, but also for real life. 1022