If areas such as Saudi Arabia or the Sahara consistently got the same rainfall every year for decades, as say Seattle, would they become suitable for farming? Would bushes and trees grow or would the land stay infertile? Thanks.
If the altered rainfall lasted decades, sure. Deserts typically don’t have a lot of organic material in their soils, but the place would be quickly colonized by plants.
Deserts occur around the world in a huge variety of underlying soils. The only common tie is the lack of water. Irrigate the desert and you’ve got fertile soil. The problem is getting that water for irrigation, and evaporation of the water leading to salinization. But with a constant influx of new water salinization wouldn’t be a problem.
Seattle actually doesn’t get that much rainfall, only 38 inches a year. That compares to NYC (42"), Boston (44"), or Miami (57"). Seattle just gets a lot of days of rain but they don’t get much actual rainfall.
Right. Seattle gets a lot of rain, but it’s mostly drizzle. Other places have sunny days alternating with downpour. It hardly ever rains really hard here, just a constant spit. And that’s only in the winter, we get drizzle all winter but sunny days in the summer.
I’m going to have to start by saying that I don’t definitively know the answer, but I’ve got some ideas about what might happen.
[ul]Given the right conditions, there are enough minerals even in the desert sands for dune grasses and other vegetation to take root and initiate soil formation. One problem is that too much rainfall will kill these grasses and another is that water and wind erosion can move dune sands around quite quickly, which can prevent the formation of soils.[/ul]
[ul]Much of the Arabian Peninsula is covered by aridisoils, which also have enough minerals to sustain plant growth - aridisoils generally are suitable for agriculture with enough irrigation. One problem is that aridisoils can have enough soluble salts to overwhelm plants if they’re all released by water - so increasing rainfall could have the paradoxical effect of making the soil too salty for farming.[/ul]
[ul]A problem in both of the soil types above is that water movement through vertically through the soils is often impeded (in the case of dune sand, impeded by the large pore size of coarse sands compared to the small pore size of fine sands - and in the case of aridisoils, by hardened layers such as petrocalcic horizons or desert pavement.) When the movement of water vertically is impeded, it will tend to flow over the surface of the impermeable layer and move as a flash flood, eroding away the looser soils and sands with it. Eventually these hardened layers will become loose if enough water is applied to them and the carbonates dissolve, but the question then becomes how much sediment erodes away before that happens?[/ul]
In short, it’s not as easy as simply increasing the amount of rainfall and then you get a lush biome (as if increasing the amount of rainfall was easy! 
) - you have to increase the amount of rainfall - but not too much for the initial pioneering grasses to get started, and prevent the accumulation of soluble salts to too high a concentration, and prevent wholesale water erosion of the soil horizons down the drainage to the sea. Throw some time into the mix, and you’d probably get some farmable land out of it.
Actually, the boundaries of the Sahara Desert move with time, according to a long cycle, IIRC. As the rain patterns shift, the parts that were desert become grassland, etc. When the rains go away, well, you get desert again.
I believe the Israelis have had some success with this in desert-like areas of Israel.
Adding water (from desalination plants) is usually all that is needed to make it productive growing land. Though sometimes they need to add a bit of good topsoil & fertilizer on top of the sand for the first year or two. Also, that helps prevent the sand from blowing away and exposing the roots of the crop. They have also tried covering the ground with grow cloths, or seeding it with hardy grasses the first couple of years, then once those have stabilized the ground, planting rows of food crops.
Note that in many cases, once you get a good crop growing in the ex-desert area, it will continue on its’ own – the the changes in wind, evaporation, ground temp, etc. that occur when the land is covered with a good crop will tend to keep it from turning back into desert. It will probably still need irrigation, but less water than was needed the first year.
A point to bear in mind: much of the Sahara as it stands to day was created by people over-farming it. That over exploitation led to a loss of tree and grass cover, severe erosion and untilmately desertification. While the area has beocme somewhat more arid in the past few thousand years it hasn’t been extreme. If we could manage to stabilise the soils then much of the area is suitbale for farmning even with current rainfall levels.
I think that’s an overly simplistic view and not one shared by climatologists presently. Do you have some citations to back up that assertion? I’d be interested reading them. 
Keep in mind how bloody big the Sahara is…