If one portal was located on the bottom of a lake, and an other one located surface of a desert ground, what would happen? How much of the water would spill out into the desert?

Uh, most of it, surely? The force of the water would push it down and then up through the other portal until there wasn't enough pressure to force the rest through. 

Most of it. 

It won't work, the portals will not even make it though the water, rather just dissapear because you can't create a portal though water.

" @Vitor: I don't think all of it, because eventually the pressure would even out on both sides... right? "

Depends on the shape of the desert (are we talking about the bottom of a valley?) and how dry the ground is (how much water will be absorbed).

If it was two identical swimming pools and the portals were placed at the very bottom, the water would end up being distributed evenly.

" @HitmanAgent47: and portals can't be on sand, since it's just a bunch of tiny rocks. "

" @Ezekeilpurger said:

" @HitmanAgent47: and portals can't be on sand, since it's just a bunch of tiny rocks. "

What if they were moon rocks? "

I like portal 2.

Even if the @Afroman269 said:

" I like portal 2. "

A portal can only be placed on a flat surface, can it not?  But even seemingly flat walls have texture...

The was a mine that built under a lake once, and by accident someone had drilled upward and pierced the bottom of the lake. This created an incredible whirlpool that ended up sucking  even a few boats down it.
I'd imagine that putting a portal at the bottom of a lake would have a similar effect. It would probably make for a really crazy fountain in the desert depending on the depth of the lake (ie, greater depth = higher fountain effect).

Science!

There's no such thing as portals

" A portal can only be placed on a flat surface, can it not?  But even seemingly flat walls have texture... "

i wanna place a portal on your MOM

The water would shoot out pretty fast, depending on how big the lake is. Every 10m underwater is another atmosphere of pressure, so if the lake was a reasonable size there would be a lot of pressure firing the water out. Then it would reach equilibrium, when the pressures on both sides of the portal would be the same. That's assuming the desert fills with water. This is all assuming Newton's laws on conservation of energy were warped to fit in with portals. There's a lot of assumptions to make this work.