I have a planet covered with a large shallow ocean, which extends over most of its surface. I want to turn this planet into a great desert, specifically a salty desert (the salts of this desert are a mixture of several chlorides, not just NaCl, there is also NH4Cl, MgCl2, CaCl2, etc.). On Earth there are small salty deserts (salt flats), which form when a lake evaporates and leaves behind all the salts that were dissolved in it. I wanted to do something similar to form the salty desert of my planet, but the question is, where will all the water go?
The surface, which is completely covered by large amounts of salts, must be depleted of water, since it would dissolve the salts in one way or another (with rain, for example), which I do not want. What mechanism could I use to remove all the water without affecting the salts?
Keep in mind that I don't want to eliminate the atmosphere (as happened with Mars) or add greenhouse gases to increase the average temperature of the planet and keep the water always evaporated in the atmosphere. Freezing the water and depositing it on the surface is not an option either. Sending the water underground is a good idea, but you should find a way not to send the salts too. It would also be possible to add some compound that reacts with water and depletes it. Things like that.