Fermentation

From my experience, that isn't an issue; eventually the salt at the top dissolves, and since brine is heavier than water it sinks, so it mixes itself. (Sometimes a gentle shake helps too.) I do it this way because the resulting salinity is a bit more consistent, than when using a brine with a fixed salt/water ratio.

A third option is to eyeball the amount of water you'll be using, weight it, do the maths on the amount of salt you'll need for the water+veggies, make the brine separately with that water, then pour it over the veggies. It takes a bit more work, but if you're worried about the salt not dissolving properly, it's a good option.