You pump water off the top of the pipe, reducing mass and pressure at the bottom and thus allowing for desalination. It's a classic distance x force trade off:

You pump water off the top of the pipe, reducing mass and pressure at the bottom and thus allowing for desalination. It's a classic distance x force trade off: it's easier to use a static membrane, and a low pressure pump then build a high pressure pump at the surface. Nothing in this system is 100% efficient, so how you organize your components can make a huge difference.