How to describe changing water flow in shower?

turn up/down the water pressure

Adjust the water pressure

In this particular context of showers and sink faucets and such, you don't normally talk about properties of _the water_, you mainly refer only to the tap itself. The most common way people do this in my experience is to say something like "open the tap all the way" or "open the tap just a little" or "open the tap some more" or "close the tap a bit" However, if you want to you can still talk directly about the water itself. In this case, you would say something about the pressure/flow of the water being low/high or strong/weak. Often you wouldn't speak about the water pressure or flow unless there was an issue with the plumbing though!

I'd probably say something around more/less water, but it really doesn't come up often. I think there are a dozen things you could say that are close enough.

Turn up/down the water pressure

Pressure. More pressure, less pressure. Maybe faster/slower. Never bigger/smaller. The erudite British comedian, Suzy Eddie Izzard, describes showers most eloquantly; https://youtu.be/0Zfsn82MF3E?t=208

'Increase/decrease the flow'

I can't say I've ever had a shower with adjustable water pressure. Turning the tap changed the water's temperature. The pressure was simply on or off. There was maybe a slightly awkward spot where you could get the shower to leak without being fully on, but it wasn't a meaningful level of control over the water pressure. I've had shower heads that could be adjusted to focus the water pressure into more or fewer streams, if that helps. You would turn a dial on the shower head to switch between labeled settings, like "jet," "massage," and "rain." A jet would have few streams, but each stream would be powerful. Massage might send the water out in pulses. Rain would be a lot of small, gentle streams. I would be inclined to describe these in terms of what they look like, how many streams there are, etc., as opposed to higher or lower water pressure, though. I think "adjust the water pressure" is the most likely to be understood. If you asked me to adjust the water pressure on a shower with no special shower head, I would simply tell you that I can't. By contrast, if you asked me to make the water stronger, I have no idea what you mean; this is never something I've been able to do or had context for. "Turn the water up" (or down) would just make me think you want the water to be hotter (or colder). If you asked for "bigger" or "smaller" water, I would again be confused. Water is water. It can't be bigger or smaller (only more or less).

Pressure: Adjust the pressure (in general)/change the nozzle (Many showers have nozzles with different settings, at least in America) Heat: Turn up/down the temperature

Increase / decrease

Make the water stronger/weaker. Turn the water up/down.

So many ways: Turn up / down the water Open the water more / close the water a bit Make it stronger / more / weaker / softer / less Turn it on more / harder / turn it off a bit Increase / decrease the flow / pressure / water More / less water There is not really one standard way to say it, and different countries and regions might have different standards.