Is there a way to turn on/off the flow of water on this water clocK?

[Moedangel]

Homework Statement

The water clock in question is this:

I currently have to build a timer for my high school physics class. I plan on using a very similar design structure as shown in the video. However, I would like to know if there is a mechanism/modification I can do to this so that I can stop/greatly reduce the flow of water by pulling/twisting/switching something. My goal is to make this device acts more like a stopwatch than a timer.

Homework Equations

No equations come to mind.

The Attempt at a Solution

Currently, I'm thinking the best way to do it is to somehow block the flow of air from one of the tubes so that the water slows down to a drip. I'm having a hard time figuring out how to do it though.

 

[Simon Bridge]

Add a tap or a spigot.
You've thought of that right?

 

[CWatters]

If you want to slow it down that suggests you want to time long intervals? Normally to do that you count how many times you have to turn it over.

If you do it by slowing down the rate you may also want to change the shape so you can measure the depth of water more accurately.