A water-thief κλεψύδρα klepsydra is an ancient tool for getting an accurate handle on passing time. It relies on the molecular uniformity of water, much as an egg-timer relies on well sieved sand, to pass through a narrow opening at a regular and reproducible rate.
The ultimate back-up is a 1m tonne UBC [(R)]set up next to the compost bins whose base is about 2m higher than the roots of the tomatoes. It is high Summer; there hasn't been more than a spit of rain for at least two weeks; and tomatoes are thirsty . . . not to mention the parsley sage rosemary and thyme which also need their wet. I have accordingly been draining the top UBC into its lower sibling to swell the reservoir of useful irrigant. I've plumbed in a half-inch pipe to the bottom spigot of the back-up UBC. It's little faster than watch paint dry, but only a little. I needed to find out how long it took to drain a set amount of water. That was I would know whether I had time to knock up a batch of scones or only time to make a cup of tea before having to nip up the steps to switch the tap to Off.Ardyspelk.