Well, there’s a fair few reasons I can think of. Here they are in the order that I think is most likely to least likely.
1. The Reservoir Has a Leak
This is the most likely reason. If the water level is dropping, then it’s probably due to a leak.
The best way to figure out if the reservoir has a leak is to fill it up and then switch off the pump.
If the water level stops dropping after a while, then you know it’s not a leak. If it keeps dropping, then you can be pretty sure it’s a leak.
You’ll have to look around the edges and see if there are any cracks. If the reservoir is made of concrete, then it might be a good idea to patch it up. It’s cheap to do, and it’ll save you from having to top off the water every couple of days.
If the reservoir is made of plastic, then you can use pond liner repair tape to patch it up. Just cut a patch, apply it to the leak, and you’re good to go.
2. The Tubing is Leaking
This is the second most likely cause of a waterfall losing water. If it’s not the reservoir, then it’s likely to be the tubing.
You’ll be able to tell if it’s the tubing if you see a drop in water level before the water reaches the bottom of the waterfall. If the water level is still the same at the bottom of the waterfall as the top, then it’s definitely not the tubing.
You’ll have to look along the tubing to see if there are any leaks. If you can’t find any, then it’s probably that the tubing has just come loose from the pump.
3. The Pump is Cavitating
This is a pretty rare problem, but it does happen from time to time.
You’ll be able to tell if the pump is cavitating by the noise it makes. If it’s loud and sounds like air is being sucked in, then it’s definitely cavitating.
There are a few causes for cavitation, but the most likely cause is the water level being too low. Check that the water level is above the “minimum water level” line.
If it’s not that, then it might be that the intake is clogged with debris. If that’s the case, then clean out the debris. If the problem persists, then it might be that the pump is about to fail. In that case, you’ll want to get a replacement.
The best way to prevent cavitation is to get a “low water shut off switch,” which you can set to automatically switch off the pump if the water level gets too low. This is a great solution if you often find that the water level is too low, and it’ll save your pump from burning out.
4. The Pump is Oversized
This is another pretty rare problem.
If you’ve found that the water level is dropping, but you’re not sure why, then you’re going to have to check if you’ve got enough water in the reservoir, and if your pump is the right size for your setup.
There are a couple of ways to fix this. The first is to simply turn down the flow of your pump. The second is to get a pump with adjustable flow, so that you can adjust it to the perfect flow rate for your waterfall.
If you’re not sure if your pump is oversized, then try turning down the flow of your pump first. If that doesn’t fix the problem, then you might have to get a new pump.
5. You’re Using the Wrong Size of Tubing
This is another pretty rare problem, but it does happen from time to time.
If you’re using tubing that’s too thin, then you might find that the water level drops before it reaches the bottom of the waterfall.
The reason for this is because the friction of the water flowing through the tubing is too high. The smaller the diameter of the tubing, the more friction there is.
The way to fix this is to get tubing that’s thicker. That way, the friction of the water flowing through the tubing will be lower, and the water will be less likely to evaporate before it reaches the bottom of the waterfall.
If you’re not sure if your tubing is too thin, then you can compare it to some 3/4" ID tubing, which is the standard size of tubing that most people use. If your tubing is noticeably thinner, then you might want to consider changing it.
