According to AquaScape, the average water lost is 1.5 to 3 inches per week. It really depends on a few factors such as splashing and temperature. In CA, the weather is fairly mild compared to the midwest. So to determine if your waterfall or the pond itself is leaking, you will have to turn off the waterfall and see if the amount of water loss stops. If it doesn't leak when the waterfall is off, the waterfall is your problem. If it continues, than it is your pond. Sometimes it is both. You have to do your math and determine how much is lost between the pond and waterfall. If it is your pond only, you can let it drain down until it stops. The hole will be at that perimeter where the water stops at. Most of the time it will be the waterfall. It is a process of elimination. Good Luck.