1. The vertical height of the waterfall inlet above the water surface of the pond
Refer to this picture. It shows the 2 points between which you must measure to get the height "H".
Do NOT measure from the bottom of the pond ... measure this in feet to use the table below.
This is called the "pond pump head".
IMPORTANT: Increase the number by 20%. For example if your head is 3 feet then multiply this by 1.2.
This means you will work on a head of (3 x 1.2) = 3.6 feet in the table below.
Whenever in doubt ALWAYS go to next higher level ... for example in this case you might want to choose say 4 feet rather than 3.6 feet. The reason for doing this is to overcome what are called pipe friction losses which you can learn more about in the advanced sections of this website.
2. The width of the water overflow point for the waterfall ...
If the stream section before the waterfall splits into a series of smaller overflows (see the picture ... this waterfall has 2 separate overflow points) but all overflows are at the same level then add all the mini overflows together to get the true width in inches. In the case of the picture shown you would add the width of the first point to the width of the second point. This width is needed to work out the water flow you will need to specify the pump. Now you can use the table below.