The yardstick makes it easy to see what's happening. I found a yellow plastic one that lasted for quite a while for my pond. The wood one sort of dissolved into the pond.
When my pond was full, I put the yardstick in upside down, so it was easy to see what I had lost. The top of the yardstick was right at the surface when full, then when it was down an inch, I would see one inch sticking up from the water. Most summers I will lose two feet of water, but in a really bad summer it was down 8 feet!!!