Assure that you are using a plastic barrel. I tried it using a metal barrel during a Texas summer. That didn’t go well so I got a plastic barrel. It lost a lot less than the metal barrel during a Texas summer.
In the big Texas sun, the pan evaporation rate exceeding the evaporation rate in the pond - proves that your metal barrel has a leak!
DD1 is absolutely correct though, the closer the conditions in your "pan" match your pond, then the more accurate the test.
