ok i was told to try this.
put a 5 gallon bucket in the pond water close to the edge. fill the bucket with water to the water level of the pond.
check each day the water level of the bucket as compared to the pond. the difference in water level between the bucket and pond is the leak amount by the pond assuming that both the pond and bucket evaporate at the same rate which i believe they should.
will begin this experiment tomarrow and post results soon.
