Outdoor ponds are wonderful additions to any garden landscape, yet they can be high maintenance. With a few proactive steps, however, repairs can be mitigated. Calculating a pond's evaporation rate is the key to determining if a falling waterline is due to a leak or just evaporation. Establish a baseline for your pond's evaporation rate when it is first installed. This will give you an accurate gauge of how long it takes the pond's water level to fall 1 inch through ordinary evaporation.
Measure the depth of the pond using a yardstick. After your pond is installed, place the yardstick in the water at a location you can use again in the future to track your evaporation rate. Determine the depth and record the measurement. Turn off any auto-fill mechanism you may have installed for the duration of the experiment.
Return to the pond each day and measure the depth in the same location you used for the initial measurement. Continue returning and measuring until the water level has fallen 1 inch. If the pond is exposed to heavy rainfall during this period, start again, since you will not be getting an accurate reading of the evaporation rate.
Determine the number of days it takes for the water level to fall 1 inch. Divide 1 inch by the number of days, and this is your baseline number for future measurements.
If you find your pond's waterline to be falling at a rate greater than the baseline, you may have a leak.
A small leak can become a large leak quickly. Do not leave fish in a pond that you believe is leaking.