If your pond's water level is decreasing, you are going to have to establish if it is natural evaporation or a leak that is the culprit. This can become a tricky situation if you have aquatic plants or fish in the pond, as marking the waterline with paint or a marker can poison the water and bring about more problems than just a decreasing water level. Following a few key steps can help you establish how your pond water is disappearing.
Note the location of your pond. New water gardeners often believe that direct sunlight is their biggest enemy when it comes to pond water evaporation. This is not always the only culprit. A pond located in a space that is subject to consistent wind can also lose water to evaporation quickly. If your pond is consistently rippling with a breeze, is subject to direct sunlight, or has fountains or sprayers that aerate the water, evaporation is likely your issue.
Tie knots in the length of rope, uniformly, at intervals of approximately one inch. Leave a length of the rope unknotted at one end. Ensure that you have enough to wrap around the length of wood and tie the rope securely. You can also use electrical tape or duct tape in place of the knots. You want something with high visibility, even when the rope is a few inches under water.
Tie the rope around the length of wood and lay the wood across the pond, so that the rope is sitting in the water. To get the rope to sink tie a few small weights, made of a rustproof material like stainless steel, to the end of the rope. Allow the rope to hang down in the water.
Note the water level against the length of rope. Each day, for a period of three or four days, note the rate at which the water level is decreasing. You may want to wait until the pond water recedes one full knot or length of tape, and note how long this process takes.
Refill the pond's to its original level. Cover one half of the pond with a tarp, securing it above the water line using weights or stones. You are looking to eliminate the effect of wind and sunlight on evaporation for approximately half of the pond. Note the rate at which the water level decreases now, with the pond partially covered.
Determine if the pond has lost water at a significantly slower pace now that it is covered. If so, evaporation from wind and sunlight is likely the culprit. If you see no substantial decrease in water loss, you may have a leak.
Do not cover more than half of the pond if you have fish. Ensure that the tarp does not fill with water and sink in to the pond, as fish can become trapped.