Pond Leaks are not always what they seem to be!

There are several reasons why ponds 'lose' water without there necessarily being an actual structural leak. These should all be investigated before the lengthy - and messy - process of leak repairs gets underway. If you can eliminate the following, then - and only then - your should prepare for repairing a leak in your pond.

Evaporation. Substantial water reduction can take place - in a short time - simply by the process of evaporation of the pond's surface. Drying winds wafting across the surface can lead to a significant water loss - resulting in a lowering of the pond water. Evaporation is further increased with the use of fountains and waterfalls. If the pond surface remains at a constant level when waterfall or fountain is not running, then these can be suspected as the primary cause.
Spillage. Waterfalls are often the culprit with a noticeable loss of water in the pond. Any spillage from waterfalls should be noticed fairly easily if it is seeping over the sides. However, if the water is leaking through a joint in concrete and stone fall, then it might be more difficult to spot if the water is seeping through the bottom of a fall structure. Stop the fall for a few days and firstly make sure that intermediate pools are not leaking. If these are ok - and the main pond area seems stable, then suspect leakage from between the intermediate pools and in the structure of the fall itself.
Once certain way to see if your waterfall is the leak culprit. Simply turn off your pump for a day! If your pond ceases to stop 'leaking', then the problem is most certainly in the waterfall area.
Spillage from fountains can also be quite considerable over time. This should easily be noticed if the fountain is being blown onto a paved surround, but not so noticeable if being blown/spilled onto surrounding vegetation.
Outside factors. Ducks can often cause havoc with the pond levels if they are taking a sneaky early morning bath! They are sometimes active before or around dawn - or even in the night - and are quite prone to throwing a bit of water about - not all of it landing back in the pond!