Before you noticed your leak and the larger drop in water level, what were you experiencing in evaporation loss? How often and how much were you have to fill the pond up?

The actual evaporation rate at the surface will depend on several factors such as the size of the pond's surface, the relative saturation of the air above the surface, the temperature of the water (faster the molecules are moving, the more energy they have and easier for them to break away into the air), and the rate at which wind is carrying away the air above the surface. Would be easiest to just compare to previous experiences of evap and see if the climates are similar (air temp, water temp, humidity).

If you're that serious about figuring out how much water loss you are getting from the pipe, then over the course of the day at several different times go catch the water drops in a measuring device and time how long it takes to fill it up.

 

Okay, this has got to have gotten out of hand. Intel really has the right idea. Set a measuring cup up and catch the drops for a set period of time then multiply. By the way, the drop size really only depends on the surface tension of the liquid providing the hole is not large enough to just let it stream out.

I had no idea that there were so many other chemists on this site.