How to trace leak

Help,

Last year I moved into a house which has a koi pond. I had no problems
with the pond last year. This year I am getting a slow leak. I can't
seem to find out where the water is going. I have checked around the
pond and I can't find a wet area which would indicate that the water is
spilling over to that area.

I have a pond which has a sump at one end. The water flows into the
sump and then a pump pumps it up to another sump which when it
overflows allows the water to move down a waterfall. I have filled up
both sumps and checked to make sure the water is not leaking from any
of the connections on the pump.

If I fill the pond and lower sump but I do not turn the pump on the
water level remains constant. It is only when the pump is turned on
that the water level drops, about 1" per hour.

As I have said I checked the ground between the two sumps and it is dry
when the pump is working. I have checked the ground on either side of
the waterfall and it is dry.

The only thing I can think of doing is buying a seperate hose to hook
up to the pump bypassing the upper sump. I was going to move it slowly
up the waterfall to see at what point the water in the pond starts to
drop. Just below that point I guess I have a leak.

Are there any other suggestions?

Thanks

Alex