Your water appears to be about 2.7% lower in the morning than in the
night. Evaporation may contribute quite a lot to this, although I
wouldn't rule out a leak. The evaporation rate depends on lots of
variables, like the humidity, tempeture, and wind (all through the
night). If you lived in Arizona, for example, I wouldn't hesitate to
suspect that your water would evaporate very quickly (it is very hot
and dry there). At least your location in Suffolk is sufficiently hot,
although it probably is humid as well.
Another effect is going on which can help explain things, but is
about a factor of 10 too small. Water expands when it heats up and
shrinks when it cools off. The thermal expansion coefficient for water
is 0.0003 per degree C. This number is the fractional change in volume
per degree C temperature change. So for a 10 degree change in
temperature, you can expect your water to shrink by 0.3%, accounting
for a tenth of what you see.
There may also be local wildlife which are hot and thirsty, and
may take splashy baths in your pond at night. Is the ground near the
pond wet in the morning?
One experiment you can do is to cover your pond with plastic to
see if that stops the water loss. If it's evaporation, then the
concentration of impurities in the water should go up over time, but I
wouldn't suggest adding anything to the water to check this out. I
still wouldn't rule out the leak hypothesis.
(published on 10/22/2007)