I don't know.

I have done some tests on the amount of water Canna use. The test was in Phoenix, 95-105F and 10% humidity. There were about 8 plants in each container which used about 1 gal of water per day for all 8 plants.

I thought that was a lot of water.

For your size pond it would be about 135 gal / inch of drop. I don't know how much of a drop you're seeing or how many shrubs you have doing this, but we'd be talking a lot of water.

I also don't know how big your Forsythia is, they sure can be huge. But I'd assume most of the water being transpirated would still be coming from the roots in the soil.

So it doesn't seem likely to me.

Yea, that was my first thought, but now I'm hedging...LOL! I also have a lot of other water plants that are now waking up and poking green sprouts up. Still does seem like evaporation and plants wouldn't use anywhere near 135 gallons. Oh well. Another pond mystery...

It is an interesting problem. Would need more clues to guess.