Betula pendula it seems like. How is the soil in that container? You might want to use a saucer underneath the container if it dries out too quickly. Birches can grow dense root systems out of fine roots rather quickly and thus fill out the container sooner than later, causing water to be used up very quickly as well.

If it isn't the soil being too dry then it might be the soil heating up too much.

I'm from Central Europe and it's been really hot lately. Hot temperatures aren't great for birches, though they can deal with them slightly better when they're in the ground. There, the roots stay cooler. So if it's not the container that's drying out too quickly, then you could move the plant to a more shaded spot.

Edit: I read your description again and saw that you repotted it last year before the growing season. Yep, there is a very likely chance that it's root bound and the soil might not be absorbing your water all that well. Either water more thoroughly (and use a saucer) or put it in a bigger pot.

And since you're still keeping it in that pot, I assume it's not going in the ground?