It is my understanding that the water iris can handle dry soil once they become settled in their new spot. I have Siberians & the other type of water iris, can't think what it is. Anyway, both seem to handle drought well, although they might bloom better if they had more water. I was told to water them in well until they adapted to their new home, a few months or so. I only have one type of actual Japanese iris, but they are in an area that gets more water, so don't know about them.

Most likely Iris pseudacorus which, although it will grow in water and may prefer it, is just as much at home in ordinary garden soil and under relatively dry conditions. In fact, I'd recommend going to great lengths to keep it AWAY from any sources of water, as this plant is listed as invasive across most of the US as it invades and travels waterways and displaces natural growth.

Here is a link that might be useful: iris pseudacorus