I do water my broms in spring and fall (when rain is scarce here) with my (very hard) well water and it doesn't seem to affect them too much. Their major growing season here is summer when we get lots of rain so they seem to do fine as long as they get rain water sometimes.
The important thing to remember about broms is that the roots really don't do much for the plant except anchor it. The major intake of water and nutrients is through the leaves, so for instance fertilizer applied on the ground around a brom doesn't do much for it. I fertilize mine with my soluble orchid fertilizer sprayed on the leaves in the summer and not at all through the cool weather.
