Typically, cutting back a plant perennial plant at this time of year will trigger a strong growth response. More top growth would mean a greater water requirement.

Incidentally, a big increase in top growth also reduces your plant's ability to store essential energy (carbon) reserves in the root system as well as drastically stealing the energy your plants need to maintain a strong root system in a very difficult time.

I'm so sorry for the drought conditions. I know first hand how difficult going though such times are.

as to which to compost .. well ... the ones that die ... i dont understand why you would kill something that is still alive ... to save it from drought....

ken

Mulching with non-decomposing organic matter seems better than pruning to me, at least that's how SoCal often handles our 6-7 months with no rain. You don't want a flush of nitrogen to stimulate new growth during a drought, as Rhizo noted above. So this is one of the rare situations where a few inches of bark or pine needles or clean straw mulch would be better than great, N-rich compost or soils.

When our drought-resistant natives have finished blooming in summer, many of us in SoCal prune some then, but not more than 25-35% or so. Sorry, I don't know your conditions & plants well enough to be of much help