hi Matt-
first off, good on you for even considering that there is an ethical question here.
I guess, even independent of the water questions, there is the electricity question. pumping that much water 24/7 is going to burn a huge amount of power.
Thinking about it in terms of ethics, one perspective I like is Kant's Catagorical Imperitive. This indicates that if an act were to be universally followed by all people, would it be a good thing in total? In your case, if everyone in your situation (rainfall, geography, etc) pumped groundwater to keep their pastures green, would the outcome be a positive one?
One way around the entire quandry might be to consider building a water catchment
pond. You could then irrigate from this
pond through your dry summer. I know many people in a similar situation to you, in Northern California, who do just this. Your pond would enrich the
local ecology while providing
enough summer water to meet your basic
gardening needs. Maybe not acres of lush green pasture, but certainly enough for a nice garden and some
trees. Any surplus could be used to supplement your pastures as well.
Sheep do really well on dry pastures, consider the Navajo and their sheep ranching. It does take a lot of
land area per animal though. But maybe if you are only trying to raise a couple lambs for your personal use, following
Alan Savory's Holistic Mangaement guidelines, you could be successful on your acreage.
Chickens certainly wont mind the dryness, so I think you can raise
chickens regardless.
There are lots of examples around the world of successful agriculture in seasonally dry climates; imitating these examples might be a better approach than pumping groundwater to try and alter your geographic circumstances. In your environment, it is the rainy winters that are your active growing seasons, and the dry summers are the dormant seasons. This doesnt necessarily suit our cultural preferences regarding vegetable production, but it certainly can work great with pastures for sheep. I think that you could find a good balance between winter pastures, pond resevoirs, and summer gardening. Matching our farming to our environment is the primary challenge we all face. There is a good solution, good luck in discovering it!