Water Harvesting

When I begin my building project, I think I will be installing some serious underground water tanks. I'll probably tie my home's rain gutters to underground pipes which will fill up the tanks. I think a 1" rainfall can collect about 1000 gallons for a 1300 sq ft house. I want to do this because in 2011, we had the worst drought on record here in Texas, receiving only 15 inches of rain all year. I'm curious how much water would I need to harvest in order to sustain 5 sheep, a dozen or so chickens, 1/2 acre garden etc.

Also, just for the sake of interest, let's say a 4-person family had no access to other water sources, how much more water would be needed for them?