It's not like Walmart is going to send semi-trailers full of bottled water to the disaster area just for free
I did some rough math based on the following assumptions (all numbers are guestimated):
A 63' long semi trailer is 8 feet wide by 10 feet tall. Pallets of water are 6' tall.
A flat of water has 24 bottles and is 18" long by 12" wide by 8" tall.
Pallets are 3' x 4'
1 layer of water flats on a pallet consists of a 3x5 pattern, 15 flats of 24 bottles or 360 bottles per layer.
One semi truck holds about 30 pallets in a 2x15 arrangement, or 98,400 bottles.
Roughly 100,000 bottles per 63' semi truck.
So we're talking about 50 to 60 semi trailers full of water.
That's really not that much. I imagine it will be stored in probably half a dozen different distribution centers around the US. And that FEMA/DHS anticipates being able to depend on support from Walmart, Waffle House, Home Depot and other major chains that provide prominent assistance during disaster relief.