I'm planning on hiking my first r/t day hike this summer (prior trips have been from the back route). Based on my calculations, I'm going to carry 4L from the TH, again knowing I'll probably be out by LP Lake on the return.
If I really need to, I can treat some water there before heading down, or just gun it and rely on the store's abundance of good things to eat/drink when I arrive.
Hobbes, carrying 8 lbs of water from the trail head is not the best approach. That weight is unnecessary and will slow you down. And you probably will run out so you will need to dip more along the trail anyway.
Why don't you check this, thanks to volunteer ranger Bob Rockwell:
Reliable Water Sources Along the Mt. Whitney Trail (a PDF document)
related pictures:
Water availability along the Mt. Whitney Trail
