The most important thing for you to remember is that whatever amount of water you THINK you need- double that and you got the right amount to take. There are many ways you can save weight but shorting on water should not be one. You never know what can happen and makine one mistake could be your last.

If i am going out for half a day i take 2 liters. I also carry a little pack of drink mix (something with electrolytes)in case i get dehydrated. One lastly important thing to remember, drinking water is not enough- you must eat something, even something small, at least every two hours while hiking to keep your body in check. Otherwise, you will not be able to hold your water. A snicker and a pack of crackers or trail mix is always a good bet. Stay away from beef jerky. Its not good for hiking. You do not need protein you need carbs and sugar.

Lastly, do not overlook the importance of carrying enough water. Trust me, i have not had enough before and been so thirsty, puking, dehydrated, ext. Do not take a chance. Bring 2x what you think is the most you'll need. On a personal note- i never wear anything hanging on my neck when i am in the woods. There are plenty of good lightweight back packs and wearing them you will hardly notice the weight of a extra ltr or two of water.