Can't wait to go on this trip.  Taking the whole family and my hyper active kids will probably need more water and food than what's provided. 
I see that you prefer the water and food.  We are heading out to Joshua Tree for some hiking.  We really need the extra water, and, there is no firewood to collect out there, so extra firewood would be very helpful for the cold, windy nights.🙂
Water is my main concern.  Keeping the little ones hydrated is a full time job.  I'm kind of concerned with how cold it might get at night.  Some extra firewood might be helpful.
We have a good size group; we are expecting 12-15, and some of those are teenagers, so the water, food, and firewood will all be helpful for us too.  I would like to take the waters and firewood.  Perhaps there are places for you to pick up some extra water, perhaps natural water sources? Out in the desert, we have no natural water sources to filter water or anything like that.🙂
I'm going to need 2 waters and at least 1 firewood.  Some food also.
😮 I see.  I actually need at least two of the waters also.  I can leave you two food, one firewood, and one water.  I have a larger group than you, and I think my need is greater given that we are in desert conditions. Given the need, I submit that this is a fair offer.
Group size isn't a concern since the basic supplies were enough for your trip.  If you want 2 waters, I'm going to have to get 2 firewoods and 2 foods along with a water.
☹️ Group size is an issue. The 'enough' is only an estimate.  A package of extra water will go much less further with 4 people than with 15.  Your offer will not work for me.  I ask you to reconsider my last offer.  I am ready to finalize, but if we cannot come to agree, then we will both suffer.  
It's a give and take.  We both want water the most. I gave in on that and agreed you could have 2.  Now you'll have to compromise a bit and let me have 2 firewoods and 2 foods.  It's only fair we both give in some. 🙂
You might consider that fairness is not just about distribution, but about need. You haven't really convinced me that your need is as great as mine so that you should get the the greater of extra supplies. My final offer is twofold: to take 2 water and 2 firewood and one food, or, two firewood and three water.  If neither of those will work for you, or unless you have a better offer, then I will have to walk away.
Walk-Away