I'm still trying to sort out how to easily track/determine how much water the party has had access to over the course of the day. I don't want to constantly be asking, both because it seems tedious, but also because if they know the drill, they will always tell me the "right answer." And that wouldn't take into account potential mishaps ( the raincatcher collapses, they couldn't keep a fire going to boil water, etc.).
I'm thinking about some sort or daily check, with modifiers for things like the use of rain catchers, the amount of rain that day, etc. Casting purify food and drink could be used to add a bonus to the roll if it "fails." (Not a guarantee of success, since it's possible they couldn't find enough water to purify) Casting Create food & water would either eliminate the need for a check or negate a fail. (Although at the moment that spell is not available to the party as no one is 5th level, and even when it is, it will cost a precious 3rd level slot)
Something like this: a straight DC 15 for the group, with the following modifiers:
-No Rain or only Mist all day: -2
-Heavy rain or Storm:
-+1/period* of heavy rain/storm if no rain catchers
-+2/period* of heavy rain/storm if 1 rain catcher for every 2 creatures (including pack animals)
-+3/period* of heavy rain/storm if 1 rain catcher for every creature
-River or lake: +5 if on/by river or lake (access to water, but it has to be boiled)
Purify Food and drink: Auto success if on/by River or Lake (getting enough water to purify is a given)
+5 otherwise (and can be cast after the group knows the roll failed; this may argue for keeping the roll result hidden)
*I split the day up into 3 segments/periods, and randomly determined weather for each period.
I'm also thinking that those in medium/heavy armor will need to make an exhaustion check each day, regardless of water intake. But on low-water days that check is at disadvantage.
What do the rest of you think about this approach?