The rain catcher item states:
"[The rain catcher] can catch 2 gallons of drinking water per inch of rainfall and hold up to 8 gallons"
My question is: How do I calculate how much rain has fallen in a given period? The adventure states that it is almost constantly raining in Chult, but I don't see any system to generate exactly how much rain falls. For example, if my party wants to set up a rain catcher in their camp each night for 10 hours, how do I calculate the water collected?
To calculate the water gathered by a rain catcher each night in gallons, multiply the number of inches that fall during the night by 2. This is because the rain catcher gathers 2 gallons per inch of rainfall. It also holds up to 8 gallons.
To figure out how much rainfall there is, there's no answer given in Tomb of Annihilation (ToA). The ToA DM Screen published by Gale Force Nine includes the same table found in the Dungeon Master's Guide (DMG) for rolling for precipitation, but the table isn't tailored for Chult's climate.
It rains in a rainforest about 250 days each year, or about 70% of the days each year. Rainforests average 80 inches of rainfall a year, and the wettest place on Earth receives 467 inches of rainfall a year. One other useful piece of information is that Heavy Rain is considered three or more times as much rainfall as Light Rain.
The chart in the DMG only has it raining 40% of the days. To match a rainforest, roll on the table for every 8 hour period, so once for the 8-hour day and twice during the 16-hour night. This makes it rain 78% of the days each year, but doesn't increase how often the players will encounter daytime Tropical Storms.
Roll on the chart 3 times every day, once during the daytime traveling, and twice during the resting hours. Note that the collection is for an 8 hour period, so players should get to collect twice during the long rest. This chart tries to keep things as close to the DMG and DM Screen as possible.
d20 Intensity Rainfall 8-hr Collection
1-12 None 0 0
13-17 Light 0.031" 0.5 gallons
18-20 Heavy 0.094" 1.5 gallons
^- 25% Tropical Storm 0.19" 3 gallons
Using this table results in annual rainfall of 222 inches, but also tries to give the players a good chance of getting some rainwater by not dumping it all on them during Tropical Storms. Rainfall in the chart is per hour. By rolling 3 times, some days it might be worth hunkering down to catch some rainwater since there's no guarantee the rain will still be there come nightfall!
An alternative chart is here which requires only two rolls, once for the day and once for the night. This version results in 227 inches of annual rainfall, and rain on 75% of the days.
d20 Intensity Rainfall Nightly Collection (16 hr)
1-10 None 0 0
11-14 Light 0.016" 0.5 gallons
15-17 Medium 0.047" 1.5 gallons
18-20 Heavy 0.078" 2.5 gallons
^- 25% Tropical Storm 0.19" 6 gallons
And finally a once per day chart. Roll once for the whole day! One change is that to determine Tropical Storms, only check on a roll of 18+. This chart results in 228 inches of annual rainfall, and rain on 75% of the days.
d20 Intensity Rainfall Nightly Collection (16 hr)
1-5 None 0 0
6-10 Light 0.016" 0.5 gallons
11-14 Medium 0.03" 1 gallon
15-20 Heavy 0.047" 1.5 gallons
18+,25% Tropical Storm 0.094" 3 gallons
Here's an extra simple method for generating rain each night. Just roll 1d4 and lookup how many gallons were gathered.
d4 Intensity Nightly Collection (16 hr)
1 None 0
2 Light 0.5 gallons
3 Medium 1 gallon
4 Heavy 1.5 gallons
This method results in annual rainfall of 205 inches, and rain on 75% of the days.
To get more rainwater each night, parties should bring more rain catchers. Characters may also be able to forage for water (see rules in the DMG).
As far as I know there aren't any actual guidelines for how much it rains in Chult except for "A day without rain is rare." Legendarypants had a blog post about this and they ended up using a combination of Wikipedia and the DMG:
I looked up a rain forest on Wikipedia, found out it rains something like 275 days of the year, did a quick calculation, and figured out that rolling twice per day on the precipitation table from the DMG is about right.
http://www.legendarypants.net/tomb-of-annihilation/
The weather table on page 109 of the DMG just lists "light rain" or "heavy rain." Google tells me that light rain is up to 0.10in/0.25cm per hour and heavy rain is in excess of 0.30in/0.76cm per hour.
That would mean in a light rain it would take at least 40 hours to fill up your raincatcher. In a heavy rain it would be 27 hours or less.
One more thing to note if you end up using the chart from the DMG. Page 11 of ToA tells us that on a heavy rain day, there's a 25% chance of a tropical storm.
Using those numbers, here is what I generally came up with:
On any given day, roll 1d4.
1 = No Rain
2 = Light Rain (0.1 inch per hour)
3 = Moderate Rain (0.2 inches per hour)
4 = Heavy Rain (0.5 inches per hour)
On a heavy rain day, roll 1d4: on a 4, it's a Tropical Storm (1 inch per hour)
Travel is assumed to be 8 hours travel and 16 hours camping. Rain catchers need to be stationary to set up, and collect 2 gallons per inch of rain. I'm also rounding down to the nearest whole number to accommodate for set-up and rain intensity shifting throughout the day. So in one day (16 hours of camping), a rain catcher collects:
Light Rain: 3 gallons
Moderate Rain: 6 gallons
Heavy Rain: 16 gallons*
Tropical Storm: 32 gallons if traveling, 48 gallons if camping for 24 hours*
*Heavy Rain and Tropical Storms require someone to empty the rain catcher one or more times to get full amount, or else they hit the 8 gallon max.
Obviously this isn't EXACT to the real world, but is a much simpler version that will speed up the process of figuring it all out.
