|Thirst and Starvation as handled in Outdoor Survival|
Each night without water requires a DC10 CON saving throw. Starting on the third night, this roll is at Disadvantage. If you fail, you take 1d4 CON damage. The player should note that this damage came from dehydration. Each night that the PC has water, they can recover 1d6 of this damage.
Follow a similar procedure for starvation, but the CON damage is only 1 instead of 1d4. Note that starvation damage needs to be tracked separately from dehydration damage, although both result in CON damage.
When a player loses 5 CON, they are at Disadvantage on all attacks and physical ability checks. When they have lost 10 CON, they also move at half Speed. Of course, when CON reaches zero the PC dies.
Horst the Cleric has a 13 CON and is without food and water trapped in a dungeon cell. Let's say there's a 10% chance per night he'll get rescued. The rolls shown below are modified for CON and Disadvantage if needed. Will he survive?
Day 1, CON 13(+1), rolled 18 for water, 13 for food, no damage
Day 2, CON 13(+1), rolled 20 for water, 4 for food, 1 CON damage
We now apply Disadvantage to subsequent rolls
Day 3, CON 12(+1), rolled 3 for water, 7 for food, 2 CON damage
Day 4, CON 10(+0), rolled 7 for water, 8 for food, 4 CON damage
Day 5, CON 6(-2), rolled 6 for water, 3 for food, 4 CON damage, Disadvantage on all physical rolls
Day 6, CON 2(-4), rolled 9 for water, 5 for food, 3 CON damage, Speed halved
Unfortunately, help did not arrive in time and Horst has died of dehydration. If only he had the Create Food and Water spell.
Hopefully the PCs will be clever enough to avoid having to go through the agony of dehydration or starvation, but it's good to know that these rules are tucked here in my blog just in cast the unthinkable happens.