Legacy of Fire!
Heat deals nonlethal damage that cannot be recovered from until the character gets cooled off (reaches shade, survives until nightfall, gets doused in water, is targeted by endure elements, and so forth). Once a character has taken an amount of nonlethal damage equal to her total hit points, any further damage from a hot environment is lethal damage.
A character in very hot conditions and active (above 100° F) must make a Fortitude saving throw each hour (DC 15, +1 for each previous check) or take 1d4 points of nonlethal damage. Characters wearing heavy clothing or armor of any sort take a –4 penalty on their saves. A character with the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the skill description). Characters reduced to unconsciousness begin taking lethal damage (1d4 points per hour).
In severe heat (above 115° F), a character must make a Fortitude save once every 10 minutes of activity (DC 15, +1 for each previous check) or take 1d4 points of nonlethal damage. Characters wearing heavy clothing or armor of any sort take a –4 penalty on their saves. A character with the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the Survival skill in Using Skills). Characters reduced to unconsciousness begin taking lethal damage (1d4 points per each 10-minute period).
Note that shade, well watered, etc can avoid saving throws completely while resting with GM approval for temperatures up to 135 degrees.
A character who takes any nonlethal damage from heat exposure now suffers from heatstroke and is fatigued. These penalties end when the character recovers from the nonlethal damage she took from the heat.
Katapesh is a mixture of Rocky Desert and Sandy Desert
Deserts are harsh and survival is difficult. A lifesaver is the create water spell, which gives you 2 gallons of drinking water per caster level. Unless you have multiple characters in your group that are capable of casting create water, a group with one 1st-level caster is limited to 6 gallons of water per day (unless you start using higherlevel spell slots on the spell), so you’ll need to provide other means of finding drinking water—which probably involves carrying it on your back.
If create water is your right hand in the desert, then endure elements is your left. A single casting of endure elements allows you to exist in most desert conditions as you would in the rest of the world, which means you don’t need extra water, don’t need to make Fortitude saves for very hot or severe heat conditions, and can wear whatever clothing and armor you like without concern for how the environment affects you.
While these two spells are immensely helpful, keep in mind that they also take up spell slots, and can easily sap your party’s combat strength just to stay comfortable. Potions, scrolls, and wands of these spells allow you to travel in relative comfort and still be ready for battle.
Gear and Magic Items
When traveling to the desert, you should select that gear which is best suited to the task: clothing and armor that keeps you cool, packs that support the most weight, and as much water as you can possibly manage to carry. A bag of holding can carry water, but the amount of water it can hold is limited not so much by size as by weight. A type I bag can hold 250 pounds of water—about 30 gallons. A type II bag can hold 60 gallons, a type III bag 120 gallons, and a type IV bag 180 gallons. If you’re willing to use all the pockets of a handy haversack, you can manage about 14 gallons of water. For a cheaper alternative to the largest bag of holding, consider a decanter of endless water, which is a more efficient means of obtaining more water than a typical adventuring group needs for survival.
Aside from food and water, you should bring a tent to shield you from wind and sandstorms, empty waterskins or other containers to gather water, and (if you can spare the weight) extra arrows for hunting—there is no useable wood for replacing spent arrows in the desert.