While planning a day trip to a dry country it's worth remembering that a single puncture or other mishap could result in a very long walk for what appears to be a small reason. When out on a scouting trip to assess the habitat and glass a distant ridge for example the water in your vehicle should suffice to drink, to clean up any spills as well as to provide a margin of error should the return journey take longer than anticipated. As such the water reserve in a vehicle is best planned around the intended route, the time of year, number of people in the vehicle and how the outing might go wrong.
Start with the day you are actually planning
A typical day trip might involve two people to leave before daylight to check out the access road to public land and to check out some of the trails, and then head out to the end of a dry basin in the late afternoon. A road closure, or a long search for an appropriate place to park would make for a very long day. You need to plan the water reserve for the worst case scenario for the day that you are planning.
Plan for the worst case scenario, not a 'best case'. A day trip with two people, driving on public-land managed roads, is very different from a deep-backcountry hunt with several people and dogs. Even road tripping, as opposed to camping, warrants different planning and supplies for the unexpected. When planning a day trip, plan for the worst case for each person (including dogs), for the time that you will be away from services, for the heat, for the distance that you will be walking from your vehicle. Don't plan for an early return – plan for the worst.
In addition to drinking, a reserve of water will also need to be sufficient for washing of eyes and other parts of face and body for removal of dust etc., washing of small wounds, other medical requirements, washing of cups and other eating utensils etc. Drinking water must be kept separate from other utility water.
Choose containers for access, not appearance
Small containers are far better than large ones for storing water for a long country day trip. Not only do they make very light packs but also one person can fill them all up rather than trying to manage a number of large containers. It is also far easier to get the water into the cab of a 4×4, into the rear of a car or into a rucksack.
This should be containers made for potable water to be used for water meant for drinking. These need to close securely, not be too heavy to be able to lift, not crush and have a opening that can be cleaned. A narrow neck is best for stopping spillage. A wide opening is better for cleaning the container and filling up.
It's better to distribute the reserve in several containers (instead of one). In case one of the caps leaks or a vessel is punctured or contaminated with something, you'll still have a suitable amount of water for drinking during short stops en route.
Always mark your drinking water containers and assign them a fixed location in your vehicle, so that in an emergency situation you and your fellow travelers will be able to recognize your potable water and utility water containers.
Protect the supply from heat, dust, and movement
Store the water in a shaded part of the vehicle and use a container with a secure seal to prevent damage from movement of the container within the vehicle. A box or a portion of the cargo area behind a cargo barrier in the cargo area would be a suitable location for storing water in the vehicle.
The protection afforded to the reserve should not make it inaccessible. While it may be prudent to store the greatest portion of the reserve in the basement of the truck, having some portion of it available in an easily accessible location could prove to be wise. Storing it behind the cooler, or beneath the recovery gear and hunting gear in the back of the truck would not be very prudent.
In addition to being inaccessible to wildlife, store containers in locations that are protected from sharp edges of the truck, leaking fuel containers, loose tools, etc. It only takes a small amount of dust on the cap of a container to transfer to the water inside when the cap is removed for consumption.
Build a simple drinking and dispensing system
When a reserve of water is carried for drinking then a means of drinking from it and dispensing the water must also be carried. To prevent spills from use of a hand pump etc then a clean cup or bottle for each person should be carried rather than a shared mouthpiece.
Take the same amount of water for active walking as you would take for driving, i.e. a separate bottle for each team member. People often underestimate the amount of water they need for longer periods of time when it is not as easy to carry it. Make the hydration of your team part of your movement plan and do not make it a afterthought at the truck.
A little utility water (to wash out mud or other dirt, clean hands etc.) can be carried in a separate container, to be used in emergencies. This does not have to be full of water and could consist of hand sanitizer and a small supply of biodegradable soap for washing. However, this does not replace the need for water for proper physical rinsing where required.
If people taking regular medication are coming with you, check how their medication has to be stored and if they can take it with them. Simple things like this can go wrong in hot weather or in dusty places, and cause more problems than needed on a simple day out.
Match the reserve to the vehicle and route
Remember the water reserve must also fit in the car. Consider that a reserve in a pickup is different from a reserve in an SUV or even in a small car. Consider weight and the space to put the water reserve, ideally high and away from other gear or even on the opposite side.
On a day trip such as this example, the water supply could be split between being left in the passenger compartment of the vehicle and the remainder being stored in a protected location of the cargo area. One container to be able to use on the road at all times and the rest as a backup in case of an accident or other problem where it may not be possible to return to the vehicle for some time.
Re-check the route for: 1) areas with no service, 2) very sharp turns that you may not be able to turn around in and 3) where you may get stranded after dark. Don't rely on old map symbols, signs or even remembered water sources (including springs). Remember that conditions can change, and rules regarding access, collection etc. will change over time – always check with current official sources for latest information.
Check the reserve before every departure
A quick check can reveal lots of potential problems with what you've brought for the reserve supply. Does it still fit in the space you have? Are any containers damaged in any way? Have the caps been left loose? Is it going off or looking cloudy? Has the strap on a container for carrying broken? Is there any sign of dust entering the container?
Store water according to the instructions of the container maker. Check local health for updated advice on water storage. Do not top off a questionable container even for a short trip. Inspect and clean and dry all reusable containers before refilling them.
Tell someone where you are going, when you expect to return, and what vehicle you will be using. Bring a charged phone with you, but do not rely on coverage to get you out of a crisis. Bring a map, first aid supplies, means to create shade, and signals for help. All of these items are part of your general crisis preparation and should not be left behind just because you are going for a day trip in the country.
No need to go on an expedition to get out to the country. With a little planning for unexpected detours and bringing enough water to keep people calm and hydrated until they can get back to their vehicle, it should be possible to have an enjoyable day out without becoming a future rescue incident.