Marco Polo

We shall all die of thirst.

A body lies in the desert sands. A desiccated corpse, stretched out in the vast, baking emptiness. A lost traveller, found by chance. Found too late.

The body has scant clothing and few possessions. Everything that was not essential long since discarded in the exhausting struggle against the desert heat. Only one precious object remains – a water bottle.

It’s still half full.

This is more common than you might think. People often die of thirst in the desert long before they run out of water. This is because they make the mistake of rationing their water supply. It seems like common sense: you only have so much water, and you want it to last as long as possible. But if you’re sweating water out and not replacing it, you will get more and more dehydrated, and eventually die.

Water isn’t like food. If you ration out your food, you’ll feel hungry, sure, but you can keep going for a very long time while taking in fewer calories than you are expending. Your body just starts using up its reserves, extracting energy from stored fat to make up the difference. You lose weight, but you stay alive. Even when all the fat is gone, your body will keep going by cannibalising its own muscle tissue. In the end, of course, you will die if you don’t get enough food, but if you carefully eke out your remaining supplies you can put that day a long way off.

When it comes to water, you have much less room for manoeuvre. Your body temperature must be kept within a fairly narrow band, within about half a degree of 37 °C. If it gets much higher than this, you begin to suffer heat exhaustion and eventually, if it gets past 40 °C, heatstroke. At this point, you either get emergency medical treatment to cool you down rapidly, or you die. (Getting too cold can be just as dangerous, but we won’t deal with that here.)

There are three main ways a body can lose heat: radiation, convection and evaporation. Of these, there’s not much your body can do about the first two. The rate at which a body radiates heat is (to a good approximation) simply a function of its surface temperature and surface area, and there’s not a lot you can do to change those. Convection is a little more hopeful. This is when your body transfers heat to the air next to the skin, and as the air moves the heat is carried away. A good breeze will help with this, if you can find one, or a fan – although fanning yourself will generate more heat than it carries off. When you’re in the desert, your best bet to maximise convection is to wear loose clothing and hope for the best.

That leaves evaporation. Your body emits droplets of water from the skin, and as these evaporate they carry away heat. Crucially, this is something your body is able to control directly, increasing the rate of water emission in response to heat, so as to keep its core temperature within that narrow band of safety.

In other words, when it’s boiling hot, you sweat buckets.

This brings us to the crucial point. You need to sweat a certain amount to prevent heatstroke, and if you deprive your body of water you deprive it of the means to regulate its temperature. There is no sweat reserve that your body can use in an emergency, as it uses up fat reserves when food is scarce. If you sweat out more water than you drink, you will die pretty quickly. And before you die you’ll suffer the early symptoms of heatstroke, including confusion and disorientation, making it all the harder for you to correct this mistake in time.

So you shouldn’t ration your water, but when you’re out in the desert and you use up your water you’re going to die anyway, so what should you do? Apart from “be somewhere else”, which in fairness is the obvious solution.

The answer is to reduce your body’s need to sweat. That way, you can keep going longer with less water, because your body isn’t using so much to keep its temperature down.

The single simplest way to do this is to rest and sleep during the hot day, in as much shade as you can find or contrive, and do your travelling in the cooler periods of early morning, late evening and night. Keeping your mouth closed as much as possible will help you to retain moisture – one traditional trick is to suck on a small, smooth round pebble. It also helps if you can avoid eating: digestion requires water, and you need to save as much of your water as possible for sweating.

You should certainly avoid the temptation to drink your own urine. Your body will just use up even more water trying to flush out all the excess salts you’ve just consumed. That’s not to say your piss is useless, however. If you can save it up until you are ready to rest for the day, then pee into some small depression and rest on top of it, the damp ground will help to keep you a little cooler.

We don’t see these techniques in use when Marco Polo is dragging our time travellers through the Gobi Desert, and in some ways that’s just as well. The sight of the Doctor settling down for the day in a bed of his own piss might have been educational, but it is unlikely to have been welcomed. Instead, our intrepid heroes manage to survive by extracting water from their surroundings using the phenomenon of condensation.

There is a way you can do this in the real world. It’s called a condensation trap, and it works like this. Dig a decent-sized hole, about a metre across, deep enough that it goes down into damp ground. You can even pee into the hole for extra moisture. Pop a cup down at the bottom of the hole, somewhere near the middle, and cover the hole with a clear plastic sheet. Make sure the sheet is weighted down with stones all around its circumference so as to seal the hole, and place a rock on top of the sheet above the cup. Then wait.

As the sun heats the damp earth, water will evaporate, then condense on the underside of the plastic sheet. It will drip down from the low point created by the rock, and be caught in the cup. At the end of the day, uncover the hole and have a good drink.

It’s a sound enough theory, and popular in survivalist circles, but unfortunately it’s not all it’s cracked up to be. It generates water, sure, but you’ll be doing well to get more than 100 ml or so out of it – and you’ll sweat out more than that digging the damn thing in the first place.

Still, this seems to have provided the inspiration for the Doctor’s life-saving discovery of condensation in the Tardis. And it also gives us some indication of why that doesn’t seem to make a whole lot of sense. For a start, you need a source of moisture, and it’s not clear where that is coming from in the Tardis. (Viewers of later series might suggest the Tardis swimming pool, but if the Tardis has a swimming pool at this point then why not just drink directly from that?) Secondly, how do you collect this condensation from the Tardis walls? Mop it up with J-cloths and wring it out into a pint mug? All suggestions gratefully accepted.

So if you must head out into the desert, plan ahead to avoid having to resort to these desperate measures. Take enough water for your daily consumption, and enough transport to carry it all. And avoid travelling with sinister villains if you can at all help it. That never goes well.