Man has three major thirsts—his own and those of industry and agriculture. We need to appreciate their relative importance. Drinking-water is man’s most urgent requirement, for although he may survive several weeks without food, he dies within a few days when deprived of water. Man is actually over 60 per cent water by weight, distributed as 26^ litres inside the cells, 12 litres flowing between the cells, and 3£ litres in the blood . We cannot survive long without water because it continuously escapes from the body and therefore needs to be quickly replaced.
The fact that we are over 60 per cent water may seem strange, but not when we remember that it is the most versatile of substances. It transports to the cells all the necessary nutrients in solution. When these nutrients reach the cells by way of the blood and of the fluid between the cells, they pass into the cells to become part of an active chemical factory in which all reactions occur in water. Here water may play an active role by participating in chemical reactions. It sometimes also acts as a catalyst—that is, a substance that speeds up chemical reactions without itself being consumed.
Water also dissolves the oxygen and carbon dioxide that circulate through the body. Cells need oxygen to burn food for energy, and in this process they produce waste carbon dioxide. These gases enter or leave the body through the lungs, but they can do so only if they first dissolve in the moist lining of the lungs. But, as this lining is permanently moist, we lose water vapour every time we breathe out, amounting to about 400 cc. a day.
Another special property of water made use of by the body is its high specific heat. Water helps to moderate the climate by absorbing large quantities of heat without showing a large rise in temperature. On a smaller scale, the same applies to the body, where heat produced by cellular activity is absorbed by water without unduly raising the cellular temperature. This is very important because large temperature fluctuations affect the catalysts, or enzymes, on which all the body’s chemical reactions depend. Enzymes, in fact, only work properly at about 31°c; below this temperature, they act so slowly that the body becomes inactive, while at temperatures much higher than 37°c, enzymes are destroyed and all bodily functions cease.
It often happens that so much heat is produced in the body that the excess cannot be removed by normal processes of radiation and conduction from the skin. The only way to remove this excess heat is by perspiration, where a watery secretion pours on to the skin’s surface, and evaporates. During this change of state, latent heat is absorbed from the skin, and hence from the body—540 calories for every gram evaporated. The average sedentary man in a temperate climate loses about 500 cc. of water a day by perspiration, but in very hot climates the loss may rise to as much as 11,000 cc. This is a large water loss, but is necessary to prevent a large rise in body temperature.
In temperate climates, the main loss of water from the body is by excretion, amounting to about 1500 cc. a day. In this process the kidneys isolate waste substances from the blood and flush them out of the body in a watery urine. One waste substance is urea, which is produced from excess protein, but there are many other toxic or useless substances formed during metabolism, or eaten with solid food. The amount of water lost in the urine is partly related to the quantity of these excretory substances; a more important factor, though, is the relative amount of water and salt in the body. These concentrations must be kept within very narrow limits, otherwise cells dehydrate or burst. So when water or salt is present in excess, the urine is dilute, and when too little is present, the urine is concentrated. AU of which leads to an interesting point—because regulating the concentration of body fluids is all-important, the body cannot store water to tide it over periods of drought in the same way that it can store solid food for periods of famine.
Our daily water loss thus averages 1500 cc. as urine, 500 cc. in perspiration, 400 cc. in exhaled air, and 100 cc. in the faeces, totalling 2500 cc. Since a reduction in the body’s water of anything over 10 per cent is fatal, roughly 2500 cc. must be taken into the body daily. We could, of course, drink this amount, but the average man drinks only about 1300 cc. The remaining water he obtains from food; a dry biscuit, for example, contains 5 per cent water, while fruit and vegetables are about 80 per cent water. This ‘free’ water provides 850 cc. daily. The other 350 cc. are produced as a by-product of metabolism when food is burnt to produce energy.
Although we drink only two pints daily, the domestic consumption of water may be over 40 gallons per head per day. Much of this is used in the interests of hygiene: for baths, washing clothes, and flushing away waste into the sewers. The remaining water we use in various ways, such as watering the garden, and cleaning the car; these are really luxuries when we compare today’s life in the western world with that of 100 years ago.
If domestic water is man’s most important need, water for crop irrigation comes a close second. In countries like Pakistan, the low rain- fall supports only a few crops, and so irrigation is indispensable. Then there are areas like the south-east of England, where a large variety of crops can grow without irrigation, but grow much better with it. Here the purpose of irrigation is to add to what is already a reasonable rainfall in order to give a maximum crop yield. Unfortunately, the amount of water required for successful irrigation is enormous; most of the water evaporates and transpires from soil and vegetation, and most of it cannot be re-used.
Lastly, large quantities of water are used in industry. Some industries are not essential for survival, but serve to provide man with amenities that are now considered one of the hall-marks of civilization. Other industries, of course, are essential, e.g. the manufacture of agricultural equipment, the production of fuel, and making pipes for water supplies. The demands of industry and agriculture are such that the total consumption per head per day is now about 140 gallons in Europe, of which only about 40 gallons are supplied for use in the home. In America, the figure is in excess of 1000 gallons.
It is one thing to state our requirements, and quite another to satisfy them. To do this successfully, we need, apart from money, a more detailed knowledge of the hydrological cycle— the process that involves the circulation and distribution of fresh water on earth. We shall now give a brief outline of the cycle. The hydrological cycle begins and ends in the sea; from this there is no permanent deviation, but there are many different routes, or subcycles, that water can follow. Today, however, the cycle is not quite the same as it was, because man removes large quantities of water from rivers and lakes, and from underground, so that the old subcycles are complicated and new ones formed. It is thus no longer relevant to describe the hydrological cycle without describing the actions of man.
Although the hydrological cycle has no beginning or end, it is convenient to begin with evaporation from the sea. Evaporation continues as long as the air above the surface of the sea is not saturated with water vapour, and is quickest in the tropics, where the sun is most effective in warming the surface of the sea. Evaporation is also quickest when the air above the sea is warm, for the warmer the air, the more moisture it can hold.
The warm, moist air, being light, rises and is replaced by cold, dry air, which is heavy. As the air rises, it cools on meeting the cooler air above; it also cools as it expands at the lower atmospheric pressure. When the air temperature has fallen sufficiently, its vapour either condenses into droplets or freezes into ice crystals. These merge to form larger droplets and crystals until they are heavy enough to sink as rain, hail, or snow. Most of this precipitation falls back to the sea, but one eighth is swept across the land by winds. In so doing, this maritime air meets considerable amounts of vapour that have evaporated from the land, and together they provide us with the precipitation that is our sole source of fresh water.
Not all precipitation on land benefits man. Some freezes on the ice-caps of Greenland and Antarctica; this accumulation of solid water accounts for 70 per cent of all fresh water on earth. Another process over which we have little control is evaporation, which removes about 30 per cent of the precipitation over land. Evaporation occurs from rivers, lakes, and wet soil, and also from plant leaves, which intercept a large proportion of the rainfall. In countries with high temperatures, high wind speeds, or low air humidity, evaporation often exceeds rainfall, so that the soil dries up before rain has had time to promote plant growth.
After evaporation, the remaining water may infiltrate into the soil, in vegetated regions, much of this infiltrated water is absorbed by plant roots; it travels up the stems, and evaporates, or transpires, into the air through pores in the leaves. A tree may transpire about 50 gallons daily, very often removing all the infiltrated water. The combined effect of evaporation and transpiration, which we call evapotranspiration, can sometimes be reduced, for example, by covering water surfaces with substances that reduce evaporation. Deforestation also reduces evapotranspiration. But there are practical limits to how far we can reduce this process, one being that man needs plants to survive, and no plant is able to grow without transpiring.
The water that is not absorbed by plant roots slowly sinks through the fine network of channels in the soil. On its way, some is absorbed by clay particles, while some is held by surface tension as a thin film on the soil particles. This soil moisture evaporates very slowly into the soil channels, and soil particles remain moist for some time after rain stops.
When infiltration is heavy, some water may sink deeper into the soil until it reaches an impervious layer, above which it accumulates, saturating overlying porous layers, or aquifers. In regions of bad drainage, this ground water may remain static for thousands of years, though in general it slowly moves through the aquifers and sooner or later finds its way to the sea. It may do this by emerging from springs in the sea floor, or a spring may appear on the side of a hill where an aquifer is bared, and reach the sea by flowing into a neighbouring stream. Ground water may enter rivers and lakes that cut through aquifers, thereby increasing their flow. The level of some rivers would fall seriously, were it not for this ground-water seepage.
The total amount of ground water far exceeds the water contained in rivers, lakes, and reservoirs. Ground water has always been, and still is, a cheap and vital source of water for public water supplies, irrigation, and industry. But much of it remains unusable because it is often heavily mineralized, or too deep. Ground water also often moves very slowly through aquifers, and replenishment from precipitation may take anything from 1 to 50,000 years.
Up till now, we have assumed that the soil absorbs all the precipitation that reaches it. Much of the land is relatively impermeable, however, and there are also times of year when normally permeable soils are so saturated with water that they are unable to absorb any more. In both these cases, precipitation forms runoff— that is, water that flows over the surface of the land in sheets or streams. Runoff finally enters the tributaries of small rivers, which in turn drain into larger rivers. Each continent has a few rivers whose tributaries drain immense areas of land. The Mississippi, for example, drains 40 per cent of the U.S.A.; a large part of South America is drained by the Amazon, and much of Africa by the Congo. Rivers provide a concentration of water that is invaluable to man, and are often the only source of water in areas of low rainfall, such as Egypt.
Also important as a source of water are lakes, which are formed when rivers flow into relatively impermeable basins. Most of this water occurs in a few large lakes, such as the Great Lakes of North America, Lake Baikal in Russia, Lakes Tanganyika and Nyasa in Africa. Lakes store much of the surplus runoff during periods of heavy rainfall; this stored water then helps to increase the outgoing river level during periods of low rainfall. Not only does this increase the amount of water that man can remove from rivers in drought, but it also reduces the intensity of river floods.
In many areas, engineers have investigated the relationships between rivers, lakes, ground water, and evapotranspiration, while mathematicians have tried to discover the rules that govern the behaviour of these different parts of the hydrological cycle. But the results of detailed research in one region cannot necessarily be applied to another, because each area is a unique part of the hydrological cycle. Even a small country like England consists of many areas with different rainfalls and amounts of ground and surface water. To understand fully the distribution and behaviour of water in a particular area therefore requires a thorough knowledge of the principles of hydrology, together with a careful study of local conditions. One striking fact that emerges from our discussion of the hydrological cycle is that the water we use and drink has passed through many different subcycles over a period of many thousands of years. A drop of water may travel from the sea, across land, and back to the sea again, without ever falling to earth, yet the following week the same drop may take a different route, to be locked for centuries in Antarctic ice. Vapour from the sea may be swept across a mountain range, condense as rain, and fall into the head waters of a river. From here, it may flow down to the sea, or it may seep through the river bed to form ground water. If it is pumped out of the river or ground for irrigation, it may end up as the moisture content of a grain of maize, and be eaten. If this same water then leaves the body in the urine, it becomes part of a sewage that discharges back into the river. Man obtains water by intercepting these subcycles at various stages, but this does not always produce enough water. The only answer for some parts of the world is to by-pass all these subcycles by going directly to the sea for fresh water, an expensive method.