Oceanography

In the United States about one third of the flow of all streams is used at least once, so that we are beginning to use a significant fraction of the major readily available water supply. At the present rate of growth of use and of population we will come pretty close to using an amount of water equal to total stream flow by the year 2000. But use does not necessarily consume water. In fact we have not even included under use the water that makes electricity at hydroelectric stations, where water turns the turbines and is not changed chemically even though it has performed an important service. Water is consumed only if, as a result of use, it does not return to the stream system, from whence it can be withdrawn again. Eventually, of course, even though it has evaporated or has been taken out of circulation in manufacturing, it will return as rain or snow to land or ocean, and so find its way back some day to the stream supply. It is obvious that some water can be used more than once. It can be withdrawn from streams, used, returned to the stream and used again. The water of the Ohio River is used three times over in its course to the Mississippi, and one of its tributaries is used seven times. If there were no losses by evaporation when water was withdrawn, and if everyone returned the used water in the condition in which he found it, the present supply would be sufficient forever. If it were not for evaporation losses, treatment of used water, perhaps polluted by minor contaminants but not salty, could almost solve our problems. In theory, every house could have a tank and all the water from dishwashing and bathing could go through a purification plant and be returned to the tank. But there would be a net loss; water is drunk, used for gardening, evaporates from the swimming pool, is lost as steam from the teakettle.

The best estimates are that water consumption today is only about one third of water withdrawal. Most of the consumption is attributed to evaporation from irrigation. Little of the evaporation loss is directly from water that is spread on the fields, but is due to evapo-transpiration of growing plants. An acre of com can withdraw three thousand gallons of water from the soil each day and send most of it out into the atmosphere through the leaves as water vapor. Of course what has evaporated is available again at some future time. The general water cycle is one of income, storage, and outgo, much like a checking account.

There are quite reliable figures for the water loss during irrigation; only 40 percent of all the water used is returned to streams. This is in contrast to municipal use, in which 90 percent is returned, or industrial use, in which 90 percent is returned. The problem of how much water is lost is still a tricky one because no one knows how much of the water evaporated from a lettuce field in California ends up in the Mississippi River after it has condensed and fallen as rain in eastern Iowa.

But most predictions of the growth of irrigation indicate that it will soon begin to lower significantly the volume of the stream flow of the whole earth.

The degree of reuse of water is determined to a large extent by the change in composition that takes place when water is withdrawn, used, and returned. Water used for baths or showers is little altered. It has had a little dirt and soap added, but it is otherwise unchanged. If bath water could be isolated from the household waste water and treated it would cost little to restore it to its original condition. In contrast, consider the water that emerges from kitchens that have various types of disposal units that grind garbage into sludge that is flushed into drains. This water is loaded with a variety of ground-up organic materials that must be oxidized away or settled out before the water returns to potable quality.

Industry poses the same variety of problems. Water used as a coolant in steel making returns to its source unchanged, except for added heat. However, water used in some of the chemical treatments of iron and steel becomes rich in chromium, sulfuric acid, or other chemicals. To clean up cooling water costs little; to clean up chemically altered water costs a lot. There is no simple solution to the reuse problem.

To return water to streams in a condition similar to that in which it was taken requires separation and special treatment of the water, according to the specific uses to which it was put. The cleaning treatments cost money, so that the problem is one of economics, rather than merely of supply or methods for cleaning.

In addition to the constant supply of water that comes from rain we must consider the amounts stored at present in lakes, in glacier ice, and underground. All of these natural reservoirs can be “mined.” By mining we mean using a supply for which rate of withdrawal exceeds rate of replacement. We cannot view mining of water reservoirs as a practical approach in the long run. How long could we exist if we did not use the normal daily atmospheric supply, but relied entirely on melting glaciers, or on draining the Great Lakes to supply water to Arizona? If we do not insist on a perpetual water supply and decide to take water only from glaciers, lakes, and underground, how long could man survive before all these reserves were depleted?