Oceanography

Irrigation is one of man’s oldest engineering endeavors. From hand carrying of water to thirsty crops man progressed to digging ditches and building dams to store and diverts water. Perhaps the beaver supplied the complicated model for the first man-made dams. Hammurabi, the ancient Babylonian king, described irrigation when he wrote, “I brought the waters and made the desert bloom.” We are doing the same thing today, but we have moved to engineering of such magnitude that the giant and far-reaching Feather River project of modern California will eventually have the capacity to deliver two thousand billion gallons of water a year, much of which will benefit agriculture.

The temperature of irrigation water is significant. Warming basins are often provided to bring water to the proper temperature for irrigation; this varies with each crop so that no average figure can be given. When water was first released from Shasta Reservoir for summer irrigation of rice in California, crop damage resulted from the forty-five degree water, sixteen degrees colder than that previously used.

There are approximately four hundred and sixty million acres of cropland in the United States and more than forty-four million of them are irrigated. This 10 percent produces roughly 25 percent of the total value of crop production. Although many areas are already using all the water available to them, irrigation is expected to increase both in the United States and abroad.

Plants as well as animals are biological factories for food production. The water they use affects not only the product but also the factory efficiency. The water they are given must be tolerable to them and safe for human consumption at the end of the food chain. The recommended limit of dissolved solids in water for agricultural uses is about 700 parts per million whereas drinking water for man may have as many as 1,000 ppm dissolved solids.

The amount of water required for agriculture will become greater in the future as population increases. There are already three and a half billion people in the world and by the year 2000 there may well be seven billion. It is predicted that the 46 percent consumption of water by agriculture in the United States will increase to 62.6 percent by 1980. Already Japan, hard pressed by a high population, uses 80 percent of her water to grow rice, and Israel, now using most of her water for agriculture, depends entirely on irrigation for food production.

As an indication of how much water is needed to produce the world’s foods, six hundred and fifty thousand gallons of water are required to produce thirty-two bushels of wheat; three hundred gallons of water are needed to produce two and a half pounds of bread; a pound of beef represents twenty-three hundred gallons of water. These figures do not include the amount of water used in processing or transporting food products.

As bacterial spores are perhaps the only vegetable organism having as little as 50 percent water, we can say that plants are mostly water. Lettuce, cucumbers, spinach and asparagus are 95 percent water; tomatoes and carrots are 90 percent water; p0tatoes are 80 percent water.

Anyone who has weeded a garden has some idea of plants’ extensive water collecting system. Most root zones are roughly six feet below the ground surface, although some plants put down roots too deep to obtain water. Trees with roots reaching down forty to eighty feet are not uncommon.

Other plants can never reach down far enough to find ground water and their roots may instead spread horizontally beneath the surface. A desert cactus, for instance, may have twenty miles of roots, lying close to the surface so that quickly penetrating rain can be absorbed before it rushes down beyond the root zone. The tiny root hairs may take up many miles in search of water; a single rye plant may have 380 miles of these root hairs.

Another agricultural need for water is for livestock. Although the amounts vary with climate, animals require much more drinking water than humans. Sheep can survive on one and a half gallons a day, while cattle need eight or ten gallons a day and horses as many as fifteen gallons a day. The great herds of animals grazing in the western states alone require staggering amounts of water. One estimate says that there are a hundred million cattle and ten million horses among the United States livestock population. Even with these numbers, the amount of water used for livestock is small in relation to other uses.

Animals can tolerate much more salt in drinking water than can humans. Poultry can survive on water with 3,000 ppm salt and sheep can take water with 10,000 ppm salt. A low salt intake from water is often supplemented with salt blocks. An awareness of animal tolerances of each element in the water supply is as necessary as is this knowledge for control of drinking water for man.

Particularly stringent water quality control is necessary in processing of dairy products. Water for milk processing must be free from all harmful bacteria, yeasts, and molds. And because the flavor of milk is so easily influenced, processing water must be free of tastes. Another strict requirement has been discovered for water used in egg-washing machines, where water containing more than 1 ppm iron may accelerate spoilage.