Part 19 (2/2)

The heavy rainfall in western Was.h.i.+ngton, Oregon, and northern California results in dense forests. To the south, the rainfall upon the lowlands is not sufficient to produce forests, but as it is greater upon the mountains, trees thrive upon their sides.

The elevation at which trees will grow becomes higher and higher as we go into the more desert regions, until in northern Arizona it is found to be above six thousand feet. The high plateaus are generally treeless, but are covered with such shrubs as greasewood and sage-brush.

We see now that our climate is the product of many factors. It frequently varies greatly in places only a few miles distant from each other. Consequently there may be a great variety of productions and industries in one small area, while in other regions the climate and productions are almost unchanged for hundreds of miles.

SOMETHING ABOUT IRRIGATION

Travellers from the Eastern States who visit New Mexico for the first time are attracted by many unusual sights. There are the interesting little donkeys, the low adobe houses of the native Mexicans, and the water ditches winding through the gardens and fields, which are divided into squares by low ridges of earth.

If the fields are seen in the winter time, when dry and barren, the meaning of their checkered appearance is not at first clear, but in the spring and summer one is not long in finding out all about them. When the time comes to sow the seed, water is turned into these squares from the ditches which traverse the valleys, and one square at a time is filled until the ground in each is thoroughly soaked. Afterward, when the ground has dried enough to be easily worked, the crop is put in. The seeds soon sprout under the influence of the warm sun, and the land becomes green with growing plants. The same method of moistening the ground is used for the orchards and vineyards.

What is the use of all this work? Why not wait for the rains to come and wet the earth, as the farmer does in the eastern United States? The Mexicans, who have tilled these valleys for more than two hundred years, ought certainly to have learned in all that time how to get the best returns. You may be sure that they would not water the ground in this way if it were not necessary. The fact is that over a large portion of the western half of the United States it does not rain enough to enable the farmer to grow his crops. The climate is generally very different from that of the Middle and Eastern States.

When the Mexicans moved northward into the valley of the Rio Grande River, into Arizona and California, they found a climate similar in many respects to that at home, and soon learned that it was necessary to water the land artificially in order to make it productive. Though in many places sufficient rain fell, yet the heaviest rainfall came in the late summer or winter, when the plants needed it less, while the spring and summer were long and dry. The Mexicans were not the first to practise watering the land, if we may judge from the ruins of ancient ditches constructed by the primitive Indian inhabitants. It is evident that they too made use of water in this manner for the growing of their corn and squashes.

This turning of water upon the land to make it productive is termed ”irrigation.” The work is performed in different ways, as we shall see later. Irrigation is now carried on through all portions of the United States where the rainfall is light and streams of water are available.

To one who has lived in a country where there is plenty of rain, it seems to involve a great deal of work to prepare the land and to conduct water to it. One may feel pity for the farmer who has to support himself in this manner in so barren a country. I am sure, however, that if any such person will stop to think, he will remember times when in his own fertile home the expected rain did not come, and the vegetation wilted and dried up. He may have become discouraged because of a number of ”dry years,” but probably never thought that he had the means at hand to make up, at least in part, for the shortcomings of Nature, in sending too much rain one year, and another year too little.

[Ill.u.s.tration: FIG. 113.--WATER-WHEEL FOR LIFTING WATER FOR IRRIGATION, VIRGIN RIVER, SOUTHERN UTAH]

It would doubtless have paid such a farmer many fold to have been prepared at the coming of a dry year to turn the water from a neighboring stream over his lands. This process would have involved a good deal of labor; but how the plants would have rejoiced, and how abundantly they would have repaid him for the extra trouble!

The showers come without regard to the time when growing things need them most, but with irrigation the crops are independent of the weather. The farmer may be sure that, if he prepares the ground properly and sows the seed, the returns will be all that he can wish. In many localities several crops may be raised in a year by this method where otherwise only one would grow.

Now let us see how the water is taken from the streams and what are the different methods employed to distribute it over the land.

Almost every valley is traversed by a stream, great or small. It may be a river, with a large volume of water, or a creek which completely dries up during the long, rainless summers of the West.

[Ill.u.s.tration: FIG. 114.--GARDEN IRRIGATION, LAS CRUCES, NEW MEXICO]

In rare cases the stream may flow upon a built-up channel which is as high as the valley, but usually it is sunken below the level of the floor of the valley, and enclosed by banks of greater or less height. How is the water to be sent over the land? Where the current is swift you may sometimes see a slowly turning water-wheel, having at the ends of the spokes little cups, which dip up the water as the wheel revolves and pour it into a flume that runs back over the land. At some places engines are used to pump the water from the stream and lift it to the desired height.

[Ill.u.s.tration: FIG. 115.--IRRIGATING AN ALFALFA FIELD, ARIZONA]

Generally, however, another method is employed: the water is taken out of the stream in an artificial channel dug in the earth. But in order to get the water at a sufficient height to make it flow over the fields, it is necessary to start a ditch or ca.n.a.l at a favorable point some distance up the stream, perhaps miles from the garden.

The ditch is made with a slope just sufficient for the water to flow. The slope must be less than that of the river from which the water is taken, so as to carry the stream, at last, high enough to cover the lands to be irrigated.

Visit almost any valley in the West where agriculture or fruit-growing is being carried on, and you will at once notice the lines of the ditches, apparently level, as they wind around the hillsides. At convenient distances there are gates to let out the water for the orchards and fields.

The ground may be moistened in different ways. The first method is that employed by the Mexicans, who, if we except the Cliff Dwellers, were the first to introduce irrigation into our country. This consists in dividing the land into squares by embankments and allowing the water to flood each in succession. The method is known as irrigation by checks, and can be used conveniently only upon nearly level land.

In many orchards a series of shallow furrows is ploughed between the rows of trees, and the water is allowed to flow down these until the soil is thoroughly soaked. In alfalfa fields the water is often turned upon the upper end and permitted to work its way across until it reaches the lower edge, soaking the ground as it goes. The slopes must in every case be so gentle that the current will not be strong enough to carry away the soil.

Once in every two to four weeks throughout the spring and summer, the exact period depending upon the rapidity with which the ground dries, the wetting is repeated. If the soil is light the water must be turned on more often and a larger supply is required.

It frequently happens that the stream from which the water is taken so nearly dries up in the summer, when the water is most needed, that the cultivated lands suffer severely. During the winter little if any irrigation is necessary, but at that time the streams are so full that they frequently run over their banks and do great damage.

How to preserve the water thus going to waste and have it at hand for summer use has been an important problem in regions where every particle of water is valuable. Study of the question has led to the examination of the streams with reference to the building of reservoirs to hold back the flood waters. A reservoir may be formed of a natural lake in the mountains in which the stream rises, by placing a dam across its outlet and so making it hold more water.

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