Utah and Nevada depend on irrigation more than any other States, for about 90 per cent of their farms have an artificial water supply. So dry are these States, however, that in spite of the streams coming from the great Wasatch and Sierra Nevada ranges, the area included in irrigation projects is only 31 per cent of Utah and 2 per cent of Nevada.
How Mountains Make Irrigation Possible.—Irrigation depends largely upon the presence of mountains. This is partly because mountains receive more rainfall than the dry lands at their base and partly because mountains act as reservoirs. The ground water which seeps into them in the rainy season gradually flows out through springs at lower levels. If the mountains are high enough so that the snow lasts till summer, the water is set free when it is most needed for irrigation. That is one reason why India has developed irrigation more highly than has any other country. The fact that both the Ganges and the Indus flow from mountains covered with perpetual snow makes it possible for a fifth of all the cultivated land of India to be irrigated. Northern Italy is another fortunate region. It not only has the snows of the Alps to serve as a reservoir, but some of its rivers such as the Ticino and Adda pass through the beautiful Lakes Maggiore and Como which aid in keeping their flow steady and in preventing floods.
In the preceding table see how the presence of snowy, mountains influences the figures in the second column. Nine per cent- of the lands of Colorado are included in irrigation projects because that State has great plains lying at the base of the snowy Rockies. A unique irrigation project is located in this State. The Gunnison Valley contains a large river, but only a little flat land, while the neighboring Uncompahgre Valley contains a small river and plenty of flat land. To bring the water to the land where it is needed a tunnel 6 miles long has been dug so that the water of the Gunnison River is now turned into the Uncompahgre Valley. Idaho and California as well as Colorado receive large streams from snowy mountains, and hence the figures in column 1 are comparatively large, while Arizona, New Mexico, and Nevada do not have such high mountains and are less fortunate.
The Need of Artificial Reservoirs.—Where the mountains are not high enough to give abundant water throughout the dry season artificial reservoirs must be made such as the Roosevelt Reservoir in Arizona, or the tanks of India. The chief trouble with reservoirs is that unless great precautions are taken they ultimately become filled with silt. India is full of old tanks that have thus gradually been converted into smooth plains which are now cultivated with the help of irrigation from newer tanks.
Methods of Raising Water for Irrigation.—In many places the demand for irrigation cannot be met entirely by supplies of water that flow to the fields from mountains or reservoirs. The additional supply must be raised mechanically from streams or wells. One of the most primitive methods of doing this may be seen in Egypt along the banks of the Nile. There the brown-skinned peasants fill buckets suspended from one end of a bar which moves like a seesaw. With the help of a weight at the other end they lift the water to a higher level. There it is raised again by another seesaw and so on until it reaches the level of the fields. In India the most familiar means of raising water is from wells by means of a large leathern bag, suspended by a rope passing over a pulley. When the bag is filled a pair of bullocks attached to the• end of the rope are slowly driven down an incline, thus raising the bag, which is emptied into a sluice leading to the fields.
These Oriental methods contrast vividly with the practices in many of our own irrigation projects where the water is raised by efficient pumps driven by windmills, by gasoline engines, or by elec tricity generated by the irrigation water at the dams higher up the valleys. What is needed now is some means of reducing the cost of power. If solar engines were practicable and cheap, irrigation would be possible in many places where it is now out of the question, for the constant sunshine would furnish abundant power at low cost.
Special Advantages of Irrigated Lands.—Agriculture in irrigated regions enjoys seven distinct advantages: (1) The soil of dry regions, as we have already seen, is peculiarly rich in plant foods, for the meager rain carries away only a small portion of the soluble ingre dients. (2) In addition to the abundant plant food in the original soil new food is often provided, for many irrigating streams deposit mud which serves as a fertilizer. (3) Since the sun shines much of the time in irrigated regions growth is rarely hindered by cool, cloudy weather. (4) Neither do the plants suffer from drought or from too much rain, for they can be given as much or as little water as is needed. (5) Weeds do not grbw as they do in wet countries. (6) The fruits grown on irrigated lands can be dried easily and cheaply. (7) Hay and grain always ripen perfectly and can be harvested without getting wet. In rainy lands vast quantities are often spoiled by being rained on at harvest time.