In the former nothing is done except to dig a large ditch through the field, as near the middle as is consistent with its following sufficiently high ground, or between two fields, if both are to be watered from it. Through this ditch, or zanje, a slow stream of water is kept running. It soaks into the ground and percolates or "seeps" through it and thus sub-irrigates the whole field without any lateral ditches. Of course, this occurs only in peculiar soils. Its best exemplification is in Fresno and Tulare Counties, California. Sometimes a single ditch, nearly straight, will in this way irrigate one hundred and sixty acres.
The other method is exactly opposite. The whole field is flooded. Head-gates are placed along the main ditch, and from every head-gate a dike or levee is run across the field. Levees are also run along the sides, one of them forming the outside of the ditch. If these levees ran at right angles, a field thus prepared for irrigation would look for all the world like a huge printer's case. The levees may be two to four feet high. The intervening spaces are called "checks," and may contain any amount of land. I have seen one thousand acres cut into checks of from one to ten or twenty acres. I did not see how it could pay. Nothing was grown but hay and pasturage.
The checks are leveled, if not already sufficiently level. They are flooded one at a time. In flooding check No. 1, head-gate No. 1 is opened and No. 2 closed. As soon as the whole surface of this check has been covered with water, head-gate No. 2 is opened, and the same flood runs back into the ditch and down into check No. 2, and so on. The water is kept on the land but a short time. In warm weather the flooding is done mostly at night. The basins or checks formed by the dikes are not filled with water.
Alfalfa hay is cut four or five times a year, and the land is flooded after each cutting. Twelve tons a year per acre are not a rare crop, though less is commoner than more. For wheat and other cereals one good flooding is enough.
A good deal of California land has been over-irrigated. Alkali has been brought on or brought up, the soil has been made heavy, pools have been formed from the "seepage," and orchards and vineyards have been spoiled. After a field has been irrigated for a few years it becomes saturated, and wells dug in it soon reach water. It no longer needs so much water, and its former supply may be carried on to reclaim new deserts. How much a single river will reclaim, only give it time enough, can be vaguely guessed. Thousands of acres in the San Joaquin Valley have been placed beyond the need of further irrigation. The whole valley was once a desert.
A part of it seems beyond the reach of any irrigation except
