The natural subirrigation is caused either by the whole soil filling up with water from the natural and original water level to the very top or to the roots of the vines, or from an impervious hardpan or clay, as subsoil, up towards the surface. As an example of the former we might cite the country around the irrigated plains of the San Joaquin valley, especially around Fresno and in Mussel Slough. Before irrigation was begun there in 1872, the surface water was from sixty to seventy feet from the top east of the railroad, and from forty to fifty feet west of the railroad, lower down in the valley. After five years of irrigation it began to be noticed that the soil required less water. The water in the wells began to rise, and the following year the water stood in many places near or on the top of the surface. Now the whole irrigated district around Fresno has filled up with water to such an extent that drainage ditches have become necessary in some places in order to lower the water in the wet season some four or five feet from the surface. Many more drainage ditches will be required, as in wet winters the surface water in places is not only very near the top, but actually forms ponds or swamps where formerly the ground was entirely dry.
In the old irrigated districts, water can now be found at from six to ten feet in the driest season, while formerly the wells had to be from fifty to seventy feet deep. In the older vineyards, and even in many of the younger ones, no more surface irrigation is used; all that is now required is to allow the water to run in the main ditches, in which the water sinks sufficiently to keep up the supply of the evaporation of the ground outside. Large tracts of land which have never been surface irrigated are now sufficiently moist to grow vines to the greatest perfection, and many of the best vineyards have never been irrigated at all; in fact, nothing but drainage ditches have ever been made on the land. Whenever such subirrigation exists, the water level will be found higher in the winter than in the summer, and drainage should accompany subirrigation in nearly every instance. A subirrigation like the above exists in Denia and other of the Spanish districts. Subirrigation may be also caused by either impervious subsoils, such as hardpan and clay, or by spongy subsoils, which keep the water like a sponge. Such is the case to a limited extent in parts of San Bernardino county. At Redlands, for instance, much less irrigation is now used than when the vines were first planted, and this fact is attributed to a spongy subsoil peculiarly retentive of water. A similar subirrigation exists in the Mussel Slough country, where the water rapidly fills the land and keeps it moist throughout the summer. The phenomenal moisture of the El Cajon land is probably also produced by some kind of subirrigation, either on impervious or through retentive strata; the waste water from the surrounding hills no doubt supplies much of the water appearing in the lower lands of the valley. Other valleys close by do not show this moisture, the underlying strata probably making subirrigation impossible with the present amount of rainfall.
Seepage.
—Seepage is the quality of the soil to attract moisture and retain it. Seepage soils attract the water from a ditch run through the land, the water spreading all through the soil towards all sides instead of sinking only vertically down. There is a distinct difference between such seepage soils and those that do not seep, although there is a gradation in the degree of the seepage, some soils seeping more than others. Thus the Fresno soil, or the soils on the Fresno plains, especially the red and sandy soils, do not seep or percolate. Vines planted on the sides of the ditches, or a foot or two from their banks, will die and dry up if not specially irrigated by bringing water to their very roots. Other soils, especially the river-bottom soils or the alluvial soils, seep or percolate in a great or less degree. They act like a sponge, attract water and give it away slowly, and the soil will be found wet for long distances from the ditch. This seepage capacity of the soil is partly caused by an abundance of humus or vegetable matter. The seepage capacity of the soil greatly increases by admixtures of green vegetable matters through the plowing under of green crops, such as alfalfa, peas, beans, grain, etc. The value of seepage soils is seen especially where some uneven ground is scraped off and the top soil removed to low places. Such ground often becomes useless for years afterwards, especially if the quantity of humus in the lower soils is small. Frequent irrigations will not serve to keep such soils moist, as the water sinks rapidly down, leaving the poor top soil dry. Vines planted in such places never do well, and even heavy manuring will not suffice to bring on a strong, healthy growth. Such humus-wanting soils must be treated with green crops, as stated above, in order to become fertile and moist. Thus seepage and subirrigation are often confounded. The former is caused by the retentive and communicative quality of the soil, while the latter is caused by the natural or artificial distribution of the underground water.
Subirrigation and seepage combined make the most perfect irrigation for a raisin vineyard, with advantages that can in no other way be attained: absence of distribution ditches, which take up valuable land and which cost money to keep clean from weeds; less growth of weeds on the surface of the ground; greater mellowness of the top soil and less work in plowing generally; a greater and more uniform supply of moisture, which, instead of being near the top of the ground, is accumulated deeper down, thus causing the roots to go down instead of spreading near the surface; no expensive irrigation, which will require plowing every time after the water is spread on the surface; a greater coolness of the ground and a lower temperature generally, which shows itself in a more vigorous growth of the vines, a greater supply of grapes and less danger from sunburn. These and many more are the advantages of subirrigation and seepage combined. To attain them in a raisin vineyard, no labor and reasonable cost should be spared.
Raisin-grape Picking at Riverside.
Drainage.
—Drainage is necessary in all vineyards where large quantities of water are used for irrigation, and principally at the very time that subirrigation begins. Thus in Fresno county the best raisin vineyards are those in which the land is both subirrigated and drained. When irrigation commences in any certain district, no one thinks of drainage as a possibility, and great carelessness is shown in locating ditches and other irrigation works. But in a few years, when the soil is full of water which finds no outlet, drainage becomes both necessary and desirable. A very successful and highly necessary drain has been constructed through a part of the Fresno district, which so filled up with water during the rainy winter of 1883-84 that much of the ground could not be plowed until late in the spring. The drain remedied the evil and drained the soil, and the vineyards grown there are now counted among the best and most profitable. The water thus drained off lowered the water level from six to seven feet. In very dry seasons this ditch is filled with water, and serves then to keep the soil moist through seepage or subirrigation.