tendency to excessive irrigation. This gives not only low duties, but results in over-saturation of the soil and has rendered large areas of the lower-lying lands in irrigated regions unfit for cultivation by flooding them with seepage water or by causing the rise of alkali. (See Alkali Soils.) For these reasons thorough drainage, either natural or artificial, is a necessary accompaniment of irrigation, as a protection against the harmful results of excessive irrigation.
Amount and Frequency of Irrigation. The conditions that must be taken into consideration in determining the amount of water to be applied are: (1) The storage capacity of the soil, (2) the depth to which the roots of the particular crop penetrate, (3) the rate at which water will rise from the soil below the root zone, and (4) the dryness of the soil and the subsoil. The frequency of irrigation will be determined by: (1) The amount of available moisture which the soil can store, (2) the rate at which moisture is lost by transpiration through the plant and by evaporation from the soil, and (3) the degree of dryness of the soil which the plant will tolerate without injury. Where the soil is deep and mellow the roots of plants extend to a great depth and over a wide area. Thus having a wider field from which to draw supplies of moisture and plant food, the actual percentage of moisture in the soil may be smaller without detriment to the plant than if the root-feeding was more restricted. Again, compact, clayey soils hold moisture so tenaciously that plants growing on them begin to suffer for moisture, even when the soil contains a percentage of water which in case of less tenacious, sandy soil would be abundant for the plant's needs. The aim in irrigating should be to apply simply enough water to meet the needs of the plant without loss in the drainage. It is well to bear in mind in attempting to accomplish this desired result that plants vary in their water requirements at different stages of growth. Edmond Gain reports investigations which indicate that at the time of planting the soil should have about 25 per cent. of the total amount of water which it is capable of holding, then it should fall to 15 per cent. and remain at this point until the first leaves are formed, when it should be raised quickly to nearly 40 per cent. It should be allowed to fall rapidly to about 25 per cent. and remain at this point until shortly before flowering, when it may be raised gradually to 40 per cent, and then allowed to fall rapidly to 12 or 15 per cent., where it remains during fruiting and maturity. King has found that a crop of maize yielding 70 bushels per acre can be brought to maturity in 110 days with 11.75 acre-inches of water, applied in 3 irrigations at intervals of 37 days on soil of medium texture, or in 5 irrigations at intervals of 22 days on the most open soil. With higher yields the number of irrigations has to be correspondingly increased. A crop of wheat yielding 40 bushels per acre requires 12 acre-inches of water, applied in 3 or 5 irrigations according as the texture of the soil is medium or very coarse. Barley yielding 60 bushels per acre may be brought to maturity in 88 days with 12.84 acre-inches of water applied in 3 or 5 irrigations, at intervals of 29 and 18 days, on medium and coarse soils respectively. Actual practice varies widely in different parts of the world. Three to five irrigations seems to be about the average for wheat. With maize it varies from 3 in Italy to 15 in Egypt, but 5 to 7 irrigations appears to be about the average. It is usual to give only one irrigation for each crop of clovers and alfalfa. Water meadows are irrigated as often as the water-supply will permit. The practice with potatoes is to give 2 to 4 irrigations, according to the slope and texture of the soil, beginning when the plants have nearly or quite reached the blossoming stage. In actual practice the intervals between irrigations of fruit-trees and vineyards vary from 7 to 40 days. According to Wickson, fruits in California receive 2 inches of water per month during May to August, on retentive soils, and 3 inches during the same period on coarse soils. In rice culture the land is kept flooded the greater portion of the time during the growth of the crop. According to Maxwell, it is a common practice in Hawaii to apply 200 to 250 acre-inches of water to sugar during a growing period of 18 to 20 months, although experiments have shown that 100 acre-inches is ample.
Fig. 13. MEASURING WEIR.
Fig. 14. FOOTE'S MEASURING WEIR AND SPILL-BOX.
Division and Measurement of Water.
Successful irrigation is very largely dependent upon
the judgment of the irrigator, and this in case
of an expert is probably as reliable as measurements,
in our present knowledge of the duty of
water. Measurements, however, are necessary
when many irrigators draw their water-supply
from the same source. In this case various
methods of division and measurements are used.
When the supply is small and the whole of it
can be used by each irrigator to advantage, water
is often distributed on the time basis, allowing
each user to have the whole stream a length of
time proportionate to the amount of water to
which he is entitled. By this method there is a
rotation in the use of water. When the supply
is too large to be used by a single individual,
various devices, called divisors, are used to
apportion to each user the proportion of water to
which he is entitled, or modules, measuring weirs,
and spill-boxes are employed to measure each
irrigator a definite quantity of water. This
division and measurement of water for irrigation
is controlled by law (see following page). The
right or privilege of using water from a canal, ditch,
or stream in definite quantity or upon a prescribed
area of land, is termed a water right, and such
- Vol. X— 51.
