The New International Encyclopædia/Irrigation
IRRIGATION (Lat. irrigatio, from irrigare, to irrigate, from in, in + rigare, to moisten; connected with Goth, rign, AS. regn, OHG. regan, Ger. Regen, Eng. rain, and probably with Gk. βρέχειν, brechein, to wet). In agriculture, the method of increasing the productiveness of soils by an artificial supply of water. The practice of irrigation is very ancient. There is evidence to show that works for the storage and distribution of irrigation water were constructed in Egypt as early as B.C. 2000. Extensive works, intended for irrigation on a large scale, existed in times of remote antiquity also in Assyria, Mesopotamia, Persia, India, Ceylon, China, and other parts of the earth, as well as in Peru and Mexico in the Western Hemisphere. Remains of ancient irrigation works are found in the Southwestern United States (New Mexico and Arizona). In all of these regions irrigation is necessary for successful agriculture, because the rainfall is insufficient for the needs of ordinary crops. Irrigation is also required even in humid regions for crops, such as rice and cranberries, which require a large amount of water.
The area of the earth's surface over which the rainfall is deficient (less than 20 inches or 500 millimeters), and irrigation consequently a necessity for successful agriculture, is very extensive. In addition to this vast area in which, as a rule, agriculture is impossible without irrigation, there are considerable areas in the so-called humid regions in which the irregularity of the rainfall makes irrigation profitable. The extent to which the arid lands can be reclaimed depends upon the water-supply available for irrigation. It has been estimated that there is sufficient water to irrigate only about one-fifth of the arid region of the United States, or from 150,000,000 to 200,000,000 acres. Of this irrigable area probably less than 10,000,000 acres have already been reclaimed. In Europe irrigation prevails chiefly in the south, where it was introduced by the Romans. It is most extensively and systematically practiced in Lombardy. Spain, and the south of France, but exists to some extent in other parts of Europe. Wilson estimates the irrigated area in Italy to be about 3,070,000 acres, in Spain 500,000 acres, in France 400,000 acres. Nowhere is irrigation practiced on so large a scale as in India, and the irrigation systems of that country are being rapidly extended by the British Government. According to Wilson, the irrigated area in India is about 25,000,000 acres. Egypt follows with 6,000,000 acres, although irrigation works now in process of construction will vastly increase this area. Only at a comparatively recent date has irrigation been introduced into Australia, but it is rapidly extending there. The same is true in a measure of South Africa. The practice of irrigation has declined, or entirely disappeared, in many of those regions where it prevailed most extensively in remote antiquity.
Water-Supply for Irrigation. Water for irrigation is derived from (1) natural streams, springs and lakes; (2) wells; and (3) storage of storm waters. Occasionally also the sewage water of towns is used for irrigating purposes. The simplest and most common method of securing water for irrigation is to divert it from streams by means of a dam and a ditch or canal running along the sides of the valley of the stream, at a less grade than that of the stream itself. When the fall of the valley is great the canal can be readily led off to such a distance that extensive areas may thus be supplied with water. When the fall of the stream is slight, however, the canal may wind along close to the stream for miles before any extensive areas lie beneath the canal in a position to receive the water it carries.
It frequently happens that a diverting dam or one or more large storage reservoirs must be provided. In a number of instances submerged dams have been built across the valley of a stream whose waters disappear in summer. The practice is to build reservoirs and headworks generally at the highest suitable point consistent with good location for both these structures and the upper portions of the conduits leading from them.
Reservoirs are often increased by adding to the heights of the dam as the demand for water increases. The financial resources of most American irrigation works are likely to be limited at the start, would-be irrigators following rather than preceding the development of water-supply. On this account early works are often temporary in character. Again, both labor and material are apt to be so dear as to render cement masonry construction quite out of the question. Consequently many timber or timber and loose stone dams, have been built, and more ambitious structures of the rock fill type (see Dams and Reservoirs) have been erected. In some cases even flimsy brush dams, loaded with stone and earth, have been thrown across or partly across a stream, with the full knowledge that they would not last more than a year or two at best.
Artesian wells are a most important source of supply for irrigation in many western sections of the United States. In Kansas, Nebraska, and other States very fair supplies may be obtained from comparatively shallow wells by pumping. In California water is sometimes developed by tunneling into hillsides. Where pumps have been used they have generally been simple in construction and of small capacity. Large numbers of windmills are employed to drive small pumps on the Great Plains, mostly for domestic water-supply, but not infrequently for irrigation. Many of these are home-made, resembling the simplest form of paddle water-wheel, or are more elaborate, according to the mechanical ability or ambition of their makers, serviceable and durable windmills (q.v.) are now so cheap and so much more efficient than the home-made affairs that there is little excuse for not having one wherever it can be put to good use. Small storage reservoirs are a necessary adjunct of windmills, since there may be no wind when water is most needed, and since they also make it possible to save the night pumping. Such reservoirs may he constructed by intelligent farmers at comparatively little cost by throwing up earth embankments. In some cases a concrete or asphalt lining may be required to prevent leakage, but oftentimes the earth may be so packed, or puddled, by wetting and ramming it, as to make more expensive lining unnecessary. Where available, motor power is a cheap means of lifting water. Its applications to that end range all the way from a series of buckets mounted on a wheel placed in and driven by the stream to the most modern and efficient combination of pumps driven by turbines. Hydraulic rams are also employed.
In a comparatively few instances steam pumping engines are used, particularly where large quantities of water are to be lifted to a considerable height. This is true in California, Hawaii, and elsewhere, where large areas of land are irrigated by immense pumping plants. Gasoline engines are used to drive pumps. They require but little more attendance than a windmill, and have the great advantage of not being dependent upon the uncertainties of the wind. In America the quantity of irrigating water raised by pumps of all kinds is small, but in Europe, Africa, and Asia much pumping is done. In Egjpt and in some Asiatic countries, however, most of the pumping is very primitive, the power being applied by men or animals. Pumping is increasing of late with the development of water from wells and with the demand for water to irrigate land that cannot be reached from existing low-level canals. In many instances water can be secured in this way at less expense than by gravity, since it permits the utilization of near-by sources, thus avoiding long and costly canals. The water, too, is more directly under the control of the irrigator.
Assuming that a good supply of water for irrigation is available either by diversion from streams, by storage of storm waters, by pumping from wells, or from any other source, the question to be here discussed is the best means of utilizing it for the production of crops.
Fig. 1. EARTH CANAL UNLINED.
Fig. 2. CANAL IN EARTH LINED WITH MASONRY.
Methods of Applying Irrigation Water.
Main canals and conduits are often the most
expensive part of irrigation works, owing to
their length and the difficulties encountered in
their construction. The cheapest and simplest
conduit is a ditch, heading in the source of
supply, and departing just sufficiently from the
natural contour of the country to insure a flow
of water. In the early days of irrigation such
ditches were little more than single furrows, or
channels no larger than might be formed by a
plow, leading a short distance from the banks
of a stream. To-day there are thousands of
miles of irrigation ditches, or canals, large
enough for small boats, while in India it is
quite common to build combined irrigation and
navigation canals, thus affording a ready outlet
for the products of the irrigated area and
inlets for supplies. In rolling or hilly country
canals may have to follow circuitous routes to
maintain their level, thus adding greatly to their
length. It may be cheaper, or, when a stream
or valley is encountered, even necessary, to
continue the line of the canal, changing the
construction to an elevated flume, or else substituting
a pipe or inverted siphon, laid on or in the ground. Where grades and other conditions permit
canals should be made narrow and deep
rather than wide and shallow, in order to lessen
the surface exposed to evaporation.
Fig. 3. CANALS ON ROCK SLOPE WITH RETAINING WALLS.
Fig. 4. SEMI-CYLINDRICAL WOOD-STAVE FLUME.
Seepage should be guarded against. To this
end lining with stone laid in mortar or with
concrete or with cement mortar may be employed.
A great advantage in linings, if reasonably
smooth, is that they increase the carrying capacity
of the canals by lessening the friction, and
aid in maintaining it by lessening the
sedimentary
deposits and plant growths on their sides
and bottoms. Sometimes leakage may be
diminished by throwing powdered clay into the
water at the head of the canal. The sedimentary
matter naturally carried by the water will
often reduce the leakage in a few months or
years.
Fig. 5. WOOD IRRIGATION FLUME ON TRESTLE.
Flumes are most commonly built of wood,
with a rectangular cross-section, but in recent
years steel has been employed, particularly in
precipitous rocky locations, or where crossing
streams or deep ravines. The ordinary flumes
of boards or plank are subject to leakage. To
avoid this, and also to give a channel better
adapted to the flow of water, wooden staves are
now being employed, formed into a semicircular
or other shape designed to give a curved bottom.
The staves are held in place by steel rods
or bands, so arranged that they can be tightened
by turning nuts. The steel flumes are made of
thin plates riveted together. Flumes may rest
on mudsills, or timbers placed on the ground,
but, being generally designed to cross depressions,
they are more frequently supported on
trestles. The trestles, like the flumes, are
generally of wood, but they are sometimes of steel,
particularly where the flume proper is of that
material, or where the flume support must be
in spans, as at a stream crossing.
Fig. 6. STEEL IRRIGATION FLUME ON TRESTLE.
When, instead of valleys or streams, hills are
encountered, necessitating a long detour for
canals, tunnels are often employed. They do
not differ materially from other tunnels. If lining
is necessary, as in earth, or to reduce the
friction when in rock, it may be of brick,
concrete, or stone, and resembles that for masonry
aqueducts in tunnels. See Tunnels; Aqueducts.
Headworks of some kind are required for nearly all canals, flumes, or pipe-lines. In America they are generally of timber, but in much of the foreign work they are permanent structures of stone. The essential features are a bulkhead, gates, and wasteway. Where there is a dam at the head of the canal, the headworks may be at one end of it, or form a part of it.
Pipes may be substituted for canals or flumes, either to convey water across depressions or under streams, as already mentioned, or to prevent losses from both evaporation and seepage. Either riveted steel or wood staves are the materials most commonly used for such pipes, being preferable to cast iron on account of their relative lightness, and consequent ease of transportation in rough country, remote from railways. Where the water is under little or no pressure either vitrified clay or cement pipes are sometimes used, particularly in southern California.
Fig. 7. HEAD-GATE FOR SMALL IRRIGATION CANAL.
Works for Final Distribution consist chiefly of open or closed channels, generally the former, leading from the main or branch canal or other conduit to the land to be irrigated. For the most part small ditches are employed, with permanent or movable gates, or temporary earth dams, to divert the water to or from the minor channels. Except as modified by the topography, the application of water to land is chiefly a detail of agriculture rather than engineering, depending on the crop and the soil and also the ideas of the cultivator. Subsurface irrigation is generally considered impracticable because of the difficulties experienced in securing a thorough spreading of the water, besides which the pipes may clog and the construction prove expensive. Surface application, therefore, is almost universally employed. Broadly speaking, the latter is effected either by flooding the whole surface or sending the water through furrows. Neither involves much engineering skill, but it is best to have the main channels located with the aid of a level, particularly where the areas are large or have an irregular surface. Where pipe systems are employed for final distribution thin wrought iron or steel, or vitrified pipe may be used, and hydrants must be provided for drawing out the water.
Fig. 8. TAPPOONS (WOOD AND METAL) FOR USE IN IRRIGATION.
The distribution of water by means of underground pipes, standpipes, and hose is, however, too expensive for irrigation on a large scale. The open ditch, which takes the water out of the larger canal or reservoir to the land to be irrigated, is made to follow the contour of the land, so that the flow is moderate and uniform and the water can be readily distributed to lateral ditches or flumes at any desired point. The simplest method of turning water from a ditch is to cut a hole in the side and to use earth to make a dam in the ditch. An improvement on this method is the use of portable cloth, wood, or metal dams or ‘tappoons.’ The water is spread over the land by a variety of methods, which belong, as a rule, in three main classes: (1) Flooding, (2) furrow irrigation, and (3) sub-irrigation.
Fig. 9. IRRIGATION BY FLOODING.
Fig. 10. CHECKWORK IRRIGATION.
Wickson describes the following methods
practiced in the Western United States: (1) Free
flooding, or running water on the land without
restraint, except that of the banks of the
lateral conveying it. In this method the ditches
or laterals are carried along the higher parts of
the field and the water is released by spade cuts
at intervals in the banks, or it is made to overflow
the banks by means of dams, as described
above. This is the oldest and simplest method of
irrigation. It is best suited to small grains and
forage plants which are sown broadcast, and is
most effective with nearly level ditches and on
land of uniform grade. On account of the labor
involved and the difficulty of securing uniform
irrigation, this method has been superseded in
many places by one of the check systems. (2)
Flooding in contour checks or irregular areas
of land inclosed by low embankments, the size
and shape of these areas being determined by the
inequalities of the surface. This method is best
adapted to land of very gentle slope. Its first cost is considerable, but it permits more effective
irrigation with less labor than free flooding. In
this method the highest check is filled from the
ditch or lateral, and the water is either allowed
to overflow into the next lower check, or is drawn
off into it by means of gates provided for the
purpose. (3) Flooding in rectangular checks,
or level areas of approximately the same size
inclosed by low embankments. This method is
little used except for orchard, vineyard, garden,
and rice irrigation, being largely superseded for
other purposes by the contour check method.
Unless the land is quite level, its preparation for
the method involves the shifting of a large
amount of earth, and the levees of irregular
heights which are required interfere with the use
of power machinery in cultivating. (4)
Depressed bed method, in which the ditches are
carried on the tops of the levees and the water is
allowed to soak out into the checks inclosed by
the levees. This is a garden modification of the
rectangular check system, and is used in the
growing of vegetables and small fruits. It is
best suited to porous soils, which require
frequent irrigation. A primitive form of this
method is ridge irrigation, in which plants are
grown on the sides or at the base of raised
ditches. (5) Furrow irrigation, or running
water in furrows between the rows of crops, is
the simplest, cheapest, and most widely used
method of irrigating crops which can be grown
to advantage in rows, and is adapted to a wide
range of slope and soil conditions. If the slope
is not too great to carry a small stream without
excessive washing, the rows are run straight
down the grade from the supply ditch or flume,
which occupies the crest of the highest ground;
otherwise the rows are run diagonally at the
angle giving the proper grade. The length of
furrow that can be used depends upon the
character of the soil and the head of the water. The
more porous the soil, the larger should be the
stream or the shorter the furrow. For most
field and garden crops a larger stream and a
shorter run are used than for fruit-trees.
Laterals
or supply ditches are usually taken across
the slopes of the land at intervals of about forty
rods. The laterals should be as nearly level
as possible, so that they can be kept full and
will discharge uniform amounts of water through
the openings into the furrows. (6) Raised-bed
irrigation, in which a raised bed is surrounded
by a small ditch from which the water passes
into the soil by seepage and capillary action, is
a modification of the furrow system, especially
suited to rather heavy, retentive soils in which
water moves readily. (7) Subirrigation, or
distribution by means of underground pipes with
suitable outlets, or from tile drains or blind
ditches, from which the water can rise to the
roots of plants by capillarity. The method is
expensive and of doubtful practicability, except
on a limited scale, in greenhouse and other
horticultural work. A similar method, known as
‘underflow irrigation,’ consists in opening
furrows at considerable distances apart and keeping
them filled with water until the ground water
rises so that it can reach the roots of plants by
capillarity. The method is little used. (8)
Distribution by means of underground pipes,
standpipes, and connections for sprinkling is a method
which is considered too expensive for use on a
large scale.
Fig. 11. FURROW IRRIGATION.
Fig. 12. PLAN OF IRRIGATED FARM.
The choice of a method must be determined by
the amount of water available, the slope and
nature of the land, the character of the crop, etc.
According to Wickson, a method to be of value must secure the following results: “(1) Distribution of moisture evenly throughout the soil mass to as great a depth as possible, providing it does not sink beyond the reach of the plant by root extension nor beyond recovery by capillary rise; (2) economy of labor both in aggregate time and in the feasibility of operating without employment of extra hands; (3) economy of water in the prevention of waste by overflow or evaporation or by rapid percolation, and in placing the water where it will do the most good; (4) leaving the land in the best condition for attaining with least labor a state of tilth which conserves moisture and at the same time favors thrift in the plant.” Crops sown broadcast can be irrigated only by flooding or sprinkling. Flooding is also best adapted to very loose soils. The contour check and furrow method require least labor. The furrow method is best suited to subsequent cultivation by horse-power, which is a matter of great importance, since thorough cultivation, to overcome the compacting tendency of irrigation and to secure a soil mulch, should in all cases follow as soon after irrigation as the condition of the soil will permit. Deep and thorough preparation of the soil increases its storage capacity for water, and frequent cultivation of the surface reduces loss from evaporation, thus reducing materially the amount of irrigation required and enabling the soil to utilize to the best advantage whatever rainfall may occur.
Duty of Water in Irrigation. The amount of water required to irrigate different crops under varying conditions is a matter of fundamental importance, but has never been very accurately determined, and there is no very accurate simple means of judging when a soil needs irrigation. The term duty of water is commonly used to express the number of acres which a given quantity of water will adequately irrigate and is best stated in acre-feet or acre-inches, which are the amounts of water required to cover an acre 1 foot or 1 inch deep, respectively (43,560 and 3630 cubic feet, respectively). The water is usually measured to consumers in cubic feet per second, or second-feet. A second-foot is a flow of 1 cubic foot of water in a second of time. One second-foot will supply an acre-inch in 1 hour and 30 seconds, one acre-foot in 12 hours and 6 minutes. “In 24 hours, a stream of 1 second-foot would supply 23.8 acre-inches, and would cover 7.93 acres of land with water 3 inches deep.” (King.) A common unit of measurement of water in the Western United States is the miner's inch, which is the amount of water which will flow through a hole 1 inch square in 1 second of time under a certain pressure or head (which, theoretically, is 6 inches, but which varies in different States). In California 50 miner's inches are considered equal to 1 second-foot; in Colorado, 38.4.
The duty of water, reported on the basis of area actually irrigated with a given amount of water, varies so widely in different localities and conditions of water-supply that it is of little or no value as a measure of the water actually required in crop production, however useful it may be as a rough guide in estimating the required capacity of irrigation works. A more accurate measure is the actual amount of water required to produce the maximum yield of a crop under given soil and climatic conditions, and this, as already intimated, has not received the investigation its importance demands. King gives the following as the minimum amounts of water required to bring the crops named to maturity under conditions of absolutely no drainage and the smallest possible evaporation, assuming also that at the time of planting the soil already possesses a sufficient amount of moisture:
Highest Probable Duty of Water for Different Yields of Certain Crops
| NAME OF CROP | Bushels per acre | ||||||||||||||||||
| 15 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 100 | 200 | 300 | |||||||||
|
4.5 | 6 | 9 | 12 | 15 | 18 | |||||||||||||
| 3.21 | 4.28 | 6.42 | 8.56 | 10.7 | 12.84 | 14.98 | |||||||||||||
| 2.35 | 3.13 | 5.70 | 6.27 | 7.84 | 9.40 | 10.98 | 12.54 | 15.68 | |||||||||||
| 2.52 | 3.36 | 5.04 | 6.72 | 8.4 | 10.08 | 11.75 | 13.43 | 16.77 | |||||||||||
| .41 | .62 | .83 | 1.03 | 1.24 | 1.45 | 1.65 | 2.07 | 4.14 | 6.2 | ||||||||||
| NAME OF CROP | Tons per acre | |||||||||||||||||
| 1 | 2 | 3 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | 20 | |||||||
|
4.43 | 8.85 | 13.28 | 17.7 | 26.55 | 35.4 | 44.25 | |||||||||||
| 2.08 | 4.16 | 6.24 | 8.32 | 12.47 | 16.61 | 20.72 | 24.95 | 29.1 | 33.26 | 37.42 | 41.58 | |||||||
| 1.41 | 2.82 | 4.23 | 5.64 | 8.46 | 11.28 | 14.1 | 16.92 | 19.74 | 22.56 | 25.38 | 28.2 | |||||||
While the above figures give much greater duties than are secured in actual practice, and cannot therefore be taken as absolute guides, they will be helpful in estimating the possible duty of water. The duty of water in many cases is determined very largely by the water-supply. If water is scarce the duty will be high, if abundant the duty is likely to be low.
All general statements as to the duty of water must be received with caution, owing to the variation of duty with the local conditions already mentioned as governing the quantity of water required in irrigation. Wilson's Manual of Irrigation Engineering gives a table for various countries of the world, expressed in acres per second-foot. To put those figures on a more definite basis, a column headed “Inches per ten days” has been added in King's Irrigation and Drainage. The modified table is reprinted herewith. It should be noted that it is based on measurements at the head of the canals, and therefore includes losses by seepage and evaporation, as it properly should. The higher duties in southern California are due largely to the care taken to prevent these losses and to apply the water to the crops with a minimum of waste. Such care is more feasible in that section than elsewhere, on account of the intense cultivation there employed and the high values of the products obtained. Some more recent American figures of duties, expressed in acre-feet, are given in The Use of Water in Irrigation, Bulletin No. 86 of the United States Department of Agriculture, Office of Experiment Stations, being reports on investigations made under the direction of Elwood Mead. These records showed a range of from 2.10 acre-feet for the period from June 16 to September 16, 1899, at Bozeman, Mont., to 6.30 acre-feet from April to September, 1899, at Salt Lake City. The rainfall in each case was less than a half inch during the period named. Special measurements at other localities showed a range of from less than 1 to more than 15 acre-feet, but the conditions were abnormal. Where the duty was measured at the point of use, instead of at the head of main canals, it was found that more than as much water was lost in the canals as was available for application on the fields.
Amount of Water Used in Irrigation in Different Countries
(Adapted from Wilson, by King)
| NAME OF COUNTRY | No. of acres per sec. ft. |
No. of inches per 10 days |
| Northern India | 60 to 150 | 3.967 to 1.587 |
| Italy | 65 to 70 | 3.661 to 3.4 |
| Colorado | 80 to 120 | 2.975 to 1.983 |
| Utah | 60 to 120 | 3.967 to 1.983 |
| Montana | 80 to 100 | 2.975 to 2.38 |
| Wyoming | 70 to 90 | 3.4 to 2.644 |
| Idaho | 60 to 80 | 3.967 to 2.975 |
| New Mexico | 60 to 80 | 3.967 to 2.975 |
| Southern Arizona | 100 to 150 | 2.38 to 1.587 |
| San Joaquin Valley | 100 to 150 | 2.38 to 1.587 |
| Southern California | 150 to 300 | 1.587 to .793 |
The fact should not be lost sight of that there has been in the past and still exists a general 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 belowfollowing 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 right or privilege is usually acquired either by
priority of use or by purchase. In an arid region,
where so much depends upon the supply of water,
a water right is a very valuable property. See
Hydrography; Water-Supply.
Fig. 15. DIVISION BOX.
Quality of Irrigation Water. The character of the water available for irrigation purposes is a matter of great importance. All natural waters carry more or less organic and mineral matter in suspension or solution, and thus furnish, in addition to the water so necessary for the growth of plants, a certain amount of fertilizing matter. Waters carrying a large amount of soluble matter should, however, be used with caution, since they may cause the accumulation of alkali in the soil in injurious amounts. Of the waters available for irrigation, sewage is most valuable on account of the fertilizing matter carried to the land. Sewage irrigation is practiced with success in Great Britain and on the Continent of Europe, notably on the Craigenlinny meadows, near Edinburgh; at Gennevilliers, near Paris; in the vicinity of Berlin; at Milan, Italy; and also at a number of places in the United States.
Laws and Institutions. The development of irrigation in the Western United States has given rise to many complex legal, economic, and social problems. This has resulted from lack of uniform laws regarding ownership, control, and distribution of the water-supply and uncertainty as to State and Federal jurisdiction. These complications become acute, for example, when, under State laws, all the water of a stream is absorbed for irrigation purposes, and the Federal courts assert the paramount importance of riparian rights and the protection of navigation, regardless of the use of the water in irrigation. Again serious complications arise when a stream crosses the boundaries of two or more States and each claims all the water flowing on its soil, as well as when the sum of the individual claims largely exceeds the amount of water available.
There is a strong popular demand in the West for public aid in the construction of irrigation works too large for private capital and enterprise to undertake, especially for the building of reservoirs for the storage of water, as is done, for example, by England in Egypt and India, where the largest irrigation works in the world are being built by the British Government. Another matter of vital importance in connection with irrigation is the protection of the forest growth of the watersheds of the streams, with a view to mitigating floods and securing a more uniform flow of the streams. The importance of this matter is beginning to be realized, and steps are being taken to protect the forests of these watersheds from the wanton destruction to which they have been subjected in the past. The practice of irrigation has given rise to many interesting economic and social conditions. Among other things it has been productive of small proprietors and diversified and intensive farming; it furnishes admirable training in self-government and encourages coöperation; and has removed the evils of isolation in farm life by making possible the building of homes in village centres, thus realizing a happy combination of town and country life.
Irrigation in Humid Climates. Supplemental irrigation to carry crops through periods of drought has been found profitable in many cases in humid regions, especially on light, well-drained soils and with crops of high value. To what extent the practice may be extended with advantage has not been definitely determined. The rainfall of such regions is a disturbing factor. If a heavy rain occurs soon after a thorough irrigation, more serious damage may be done by excess of water than would have resulted from drought, especially if the soil be compact and poorly drained. Irrigation should therefore be attempted in such regions only on soils provided with thorough natural or artificial drainage.
Bibliography. The more important literature relating to irrigation includes publications of the United States Department of Agriculture on irrigation; water-supply and irrigation papers and other publications of the United States Geological Survey, Division of Hydrography; Report of a Senate Committee for Irrigation and Proclamation of Arid Lands (Washington, 1890); Special Reports to Congress on Irrigation in the United States (Washington, 1891); Hinton and others, Artesian and Underflow Investigations (Washington, 1892); Chittenden, Report to Congress on Reservoir Sites in Wyoming and Colorado (Washington, 1898); Newell, “Irrigation,” in the Eleventh and Twelfth United States Census Reports (Washington, 1890 and 1900); reports and bulletins on irrigation of the agricultural experiment stations in California, Colorado, Utah, and Wyoming, and the reports of the State engineers of these and other States in the arid regions; American Society of Irrigation, Engineers' Annuals; Mead, Irrigation Institutes (New York, 1903); Dennis, Reports on Irrigation and Canadian Irrigation Surveys (Ottawa, 1894-96); Deakin, Irrigation in Western America (Melbourne, 1885); Hall, Irrigation Development and Irrigation in California (Southern) (Sacramento, 1886-88); Brough, Irrigation in Utah (Baltimore, 1898); Flynn, Irrigation Canals and Other Irrigation Works (San Francisco, 1892); Buckley, Irrigation Works in India and Egypt (London, 1893); Willcocks, Egyptian Irrigation (London, 1899); Newell, Irrigation in the United States (New York, 1902); Schuyler, Reservoirs for Irrigation (New York, 1902); Ross, Notes on Distribution of Water of Upper Egypt (Cairo, 1882); Brunhes, L'irrigation dans la péninsule ibrique et dans l'Afrique du nord (Paris, 1902); Baird-Smith, Italian Irrigation (London, 1855); Moses, L'irrigation en Asie centrale (Paris, 1894); Parral, Les irrigations dans les bouches du Rhône (ib., 1876); id., Les irrigations de Vaucluse en 1877 (ib., 1878); Wilson, Manual of Irrigation Engineering (New York, 1895); Stephens, Practical Irrigator and Drainer (Edinburgh, 1884); Stewart, Irrigation for the Farm, Garden, and Orchard (New York, 1877); Scott, Irrigation and Water Supply (London, 1895); Ronna, Les irrigations (Paris, 1889-90); Salvador, Hydraulique agricole (ib., 1900); Llaurado, Tratado de aguas y riegos (Madrid, 1884); Wilcox, Irrigation Farming (New York, 1893); King, Irrigation and Drainage (New York, 1899); Smythe, Conquest of Arid America (New York, 1900); Kinney, Treatise on the Law of Irrigation (Washington, 1894); Long, Irrigation Law (Saint Paul, 1901). Among the periodicals devoted to irrigation are Irrigation Age (Chicago) and National Irrigation (San Francisco). Bibliographies of irrigation will be found in the annual report of the Colorado Experiment Station for 1891 (Fort Collins, Col.), and in the Eleventh Annual Report of the United States Geological Survey, 1889-90, part ii., and a bibliography is in course of preparation under the direction of the librarian of the United States Department of Agriculture.
See articles on Aqueduct; Canal; Dams and Reservoirs; Flume; Hydrography; Pumps and Pumping Machinery; Sewage Disposal; Tunnel; Water-Meter; Water-Supply.