CERTAIN IMPORTANT QUESTIONS RELATING TO IRRIGABLE LANDS.
THE UNIT OF WATER USED IN IRRIGATION.
The unit of water employed in mining as well as manufacturing
enterprises in the west is usually the inch, meaning thereby the amount
of water which will flow through an orifice one inch square. But in
practice this quantity is very indefinite, due to the “head” or amount
of pressure from above. In some districts this latter is taken at six
inches. Another source of uncertainty exists in the fact that increase
in the size of the orifice and increase in the amount of flow do not
progress in the same ratio. An orifice of one square inch will not
admit of a discharge one-tenth as great as an orifice of ten square
inches. An inch of water, therefore, is variable with the size of the
stream as well as with the head or pressure. For these reasons it
seemed better to take a more definite quantity of water, and for this
purpose the _second-foot_ has been adopted. By its use the volume of
a stream will be given by stating the number of cubic feet which the
stream will deliver per second.
THE QUANTITATIVE VALUE OF WATER IN IRRIGATION.
In general, throughout the Arid Region the extent of the irrigable land
is limited by the water supply; the arable lands are much greater than
the irrigable. Hence it becomes necessary, in determining the amount of
irrigable lands with reasonably approximate accuracy, to determine the
value of water in irrigation; that is, the amount of land which a given
amount of water will serve.
All questions of concrete or applied science are more or less complex
by reason of the multifarious conditions found in nature, and this is
eminently true of the problem we are now to solve, namely, how much
water must an acre of land receive by irrigation to render agriculture
thereon most successful; or, how much land will a given amount of water
adequately supply. This will be affected by the following general
conditions, namely, the amount of water that will be furnished by
rainfall, for if there is rainfall in the season of growing crops,
irrigation is necessary only to supply the deficiency; second, the
character of the soil and subsoil. If the conditions of soil are
unfavorable, the water supply may be speedily evaporated on the one
hand, or quickly lost by subterranean drainage on the other; but if
there be a soil permitting the proper permeation of water downward and
upward, and an impervious subsoil, the amount furnished by artificial
irrigation will be held in such a manner as to serve the soil bearing
crops to the greatest extent; and, lastly, there is a great difference
in the amount of water needed for different crops, some requiring less,
others more.
Under these heads come the general complicating conditions. In the
mountainous country the areal distribution of rainfall is preëminently
variable, as the currents of air which carry the water are deflected in
various ways by diverse topographic inequalities. The rainfall is also
exceedingly irregular, varying from year to year, and again from season
to season.
But in all these varying conditions of time and space there is one
fact which must control our conclusions in considering most of the
lands of the Arid Region, namely: any district of country which we may
be studying is liable for many seasons in a long series to be without
rainfall, when the whole supply must be received from irrigation.
Safety in agricultural operations will be secured by neglecting the
rainfall and considering only the supply of water to be furnished by
artificial methods; the less favorable seasons must be considered;
in the more favorable there will be a surplus. In general, this
statement applies throughout the Arid Region, but there are some
limited localities where a small amount of rainfall in the season of
growing crops seems to be constant from year to year. In such districts
irrigation will only be used to supply deficiencies.
The complicating conditions arising from soil and subsoil are many.
Experience has already shown that there are occasional conditions of
soil and subsoil so favorable that the water may be supplied before the
growing season, and the subsoil will hold it for weeks, or even months,
and gradually yield the moisture to the overlying soil by slow upward
percolation or capillary attraction during the season when growing
crops require its fertilizing effect. When such conditions of soil and
subsoil obtain, the construction of reservoirs is unnecessary, and the
whole annual supply of the streams may be utilized. On the other hand,
there are extremely pervious soils underlaid by sands and gravels,
which speedily carry away the water by a natural under drainage. Here
a maximum supply by irrigation is necessary, as the soils must be kept
moist by frequent flowing. Under such conditions the amount of water to
be supplied is many fold greater than under the conditions previously
mentioned, and between these extremes almost infinite variety prevails.
Practical agriculture by irrigation has also demonstrated the fact
that the wants of different crops are exceedingly variable, some
requiring many fold the amount of others. This is due in part to the
length of time necessary to the maturing of the crops, in part to the
amount of constant moisture necessary to their successful growth. But
by excluding the variability due to rainfall, and considering only
that due to differences of soils and crops, and by taking advantage
of a wide experience, a general average may be obtained of sufficient
accuracy for the purposes here in view.
In examining the literature of this subject it was found that the
experience in other countries could not be used as a guide in
considering our problems. In general, irrigation in Europe and Asia
is practiced only to supply deficiencies, and the crops there raised
are only in part the same as with us, and the variation on account of
the crops is very great. Certain statements of Marsh in his “Man and
Nature” have been copied into the journals and reports published in
the United States, and made to do duty on many occasions; but these
statements are rather misleading, as the experience of farmers in the
Arid Region has abundantly demonstrated. The writers who have used
them have in general overestimated the quantitative value of water in
irrigation. The facts in Italy, in Spain, in Grenada, and India are
valuable severally for discussion in the countries named, but must be
used in a discussion of the arid lands of the United States with much
care. It seemed better, under these circumstances, to determine the
quantitative value of water in irrigation in Utah from the experience
of the farmers of Utah. Irrigation has there been practiced for about
thirty years, and gradually during that time the area of land thus
redeemed has been increased, until at present about 325,000 acres
of land are under cultivation. A great variety of crops have been
cultivated--corn, wheat, oats, rye, garden vegetables, orchard trees,
fruits, vines, etc., etc.; and even the fig tree and sugar cane are
there raised.
During the past six or seven years I have from time to time, as
occasion was afforded, directed my attention to this problem, but being
exceedingly complex, a very wide range of facts must be considered in
order to obtain a reasonably approximate average. During the past year
the task of more thoroughly investigating this subject was delegated
to Mr. Gilbert. The results of his studies appear in a foregoing
chapter, written by him; but it may be stated here that he has reached
the conclusion that a continuous flow of one cubic foot of water
per second, _i. e._, a _second-foot_ of water, will, in most of the
lands of Utah, serve about 100 acres for the general average of crops
cultivated in that country; but to secure that amount of service from
the water very careful and economic methods of irrigation must be
practiced. At present, there are few instances where such economic
methods are used. In general, there is a great wastage, due to badly
constructed canals, from which the water either percolates away or
breaks away from time to time; due, also, to too rapid flow, and also
to an excessive use of the water, as there is a tendency among the
farmers to irrigate too frequently and too copiously, errors corrected
only by long experience.
The studies of Mr. Gilbert, under the circumstances, were quite
thorough, and his conclusions accord with my own, derived from a more
desultory but longer study of the subject.
AREA OF IRRIGABLE LAND SOMETIMES NOT LIMITED BY WATER SUPPLY.
While, as a general fact, the area of arable land is greater than the
area of irrigable land, by reason of the insufficient supply of water,
yet in considering limited tracts it may often be found that the supply
of water is so great that only a part of it can be used thereon. In
such cases the area of irrigable land is limited by the extent to which
the water can be used by proper engineering skill. This is true in
considering some portions of Utah, where the waters of the Green and
Colorado cannot all be used within that territory. Eventually these
surplus waters will be used in southern California.
METHOD OF DETERMINING THE SUPPLY OF WATER.
To determine the amount of irrigable land in Utah, it was necessary to
consider the supply; that is, to determine the amount of water flowing
in the several streams. Again, this quantity is variable in each stream
from season to season and from year to year. The irrigable season is
but a small portion of the year. To utilize the entire annual discharge
of the water, it would be necessary to hold the surplus flowing in the
non-growing season in reservoirs, and even by this method the whole
amount could not be utilized, as a great quantity would be lost by
evaporation. As the utilization of the water by reservoirs will be to
a great extent postponed for many years, the question of immediate
practical importance is resolved into a consideration of the amount of
water that the streams will afford during the irrigating season. But
in the earlier part of the season the flow in most of the streams in
this western region is great, and it steadily diminishes to the end of
the summer. Earlier in the season there is more water, while for the
average of crops the greater amount is needed later.
The practical capacity of a stream will then be determined by its flow
at the time when that is least in comparison with the demands of the
growing crops. This will be called the critical period, and the volume
of water of the critical period will determine the capacity of the
stream. The critical period will vary in different parts of the region
from the latter part of June until the first part of August. For the
purposes of this discussion it was only necessary to determine the
flow of the water during the critical period. This has been done by
very simple methods. Usually in each case a section of the stream has
been selected having the least possible variation of outline and flow.
A cross-section of the stream has been measured, and the velocity of
flow determined. With these factors the capacity of the streams has
been obtained. In some cases single measurements have been made; in
others several at different seasons, rarely in different years. The
determination of the available volume of the several streams by such
methods is necessarily uncertain, especially from the fact that it has
not always been possible to gauge the streams exactly at the critical
period; and, again, the flow in one season may differ materially from
that in another. But as the capacity of a stream should never be rated
by its volume in seasons of abundant flow, we have endeavored as far
as possible to determine the capacity of the streams in low water
years. Altogether the amount of water in the several streams has been
determined crudely, and at best the data given must be considered
tolerable approximations. In considering the several streams experience
may hereafter discover many errors, but as the number of determinations
is great, the average may be considered good.
METHODS OF DETERMINING THE EXTENT OF IRRIGABLE LAND UNLIMITED BY WATER
SUPPLY.
In the few cases where the water supply is more than sufficient to
serve the arable lands, the character of the problem is entirely
changed, and it becomes necessary then to determine the area to which
the waters can be carried. These problems are hypsometric; relative
altitudes are the governing conditions. The hypsometric methods
were barometric and angular; that is, from the barometric stations
vertical angles were taken and recorded to all the principal points
in the topography of the country; mercurial and aneroid barometers
were used, chiefly the former; the latter to a limited extent, for
subsidiary work. Angular measurements were made with gradientors to a
slight extent, but chiefly with the orograph, an instrument by which a
great multiplicity of angles are observed and recorded by mechanical
methods. This instrument was devised by Professor Thompson for the use
of the survey, and has been fully described in the reports on the
geographical operations. To run hypsometric lines with spirit levels
would have involved a great amount of labor and been exceedingly
expensive, and such a method was entirely impracticable with the means
at command, but the methods used give fairly approximate results, and
perhaps all that is necessary for the purposes to be subserved.
THE SELECTION OF IRRIGABLE LANDS.
From the fact that the area of arable lands greatly exceeds the
irrigable, or the amount which the waters of the streams will serve,
a wide choice in the selection of the latter is permitted. The
considerations affecting the choice are diverse, but fall readily
into two classes, viz: physical conditions and artificial conditions.
The mountains and high plateaus are the great aqueous condensers; the
mountains and high plateaus are also the reservoirs that hold the water
fed to the streams in the irrigating season, for the fountains from
which the rivers flow are the snow fields of the highlands. After the
streams leave the highlands they steadily diminish in volume, the loss
being due in part to direct evaporation, and in part to percolation
in the sands from which the waters are eventually evaporated. In like
manner irrigating canals starting near the mountains and running
far out into the valleys and plains rapidly diminish in the volume
of flowing water. Looking to the conservation of water, it is best
to select lands as high along the streams as possible. But this
consideration is directly opposed by considerations relating to
temperature; the higher the land the colder the climate. Where the
great majority of streams have their sources, agriculture is impossible
on account of prevailing summer frosts; the lower the altitude the more
genial the temperature; the lower the land the greater the variety of
crops which can be cultivated; and to the extent that the variety of
crops is multiplied the irrigating season is lengthened, until the
maximum is reached in low altitudes and low latitudes where two crops
can be raised annually on the same land. In the selection of lands,
as governed by these conditions, the higher lands will be avoided on
the one hand because of the rigor of the climate; if these conditions
alone governed, no settlement should be made in Utah above 6,500 feet
above the level of the sea, and in general still lower lands should be
used; on the other hand the irrigable lands should not be selected at
such a distance from the source of the stream as to be the occasion of
a great loss of water by direct and indirect evaporation. For general
climatic reasons, the lands should be selected as low as possible;
for economy of water as high as possible; and these conditions in the
main will cause the selections to be made along the middle courses of
the streams. But this general rule will be modified by minor physical
conditions relating to soil and slope--soils that will best conserve
the water will be selected, and land with the gentlest slopes will be
taken.
In general, the descent of the streams in the arid land is very great;
for this reason the flood plains are small, that is, the extent of
the lands adjacent to the streams which are subject to overflow at
high water is limited. In general, these flood-plain lands should not
be chosen for irrigation, from the fact that the irrigating canals
are liable to be destroyed during flood seasons. Where the plan of
irrigation includes the storage of the water of the non-growing season,
by which all the waters of the year are held under control, the
flood-plain lands can be used to advantage, from the fact that they
lie in such a way as to be easily irrigated and their soils possess
elements and conditions of great fertility.
Other locally controlling conditions are found in selecting the most
advantageous sites for the necessary water works.
These are the chief physical factors which enter into the problem, and
in general it will be solved by considering these factors only; but
occasionally artificial conditions will control.
The mining industries of the Arid Region are proportionately greater
than in the more humid country. Where valuable mines are discovered
towns spring up in their immediate vicinity, and they must be served
with water for domestic purposes and for garden culture. When possible,
agriculture will be practiced in the immediate vicinity for the purpose
of taking advantage of the local market. In like manner towns spring
up along the railroads, and agriculture will be carried on in their
vicinity. For this and like reasons the streams of the Arid Region will
often be used on lands where they cannot be made the most available
under physical conditions, and yet under such circumstances artificial
conditions must prevail.
In the indication of specific areas as irrigable on the accompanying
map of Utah, it must be considered that the selections made are
but tentative; the areas chosen are supposed to be, under all the
circumstances, the most available; but each community will settle
this problem for itself, and the circumstances which will control
any particular selection cannot be foretold. It is believed that the
selections made will be advantageous to the settler, by giving him the
opinions of men who have made the subject a study, and will save many
mistakes.
The history of this subject in Utah is very instructive. The greater
number of people in the territory who engage in agriculture are
organized into ecclesiastical bodies, trying the experiment of communal
institutions. In this way the communal towns are mobile. This mobility
is increased by the fact that the towns are usually laid out on
Government lands, and for a long time titles to the land in severalty
are not obtained by the people. It has been the custom of the church
to send a number of people, organized as a community, to a town site
on some stream to be used in the cultivation of the lands, and rarely
has the first selection made been final. Luxuriant vegetation has
often tempted the settlers to select lands at too great an altitude,
and many towns have been moved down stream. Sometimes selections have
been made too far away from the sources of the streams, and to increase
the supply of water, towns have been moved up stream. Sometimes lands
of too great slope have been chosen, and here the waters have rapidly
cut deep channels and destroyed the fields. Sometimes alkaline lands
are selected and abandoned, and sometimes excessively sandy lands have
caused a change to be made; but the question of the best sites for the
construction of works for controlling and distributing the water has
usually determined the selection of lands within restricted limits.
To a very slight extent indeed have artificial conditions controlled
in Utah; the several problems have generally been solved by the
consideration of physical facts.
INCREASE IN THE WATER SUPPLY.
Irrigation has been practiced in different portions of the Arid Region
for the last twenty-five or thirty years, and the area cultivated
by this means has been steadily increasing during that time. In
California and New Mexico irrigation has been practiced to a limited
extent for a much longer time at the several Catholic missions under
the old Spanish regime. In the history of the settlement of the several
districts an important fact has been uniformly observed--in the first
years of settlement the streams have steadily increased in volume.
This fact has been observed alike in California, Utah, Colorado, and
wherever irrigation has been practiced. As the chief development of
this industry has been within the last fifteen years, it has been a
fact especially observed during that time. An increase in the water
supply, so universal of late years, has led to many conjectures and
hypotheses as to its origin. It has generally been supposed to result
from increased rainfall, and this increased rainfall now from this, now
from that, condition of affairs. Many have attributed the change to the
laying of railroad tracks and construction of telegraph lines; others
to the cultivation of the soil, and not a few to the interposition of
Divine Providence in behalf of the Latter Day Saints.
If each physical cause was indeed a _vera causa_, their inability to
produce the results is quite manifest. A single railroad line has
been built across the Arid Region from east to west, and a short
north and south line has been constructed in Colorado, another in
Utah, and several in California. But an exceedingly small portion of
the country where increase of water supply has been noticed has been
reached by the railroads, and but a small fraction of one per cent. of
the lands of the Arid Region have been redeemed by irrigation. This
fully demonstrates their inadequacy. In what manner rainfall could be
affected through the cultivation of the land, building of railroads,
telegraph lines, etc., has not been shown. Of course such hypotheses
obtain credence because of a lack of information relating to the laws
which govern aqueous precipitation. The motions of the earth on its
axis and about the sun; the unequal heating of the atmosphere, which
decreases steadily from equator to poles; the great ocean currents
and air currents; the distribution of land and water over the earth;
the mountain systems--these are all grand conditions affecting the
distribution of rainfall. Many minor conditions also prevail in
topographic reliefs, and surfaces favorable to the absorption or
reflection of the sun’s heat, etc., etc., affecting in a slight degree
the general results. But the operations of man on the surface of the
earth are so trivial that the conditions which they produce are of
minute effect, and in presence of the grand effects of nature escape
discernment. Thus the alleged causes for the increase of rainfall fail.
The rain gauge records of the country have been made but for a brief
period, and the stations have been widely scattered, so that no very
definite conclusions can be drawn from them, but so far as they are of
value they fail to show any increase. But if it be true that increase
of the water supply is due to increase in precipitation, as many have
supposed, the fact is not cheering to the agriculturist of the Arid
Region. The permanent changes of nature are secular; any great sudden
change is ephemeral, and usually such changes go in cycles, and the
opposite or compensating conditions may reasonably be anticipated.
For the reasons so briefly stated, the question of the origin and
permanence of the increase of the water supply is one of prime
importance to the people of the country. If it is due to a temporary
increase of rainfall, or any briefly cyclic cause, we shall have to
expect a speedy return to extreme aridity, in which case a large
portion of the agricultural industries of the country now growing up
would be destroyed.
The increase is abundantly proved; it is a matter of universal
experience. The observations of the writer thereon have been widely
extended. Having examined as far as possible all the facts seeming
to bear on the subject, the theory of the increase of rainfall was
rejected, and another explanation more flattering to the future of
agriculture accepted.
The amount of water flowing in the streams is but a very small part of
that which falls from the heavens. The greater part of the rainfall
evaporates from the surfaces which immediately receive it. The
exceedingly dry atmosphere quickly reabsorbs the moisture occasionally
thrown down by a conjunction of favoring conditions. Any changes in
the surfaces which receive the precipitation favorable to the rapid
gathering of the rain into rills and brooks and creeks, while taking
to the streams but a small amount of that precipitated, will greatly
increase the volume of the streams themselves, because the water in the
streams bears so small a proportion to the amount discharged from the
clouds. The artificial changes wrought by man on the surface of the
earth appear to be adequate to the production of the observed effects.
The destruction of forests, which has been immense in this country for
the past fifteen years; the cropping of the grasses, and the treading
of the soil by cattle; the destruction of the beaver dams, causing a
drainage of the ponds; the clearing of drift wood from stream channels;
the draining of upland meadows, and many other slight modifications,
all conspire to increase the accumulation of water in the streams, and
all this is added to the supply of water to be used in irrigation.
Students of geology and physical geography have long been aware of
these facts. It is well known that, under the modifying influences
of man, the streams of any region redeemed from the wilderness are
changed in many important characteristics. In flood times their volumes
are excessively increased and their powers of destruction multiplied.
In seasons of drought, some streams that were perennial before man
modified the surface of the country become entirely dry; the smaller
navigable streams have their periods of navigation shortened, and the
great rivers run so low at times that navigation becomes more and
more difficult during dry seasons; in multiplied ways these effects
are demonstrated. While in the main the artificial changes wrought by
man on the surface are productive of bad results in humid regions,
the changes are chiefly advantageous to man in arid regions where
agriculture is dependent upon irrigation, for here the result is to
increase the supply of water. Mr. Gilbert, while engaged during the
past season in studying the lands of Utah, paid especial attention to
this subject, and in his chapter has more thoroughly discussed the
diverse special methods by which increase in the flow of the streams is
caused by the changes wrought by man upon the surface of the earth. His
statement of facts is clear, and his conclusions are deemed valid.