WATER OF SOILS (_Continued_).
THE REGULATION AND CONSERVATION OF SOIL MOISTURE.
In view of the commanding importance of an adequate supply of water to
vegetation, the possible and available means of assuring such supply
by utilizing to the best advantage both rainfall and irrigation water,
require the closest consideration.
_Loosening of the Surface._—The first thing needful, of course, is
to allow the water free opportunity to soak into the soil, so as to
moisten the land as deeply as possible. That to this end the surface
should be kept loose and pervious by tillage, breaking up crusts
that may have been formed by the beating of rains, has already been
discussed. In the case of heavy clay soils, however, this alone is not
always sufficient. The most effectual way to loosen the land to greater
depths than can be reached by tillage, is by means of underdrains laid
at the greatest depth that is practically admissible.
_Effects of Underdrains._—That drain tiles laid for the express purpose
of carrying off surplus water should help to conserve soil moisture,
seems at first sight to be a paradox. Yet the explanation of the fact,
which has been demonstrated by long experience, is not difficult. The
effect is most striking in clay soils, for sandy soils are commonly
naturally underdrained already.
In discussing the changes of volume which soils undergo in wetting
and drying, the fundamental points in the premises have already
been mentioned (see chap. 7, p. 112). Clay soils in drying shrink
considerably, and re-expand on wetting, but rather slowly; moreover,
some clays crumble when wetted _after_ drying, while others, very
plastic when wet, crumble on drying (see chap. 7, p. 116).
It follows that while a clay subsoil when kept permanently wet, will
form a uniform, pasty, difficultly penetrable mass: when subjected
to frequent alternate wetting and drying, it becomes fissured and
crumbly, so as to resemble in its texture a tilled soil. This frequent
alternation of wetting and drying is precisely what, in the course of
time, is brought about by underdrains; rendering clay subsoils pervious
both to air and water. The consequence is that even heavy rains can be
fully absorbed by the soil mass lying above the drains, the surplus
draining off readily in a short time. Roots therefore can not only
penetrate, but exercise their vegetative functions perfectly at the
full depth of the drains. They are still at liberty to penetrate as
much deeper as their demands for moisture may require; but the depth
of four to four and a half feet is already so much greater than in the
humid region would usually be reached by them in undrained clay soils,
that commonly the moisture successively retained within that mass is
as much as is required by them during the growing season. At the same
time, their feeding roots are so far below the surface, that ordinary
short droughts do not reach them at all; while the underdrains prevent
any injurious stagnation of water around them. It need hardly be added
that the entire task of cultivation is also greatly facilitated; not
only because drained soils can be plowed within a few hours after the
cessation of rains, as against the same number of days that would have
to elapse in the undrained areas; but because tillage is easier, and
less draft is required, even when it is carried to a much greater depth.
_Underdrainage_, then, must be counted as being among the most
effective means both of utilizing the rainfall so as to prevent loss
from runoff and injury from washing, and of creating a deep, loose,
pervious soil mass, well adapted to root penetration as well as to
the conservation of moisture; rendering possible timely tillage and
cultivation, and early development of crops fully supplied with
moisture and therefore secure against loss from drought. The safety
and improvement of crops thus secured corresponds in the humid region
to that brought about by the command of irrigation water in the arid
countries. But it by no means follows that underdrainage can therefore
be dispensed with in the latter, or irrigation in the former. Both
have their proper place in both regions; but from special causes
underdrainage, as has already been stated, should be widely used in
irrigation countries to prevent the injuries otherwise but too likely
to arise from over-irrigation (see chap. 12, p. 231).
_Winter Irrigation._—In many regions where irrigation is desirable but
not absolutely necessary in ordinary seasons, or where irrigation water
is scarce in summer, much advantage is gained by insuring thorough
saturation of the land during the latter part of winter, especially
when spring or summer crops are to be sown. The not inconsiderable time
required for water to reach its permanent level or the country drainage
in most soils, often insures the retention of a certain surplus over
what the soil can permanently hold, within the period when it can be
utilized by growing crops; whose roots moreover are more likely to
penetrate deeply in land where there is a steady increase of moisture
as they descend, than when the contrary condition is encountered. The
use of _winter flood-waters to saturate the land_ is therefore in many
cases the saving clause for a dry season.
METHODS OF IRRIGATION.[89]
The manner in which irrigation water is supplied to land and especially
to growing crops exerts such a potent influence not only upon the
welfare of the plants but also upon the condition of the land, that a
brief discussion of this topic seems necessary.
The following methods are in use to a greater or less extent:
1. Surface sprinkling.
2. Flooding.
A. By lateral overflow from furrows or ditches.
B. By the “check” system.
3. Furrow irrigation.
4. Lateral seepage from ditches.
5. Basin irrigation.
6. Irrigation from underground pipes.
[89] Only a general outline of the principles of this subject is
given in this volume; special works must be consulted for working
details. Among these the volume by King on “Irrigation and Drainage”
gives probably the most comprehensive presentation of the subject for
both humid and arid climates. Also bulletins of the U. S. Dep’t of
Agriculture.
_Surface Sprinkling._—This method seems to be the closest imitation of
the natural rainfall; and yet it is in practice about the most wasteful
and least satisfactory of all. It is difficult of application on any
large scale, from obvious causes; on the small scale, in gardens and on
lawns, its disadvantages become amply apparent. As usually practiced,
from a rose spout or spray nozzle, the water falls much more abundantly
than in the case of any desirable rain, within the short time allowed
by the patience of the operator. If continued for a sufficient length
of time to soak the soil to the desirable depth, it compacts the
surface of the ground so as to render subsequent tillage indispensable.
To avoid this, amateur gardeners usually restrict the time of
application, repeating the same at frequent intervals, sometimes daily.
The result is that the very slight penetration of the water either
fails to reach the absorbent roots, so that it is of little use to
them, and is evaporated by the next day’s sun or wind; or else it tends
to draw the roots close to the surface, where, unless the application
of water is actually made daily, they are sure to suffer from the first
intermission of the daily dose. In actual practice the sprinkling
method is therefore both inefficient and wasteful of water, and exposes
the plants to grave injury from any cessation of the water supply.
Flooding presupposes land either level or only slightly sloping
naturally, or rendered so artificially; usually by means of the plow
and horse scraper.
_Flooding_ by lateral overflow from large furrows, or ditches, is very
commonly practiced where the water supply is abundant and large areas,
such as alfalfa or grain fields, are to be irrigated. The overflow is
regulated by portable check-boards, proceeding from the highest points
to the lowest, and leaving each temporary dike in place until the
ground is adequately soaked or the water reaches the next furrow below.
In heavy ground the operation may have to be repeated to insure proper
depth of percolation.
_Check flooding_ necessitates more careful leveling, and the throwing
up of small dikes, either temporary or permanent. The costliness
of the earth-work restricts the use of this method materially, and
the inconvenience caused in tillage by the dikes is objectionable,
especially in large-scale culture. For the case of alfalfa fields,
which remain permanently set for a number of years, it is however
the largely preferred method. In the case of field cultures, the
consolidation of the surface that follows flooding on the heavier soils
renders subsequent tillage necessary in all but very sandy soils; and
hence it should always _precede_ broadcast sowing.
One disadvantage of the surface flooding system is the slow penetration
of the water caused by the resistance of the air in the soil to
downward displacement; its buoyancy acting directly contrary to the
percolation of the water. In close-grained, heavy soils this objection
is very serious, on account of the loss of time involved when the
irrigator’s time is limited. On sandy lands the air bubbles up quite
livelily at first, but this soon ceases and the air is compelled to
escape sideways as best it can.
_Furrow Irrigation._—By this method it is intended to soak the land
uniformly by allowing the water to flow through furrows drawn 3 to 8
feet apart, with a gentle slope from the supply or head ditch; the
flow being continued until the water has reached the far end of the
furrows, or longer according to the nature of the soil, especially
if another ditch to receive the surplus flow lies below. The furrows
should subsequently be closed by means of the plow or cultivator; but
even if left open they are much less a source of waste by evaporation
than would be a flooded surface. The water thus, in the main, soaks
downward and only reaches the surface by capillary rise, so that the
land between the furrows is not sensibly compacted when the furrows
have been made deep enough. Evidently this is a much more rational
procedure than surface flooding, as it tends to leave most of the
surface in loose tilth, while penetrating to much greater advantage,
because of the ready escape of the air from the soil. It is the system
naturally and almost exclusively used in truck gardens and orchards,
and generally where crops are grown in drills or rows sufficiently far
apart to permit of cultivation.
[Illustration: _Furrows 6 inches deep in Heavy Loam Soil_]
[Illustration: _Narrow and Wide Furrows in Sandy Loam Soil_]
[Illustration: _Furrows 5 and 10 inches deep in Sandy Loam Soil_
_Water running, in each, seven hours_
FIG. 43.—Profiles of Water penetration in Furrow Irrigation.]
The figure annexed[90] shows the manner in which water sinks and
spreads from furrows of various depths and widths, as actually
observed in the work of the Irrigation Division of the U. S. Dep’t of
Agriculture, under the direct supervision of Prof. R. H. Loughridge of
the California Station. The mode of percolation is shown for two soils,
a heavy loam and a sandy one, both in the vicinity of Riverside, Cal.
[90] Published by permission of the Department.
The upper section shows the variation in penetration in one and the
same soil with the same kind of furrow, the broken line indicating the
cessation of the flow in the furrows; after which there was a still
farther penetration of the water to from 6 to 9 inches deeper.
The second section from above shows the percolation of the water
respectively in wide and narrow furrows of the same depth. It is
evident at a glance how much more effective is the wide furrow in
utilizing the limited time during which the irrigator usually has the
flow at his command.
The third section shows several practically important points in favor
of the wide and deep instead of narrow and shallow furrow. It is seen
that in doubling the width and depth, the penetration has also nearly
doubled. Moreover, it is seen that in the deep furrow the water has not
in the course of seven hours reached the surface at all, being still
six inches away; so that in view of the diminishing ratio of capillary
ascent, it probably would not have reached the edge of the furrow, at
the surface, in less than thirty hours. Thus all surface evaporation,
which oftentimes causes the loss of 50 % of the water entering the
shallow furrows, would be prevented; and a dry furrow-slice might
be turned into the furrow immediately after the cessation of the
water-flow, effectually obviating the need of subsequent tillage
also. The cost of the latter, together with the saving in water, and
increased efficiency of the water by deeper penetration, will much more
than offset the additional cost and trouble of plowing deeper furrows.
There is therefore every reason for doing away with the wasteful,
easy-going practice of irrigating in numerous shallow furrows, by which
the irrigator loses up to half of the water paid for, by evaporation;
is compelled to wait for the soaked surface to dry before being able
to turn back a furrow-slice into the furrows to prevent the drying-out
of their moisture; and by losing penetration of the water, is obliged
to irrigate again within a much shorter time than will be necessary if
deep-furrow irrigation be used.
A similar experiment with deep and shallow furrows was made at the
Southern California station near Pomona in 1901, as reported in
Bulletin 138 of the California Station. The results as far as they
went were precisely similar, and upon the basis of these the writer
earnestly advocated deep-furrow irrigation, and had the satisfaction
of seeing it strongly approved by orange-growers at Riverside and
elsewhere, by putting it into practice.
In addition to the saving and better utilization of the water used,
this mode of application has the advantage of preventing the roots from
coming too near the surface; it will also largely eliminate “irrigation
hardpan” or plowsole.
The results produced by long-continued shallow plowing and irrigation
in shallow furrows is well illustrated in the last of the irrigation
profiles, which shows the observations made on the same land as the
others, but where rational cultivation and deep-furrow irrigation
had not yet been introduced. It will be seen that after applying,
and of course paying for, the water for three days, its average
penetration was only about eighteen inches; so that the trees of the
orchard received very little benefit, and were supposed to be needing
fertilization when in fact they were simply suffering from lack of
water at the lower roots.
One somewhat unexpected point is shown by these diagrams, viz., the
slight sidewise penetration of the water; the wetted areas having a
nearly vertical lateral outline. This means, of course, that unless the
furrows run very near the trees of an orchard, the soil immediately
beneath the trees will remain dry; thus inducing the roots to spread
sideways and losing depth of penetration and soil. It will be noted
especially in the lower figure that here again the deep furrow offers
a material advantage over the shallow, the sidewise spread being much
more pronounced than in the shallow furrow alongside.
_Distance Between Furrows and Ditches._—The distance between the
furrows must, of course, be proportioned to the readiness with which
the water penetrates, being less as the land is of closer texture.
The distance between head ditches must, on the contrary, vary in
the opposite sense, since if these are too far apart, the water near
the head ditch will in sandy lands be wasting into the subdrainage
before the end of the furrows is reached; so that the distribution
will be very uneven. The great differences observed between crops, and
especially trees, _below_ and _above_ the head ditches, are mainly due
to this unevenness in water distribution, caused by too great distance
between successive head ditches. Each farmer must himself, however,
determine by actual trial the proper distances between ditches as well
as furrows, for his particular case; since everything depends upon the
rapidity with which water will penetrate the soil and subsoil. Actual
tests to determine this point[91] should be the first step, before
laying off the system of ditches as well as furrows. It not uncommonly
happens that the failure to do this at first, compels a subsequent
total change of arrangements in this respect. (See page 253 below).
[91] Such tests can be readily made by any one, by digging a pit to
four or five feet depth, and supplying water to a shallow basin dug
into the surface 8 to 12 inches distant from the vertical wall of the
pit. The descent of the water is then readily observed on the vertical
side of the pit nearest to the water basin. Preliminary tests with soil
probe (see chap. 10, p. 177).
Thus while in some very pervious land furrows may be six
or even eight feet apart, in other cases, in certain
finely pulverulent or silty soils such as the “dust soils”
described in a former chapter; (see chapter 6, p. 104),
furrows drawn three feet apart may fail to allow the water
to penetrate so as to prevent grain on the middle foot
from suffering from drought after the water has run for
twenty-four hours.
_Irrigation by lateral Seepage._—Is in reality a mere modification
of furrow irrigation, practiced in the case of lands very readily
permeable, and where water is abundant. The fields are laid off in
“lands” twelve to twenty-five feet wide, with a deep furrow or narrow
ditch between, from which water percolates in a short time so as to
overlap from the two sides. In this case sometimes the water does not
reach the surface visibly at all; a very great advantage where alkali
exists, as surface evaporation, and the consequent accumulation of
alkali, is thus effectually prevented; while deep-rooting is favored to
the utmost.
_Basin Irrigation._—In this method of irrigation, practiced only in
the case of trees and sometimes vines, and when water is scarce, a
wide circular furrow or basin is excavated around each trunk and water
is run either from one to the other, or sideways from a furrow laid
along the rows. The water thus applied of course percolates immediately
around the trunk first, and in practice is found to follow also the
large roots; so that it goes precisely where it is most wanted, besides
forming a vertical body of moist soil reaching to considerable depth,
where it is most desirable that the root system should follow. By
this deep penetration to natural moisture in the depths of the soil,
comparatively small quantities of water produce very marked effects.
On the same principle, the grape vines which bear some of the choicest
raisins of Malaga on the arid coastward slopes, are made to supply
themselves with moisture, without irrigation, by opening around them
large, funnel-shaped pits, which remain open in winter so as to catch
the rain, causing it to penetrate downward along the taproot of the
vine, in clay shale quite similar to that of the California Coast
Ranges, and like the latter almost vertically on edge. Yet on these
same slopes scarcely any natural vegetation now finds a foothold.
Similarly the “ryats” of parts of India water their crops by applying
to each plant immediately around the stem such scanty measure of the
precious fluid as they have taken from wells, often of considerable
depth, which form their only source of water-supply. Perhaps in
imitation of these, an industrious farmer has practiced a similar
system on the high benches of Kern River, California, and has
successfully grown excellent fruit for years, on land that would
originally grow nothing but cactus. Sub-irrigation from pipes has been
applied in a similar manner.
A combination of the furrow- and basin-irrigation system is sometimes
practiced in southern California by drawing the furrow so as to bring
the tree within a square, one side of which is left closed. The same
result may be accomplished by plowing cross furrows at right angles
near the tree and then placing check-boards so as to force the water
along the rows, zigzagging, on three sides.
The basin irrigation of orchards was originally largely practiced in
California, but has now been mostly abandoned for furrow irrigation.
The latter has been adopted partly because it requires a great deal
less hand-labor, partly under the impression that the whole of the soil
of the orchard is thus most thoroughly utilized; partly also because of
the injurious effect upon trees produced at times by basin irrigation.
The explanation of such injurious effects is, essentially, that _cold_
irrigation water depresses too much the temperature of the earth
immediately around the roots, and thus hinders active vegetation to
an injurious extent, sometimes so as to bring about the dropping of
the fruit. This of course is a very serious objection, to obviate
which it might be necessary to reservoir the water so as to allow it
to warm before being applied to the trees.[92] In furrow-irrigation
the amount of soil soaked with the water is so great that the latter
is soon effectually warmed up, besides not coming in contact too
intimately with the main roots of the tree; along which the water
soaks very readily when applied to the trunk, thus affecting their
temperature much more directly. It is for the farmer to determine which
consideration should prevail in a given case. If the water-supply be
scant and warm, the most effectual use that can be made of it is to
apply it immediately around the tree, in a circular trench dug for the
purpose. When on the contrary, irrigation water is abundant and its
temperature low, it may be preferable to practice furrow irrigation, or
possibly even flooding.
[92] See below, chap. 17.
As to the supposed more complete use of the soil under the latter
two methods, it must be remembered that while this is the case in
a _horizontal_ direction, if irrigation is practiced too copiously
under the shallow-furrow system, it may easily happen that the gain
made horizontally is more than offset by a corresponding loss in the
_vertical_ penetration of the root-system. This is amply apparent in
some of the irrigated orange groves of southern California, where the
fine roots of the trees fill the surface soil as do the roots of maize
in a corn field of the Mississippi States; so that the plow can hardly
be run without turning them up and under. In these same orchards it
will often be observed, in digging down, that at a depth of a few feet
the soil is too water-soaked to permit of the proper exercise of the
root-functions, and that the roots existing there are either inactive
or diseased. That in such cases frequent irrigation and abundant
fertilization alone can maintain an orchard in bearing condition, is
a matter of course; and there can be no question that a great deal of
the constant cry for the fertilization of orchards in the irrigated
sections is due quite as much to the shallowness of rooting induced by
over-irrigation, as to any really necessary exhaustion of the land.
When the roots are induced to come to and remain at the surface,
within a surface layer of eighteen to twenty inches, it naturally
becomes necessary to feed these roots abundantly, both with moisture
and with plant-food. This has, as naturally, led to an overestimate of
the requirements of the trees in both respects. Had deep-rooting been
encouraged at first in the deep soils of the southern “citrus belt,”
instead of over-stimulating the growth by surface fertilization and
frequent irrigation, some delay in bearing would have been compensated
for by less of current outlay for fertilizers, and less liability
to injury from frequently unavoidable delay, or from inadequacy, of
irrigation.
_Irrigation by Underground Pipes._—Where economy in the use of
irrigation water is a pressing requirement, its distribution through
underground pipes affords the surest mode of accomplishing that end,
in connection with the application of the water in accordance with the
principles just discussed. The enormous saving of water effected by
its conveyance in cement-lined ditches or concrete pipes, as compared
with earth ditches, if additionally combined with its application to
individual trees or vines, presents the maximum of economy that can
be effected. The actual use of this method is unfortunately limited
in practice by the high first cost of piping; but as its use renders
unnecessary the digging of basins and plowing of furrows and their
subsequent closing-up, it is when once established by far the cheapest
system, both as to the use of water and of labor.
The best results of this system are undoubtedly achieved
by the use of iron pipes for the distribution in field and
orchard, whatever may be the material used for the main
conduits. The use of concrete and tile in small sizes proves
in the end very expensive, because of frequent breakage,
and leakage due to varying pressure in the supply pipes or
reservoirs; as well as from even slight earthquake tremors,
undermining by water or by the burrowing of animals, and
many other accidents which do not affect an iron pipe
system. The pipes must in any case, of course, be laid
deep enough to be out of reach of the deepest tillage;
therefore not less than one foot, and preferably eighteen
inches. A proper construction of the outlets, permitting of
exact regulation of the flow and ready operation from above
ground, as well as preventing their being clogged by earth,
rust, roots or burrowing animals, insects etc., is of course
of the greatest importance. A variety of devices for this
purpose is already on the market.
QUALITY OF THE IRRIGATION WATER.
_Saline Waters._—Considering the large amount of water annually used
in irrigation, among the most needful precautions to be observed by
the irrigator is in the testing of the quality of his water-supply.
First among the points to be noted is the possible content of
soluble “alkali” salts. While in most cases what is called the “rise
of the alkali” is due to the salts already contained in the soil
and subsoil, in but too many the evil is either brought about, or
greatly aggravated, by the excessive saline contents of the water
used in irrigation. The effects of the use of saline irrigation water
(containing in this case about 100 grains per gallon, or 1700 parts
per million) are shown in the accompanying plate. The predominant
ingredients of these alkali salts were common salt and carbonate of
soda. In the lands near Corona, Cal., where this case was observed, the
original alkali-content of the soil was about 2500 pounds per acre in
four feet depth, and had been just quadrupled, with the results shown;
viz., complete defoliation of the orange trees, while on the same
land, where the trees had been irrigated with good artesian water, the
orchard was in fine condition.
_Limits of Salinity._—It is not easy to assign a definite limit of
mineral content beyond which water should be considered unfit for
irrigation purposes; partly because of the differences in the kind of
the mineral salts, partly because the nature of the soil and the amount
of water at command, materially influence its availability.
[Illustration: FIG. 44.—Orange Trees Irrigated with Artesian Water.]
[Illustration: FIG. 45.—Lake Elsinore Water, Three Years.]
Forty grains per gallon is usually assigned as the limit for potable
as well as irrigation waters. But if most or the whole of such mineral
contents should consist of the carbonates and sulfates of lime and
magnesia, the water while unsuitable for domestic use may be perfectly
available for irrigation, since these salts are either beneficial or
harmless in the amounts likely to be introduced by the water. But
if most or the whole of such forty grains should consist of “alkali
salts” proper, viz., the sulfates, chlorids and carbonates of potash
and soda, or if they should contain even small amounts of the chlorid
of magnesium, they might render the water either wholly unsuitable for
irrigation, or if used it would be needful to take the mineral content
into consideration, by regulating its application accordingly.
It has been found in California that practically the upper limit of
mineral content for irrigation water _under the ordinary practice_ lies
below seventy grains per gallon in _all_ cases; for when this strength
is reached, even though such water may bathe the roots of almost any
plant with impunity, yet accidental concentration by evaporation is
so certain to happen, that injury to crops is practically almost
unavoidable.
In South Dakota and other parts of the American semi-arid
region, waters containing seventy grains and even more of
alkali salts per gallon are annually used during the short
irrigation season. This can be done harmlessly because the
aggregate amount used is only small, and the more abundant
rainfall of that region annually washes the salts out of the
soil. But where almost the full amount of water required
by crops must be supplied by irrigation, the total amount
of salts thus introduced would speedily render the land
uncultivable.
According to the observations of Means and other explorers[93] of the
U. S. Dep’t of Agriculture, waters of much higher mineral content are
used for irrigation both in Egypt and in the Saharan region, some
going as high as 8000 parts per million, or 214 grains per gallon. The
cultivators are said to be very skilful in the use of these waters,
applying them only to plants of known resistance, and in certain ways.
These ways include doubtless a good deal more time and patience than
American irrigators are ordinarily willing to bestow upon their work.
Much depends of course not only upon the character of the salts in the
water, but also upon the long experience had in the old irrigation
regions.
[93] Bull. No. 21, Bureau of Soils; also circular No. 10, ibid.
_Mode of using Saline Irrigation Waters._—The fact that abundant
growths of native as well as cultivated plants may sometimes be seen
on the margins of “alkali lakes” where water of over a hundred grains
of mineral salts per gallon continuously bathes the roots, while the
same plants perish at some distance from the water’s edge, points the
way to the utilization, in emergencies, of fairly strong saline waters;
viz., by the prevention of their concentration to the point of injury
_by evaporation_. It is clear that when such waters are used sparingly,
so as to penetrate but a few feet underground, whence the moisture
re-ascends for evaporation at the surface, a few repetitions of its
use will accumulate so much alkali near the surface as to bring about
serious injury. If, on the other hand, the water is used so abundantly
that the roots may be considered as being, like the marginal vegetation
of alkali lakes, bathed only by water of moderate strength, no such
injury need occur; and what does accumulate in consequence of the
inevitable measure of evaporation occurring in the course of a season,
may be washed out of the land by _copious winter irrigation_.
This, of course, presupposes that the land, as is mostly the case
in the arid region, is readily drained downwards when a sufficiency
of water is used. When this is not the case, _e. g._, in clay or
adobe soils, or in those underlaid by hardpan, waters which in sandy
lands could have been used with impunity, may become inapplicable to
irrigation use.
_Apparent Paradox._—The prescription to use saline waters _more
abundantly_ than purer ones, in order to avoid injury from alkali,
though paradoxical at first sight, is therefore plainly justified by
common sense as well as by experience, in pervious (sandy) soils;
while in difficultly permeable ones, their use may be either wholly
impracticable, or subject to very close limitation.
Sometimes the alternate use of pure and salt-charged water serves to
eke out a too scant supply of the former. But in all such cases, close
attention to the _measure_ of water that will wet the soil to a certain
depth, and “eternal vigilance” with respect to the accumulation of
alkali near the surface, must be the price of immunity from injury. In
all cases the farmer should know how much of alkali salts he introduces
into his land with the irrigation water, and watch that it does not
approach too closely, or exceed, the tolerance of his crops for alkali
salts, as given in chapter 26.
_Use of Drainage Waters for Irrigation._—When lands charged with alkali
salts are being reclaimed by drainage, the question sometimes arises
whether the drainage-water may not be used for irrigation, lower down.
This of course depends entirely upon the amount of alkali in the water,
the nature of the lands to be irrigated, and the manner of applying it.
In the Fresno drainage-district of California it has been shown that
some of the drainage-water contains not more than 25 to 30 grains per
gallon of objectionable salts, and such waters could of course be used
on pervious lands with the precautions above noted.
_“Black Alkali” Waters._—As regards, however, waters containing any
large proportion of carbonate of soda, it must be remembered that
even very dilute solutions of salsoda serve to puddle the soil and
thus render it difficultly tillable. When such waters are used it
is necessary to forestall injury either by the use of gypsum in the
reservoir or ditch, or by annually using on the land a sufficient
amount of gypsum to transform the carbonate of soda into the relatively
innocuous sulfate.
_Variations in the Saline Contents of Irrigation Waters._—When
irrigation waters are derived from deep wells, there is little if
any variation of their saline contents to be expected, and a single
analysis will serve permanently. But in the case of relatively
shallow wells, from which the water must be raised by pumping, it not
unfrequently happens that after a series of seasons of short rainfall,
saline waters are brought up by the pump and may seriously injure
crops and orchards. Again, in the case of streams and rivers whose
flow becomes very small in summer, the saline content may increase to
several times the amount carried at the time of high water. Both kinds
of cases occur in southern California, in Arizona,[94] New Mexico and
other states of the arid region. The Gila, Pecos and upper Rio Grande
are cases in point, and to a certain extent the Colorado of the West.
[94] Bull. Ariz. Exp’t Sta. No. 44.
_Muddy Waters._—In the latter as well as other streams of Arizona,
there is another point which sometimes creates difficulties to the
irrigator, together with some current expense. It is the amount of silt
or mud carried by the water, which while it is a benefit to the land
over which it is spread, (“warping”) as in the classic case of the
Nile, often clogs the irrigation ditches to such an extent as to cause
considerable inconvenience and expense in cleaning them out. This is
especially the case in the streams draining pasture lands that have
been overstocked, and where the destruction of the natural herbage
allows the rain water to run off rapidly, at first forming runlets and
then gullies and ravines that originally were simply cow-paths leading
toward the watering places.[95] The devastation of lands thus caused
in Arizona is almost as great as that which has occurred in the Cotton
states, as mentioned above chap. 12. p. 217.
[95] Bull. Arizona Exp’t Station Nos. 2, 38.
These variations in the character of the irrigation water must of
course be watched by the farmer who does not receive directly from
mountain streams, or from deep artesian wells water known to have a
constant content of saline matter.
THE DUTY OF IRRIGATION WATER.—The amount of water thought to be needed
for the production of satisfactory crops varies widely in different
regions, ranging all the way from about two feet to as much as eight
annually, within the United States; while in the sugar-cane fields
of the Hawaiian Islands as much as three inches per week, or over
twelve acre-feet in the course of the year, have been thought to be
beneficial, if not absolutely required for the best crop results.
As has been stated above (chap. 12, p. 215), the rainfall limit below
which irrigation becomes, if not absolutely essential, at least a
highly desirable condition for the safety of crops, is usually assumed
to lie at about 20 inches (500 milimeters). This general statement
is, however, subject to material modification according to the manner
in which the rainfall is distributed. Thus in central Montana with 24
inches of rainfall distributed throughout the year, irrigation is
indispensable; while in the Santa Clara valley of central California,
with an average rainfall of 15 inches falling through the winter and
spring, the growth of all ordinary field crops has for fifty years
not failed oftener than is commonly the case in the humid region of
the North Central states. This is because in California the winter
and spring are the growing seasons, while the rainless summers do not
stand in the way, for crops are already harvested; and the deep rooting
of trees and vines provides these with the needful moisture from the
depths of the substrata (see chap. 10, pp. 163 to 173).
It would thus seem that twenty inches of irrigation water properly
applied ought to be sufficient for all purposes, when added to the
natural rainfall, which is rarely entirely absent. Yet in actual
practice less than 24 acre-inches is rarely used, and much more is
the rule; 72 to 96 ins. being sometimes used in Arizona. Evidently
enormous losses occur in practice, and it is of the utmost importance
to discover the causes of these.
_Causes of Loss._—Since irrigation water is commonly measured at the
distributing weirs, loss from seepage and evaporation on the way to the
fields is an obvious source of an overestimate of the water actually
supplied to the farmer. In sandy districts the loss thus incurred is
reliably estimated at nearly 50% in many cases. The apparent duty of
the water is thus at once reduced to half its effect, and four instead
of two feet of water are supposed to have been used, and are charged
for.
_Evaporation_ resulting from surface flooding or use in shallow furrows
may, again, cause the loss of from 30 to 50% of the water that actually
reaches the land; so that in the latter case, between seepage and
evaporation the irrigator may lose the effect of three-fourths of the
water he pays for.
_Loss by Percolation._—Finally, the water may be wasted on the land
itself in leachy soils by over-use, _i. e._, it may percolate to a
large extent beyond the reach of the roots when the flow is continued
too long; as will always be the case when the head (supply) ditches are
laid too far apart, so that the water may be wasting into the country
drainage just below the upper ditch long before the water in the
furrow reaches the lower one; as illustrated in the upper one of the
subjoined diagrams. That this will not happen when the head ditches are
nearer together, is shown in the lower diagram.
[Illustration: FIGS. 46, 47.—Diagram showing loss by percolation when
head ditches are too far apart.]
The means of avoiding the mechanical losses have already been
discussed, and may be summarized thus: tightening of leaky ditches; use
of water in deep furrows; and ascertaining the rapidity of percolation
(see p. 242) so as to obtain a proper gauge for the time during which
water should run, and for the distances at which head ditches or
furrows should be placed.
The importance of thus diminishing the losses of water is obvious when
it is considered that if the duty of water can be reduced to twenty
instead of forty or fifty acre-inches, twice the area can be irrigated
with the same amount of water, or the cost of water correspondingly
reduced. It should be noted that when the land is leachy it may be pure
waste to continue the flow beyond a few hours; but the irrigation must
then be more frequently repeated.
EVAPORATION.
Alongside of and supplementary to the best possible utilization of
the rainfall and irrigation water, the prevention of unnecessary
evaporation has to be considered. Evaporation from the soil’s surface
implies not only unnecessary loss of water that should have remained
for the use of the crop, but also the depression of temperature which,
as a rule, is unfavorable to the best development of vegetation. It
is only in case of extreme stress from hot, drying wind that such
evaporation and the consequent depression of the temperature of the
surface soil can be of advantage to the farmer.
The amount of water evaporating either from a water-surface, or from a
wet or moist soil, varies greatly according to the climatic conditions,
and the state of the weather; also according to the condition of the
soil-surface. There are damp climates, and days or periods when,
the air being nearly saturated with moisture, evaporation even from
a water-surface will be almost insensible. On the other hand, with
dry air and a high temperature, enormous quantities of water may be
evaporated in the course of a day. The evaporation from water-surfaces
interests deeply those who supply, as well as those who are supplied
with, water from storage reservoirs; evaporation from the soil-surface
interests deeply all farmers, and more especially irrigators whose
water-supply is scanty, or is paid for by them by measurement. Light
rains, as well as light surface irrigations, may at times evaporate
almost wholly without any effect save a lowering of the temperature of
the soil. In the case of snow, it is a well-known fact in the northern
arid regions that a light snowfall may in winter evaporate entirely
without imparting any liquid moisture to the soil. A loss of 50% of the
water actually brought upon land by surface irrigation is of common
occurrence in some portions of the irrigated region.
The dependence of evaporation upon air-temperature under conditions
otherwise identical, is well illustrated by the experiments made in
1904 by S. Fortier[96] on the Experiment Station grounds at Berkeley,
California, at a time when under the influence of the sea breeze the
average saturation of the air might be assumed at about 70%. The tests
were conducted in six tanks sunk into the ground so as to place the
water-surfaces on a level with it, and the water-temperatures were
maintained in four of the tanks by means of ice or heating lamps. The
results are shown in the following table:
SUMMARY OF AVERAGE WEEKLY LOSSES BY EVAPORATION,
WITH VARYING TEMPERATURES OF WATER, AT BERKELEY, CAL.,
IN JULY AND AUGUST, 1904.
=======================+============
Temperature of water. | Weekly
|evaporation.
-----------------------+------------
Degrees Fahrenheit: | Inches.
55.5 | 0.42
62.0 | 0.77
69.2 | 1.54
80.1 | 3.08
89.2 | 3.92
-----------------------+------------
[96] Progress Report on Coöperative Irrigations in Calif.; Cir. No. 56,
Office Exp’t Stations.
A farther illustration is given in the subjoined table, showing maxima
and minima of monthly evaporation, as well the totals of one (seasonal)
year, in three California localities where the air-saturation is
considerably below that at Berkeley, ranging in summer from 50% to 20%
and even less (at Calexico in the Colorado desert):
SUMMARY OF EVAPORATION-LOSSES FROM WATER-SURFACES, AT POMONA,
TULARE, AND CALEXICO, CAL., FROM JULY 1, 1903, TO JULY 31, 1904.
============+=================+=================+=================
| Pomona. | Tulare. | Calexico.
+---------+-------+---------+-------+---------+-------
| Month. |Inches.| Month. |Inches.| Month. |Inches.
------------+---------+-------+---------+-------+---------+-------
Maximum |Aug. 1903| 9.07 |July 1903| 12.34 |July 1903| 14.48
Minimum |Feb. 1904| 2.57 |Jan. 1904| 1.46 |Jan. 1904| 4.39
| | ----- | | ----- | | ------
Totals for | | 66.92 | | 74.68 | | 108.23
year | | | | | |
------------+---------+-------+---------+-------+---------+-------
Of these three stations, Pomona is located within reach of the ocean
winds, but distant 25 to 30 miles from the shore. Tulare is situated
in the upper San Joaquin valley, far in the interior; Calexico is in
the southern part of the Colorado desert, with extremes of temperature
ranging from 13° Fahr. in winter to 120° in summer.
_Evaporation in Different Climates._—The following table conveys some
general data regarding average evaporation from water-surfaces in
different climates. Evaporation from the soil-surface depends largely,
of course, upon the mechanical condition of the surface, the extent
to which it is wetted, and the rapidity with which moisture will be
supplied from the subsoil as the surface dries. A field plowed into
rough furrows will evaporate more water than when harrowed, because of
the larger surface exposed; and a harrowed field moderately compacted
by rolling will lose less water by evaporation than when unrolled,
other things being equal. On the other hand, a thoroughly compacted
surface, even if suffering less loss at first than a plowed or harrowed
field, will continue to lose moisture longer by withdrawing it from the
substrata by its superior capillary suction; while a loose surface,
once dried out, will prevent farther loss from the subsoil very
effectually, as stated below.
TABLE SHOWING EVAPORATION, FROM WATER-SURFACE EXPOSED IN SHALLOW
TANKS, NEAR WATER OR GROUND SURFACE.
===================================+========+====================
| Years. | Inches.
-----------------------------------+--------+--------------------
Rothamsted, England | 9 | 17.80 (16.6 to 18.4)
London, “ | 14 | 20.66
Oxford, “ | 5 | 31.04
Munich, Germany | ? | 24.00
Emdrup, Denmark | 10 | 27.09
Cambridge, Massachusetts | 1 | 56.00
Syracuse, New York | 1 | 50.20
Logan, Utah | 1 | 52.39
Tucson, Arizona | 1 | 75.80
Fort Collins, Colorado | 11 | 41.00
Fort Bliss, Texas | 1 | 82.70
San Francisco, California | | 45 to 50
Sweetwater Reservoir, | |
San Diego, California | 1 | 57.6
Peking, China | ? | 38.80
Demerara, South America | 3 | 35.12
Bombay, East India | 5 | 82.28
Petro-Alexandrowsk, West Turkestan | ? | 96.40
Kimberley, South Africa | ? | 98.80
Alice Springs, South Australia | ? |103.50
-----------------------------------+--------+--------------------
This table, the data for which are taken from various sources,
exhibits clearly the enormous variations in evaporation in different
countries, and even in localities not very remote from each other. The
low evaporation near London is doubtless due to its foggy and hazy
atmosphere, but it is not clear why Rothamsted should show so low an
evaporation compared with Oxford. Tropical Demerara stands nearest to
Oxford in its evaporation; Bombay indicates its location on the hot
and arid west coast of India, despite its nearness to the sea. The
inland localities in the desert regions of South Africa, Australia and
Western Turkestan, show how enormous may be the losses from evaporation
of irrigation water, unless the latter is applied with special care
for their prevention. Thus, with the wasteful methods of irrigation
prevailing in portions of the American arid region, it is certain that
in many cases 50% and more of the water evaporates before it reaches
the crops.
_Evaporation from Reservoirs and Ditches._—The evaporation from
water-surfaces especially may, in many cases, exceed the rainfall of
the year, so as to materially diminish the available water-supply
in reservoirs. Thus the annual evaporation from the reservoir-lakes
forming part of the water-supply of the city of San Francisco, ranges
from 40 to 50 inches, while the rainfall averages less than 24 inches.
Were it not, then, for the prevention of evaporation by a covering
of dry earth during summer, no moisture would remain in the ground
to sustain vegetation. In the cool coast climate of Berkeley, Cal.,
directly opposite the Golden Gate and subject to its summer fogs,
evaporation from a water-surface maintaining the average climatic
temperature of 60°, was found to be ¾ inch during the month from
the middle of July to the middle of August, 1904. But at the high
temperatures and low degree of air-saturation prevailing in the great
interior valley, or in the Colorado desert, the evaporation from
water-surfaces is enormously increased, exceeding even the figure given
in the table for Bombay. Hence the great importance of preventing all
avoidable evaporation, particularly in the use of irrigation water.
_Prevention of Evaporation; Protective Surface Layer._—The loose
tilth of the surface which is so conducive to the rapid absorption of
surface-water, is also, broadly speaking, the best means of reducing
evaporation to the lowest possible point. For while it is true that the
floccules of well-tilled soil permit of the ready access of air, and
therefore of evaporation, it is also true that these relatively coarse
compound particles are incapable of withdrawing capillary moisture
from the denser soil or subsoil underneath; just as a dry sponge is
incapable of absorbing any moisture from a wet brick, while a dry
brick will readily withdraw nearly all the water contained in the
relatively large pores of the sponge (see chap. 11). A layer of loose,
dry surface-soil is therefore an excellent preventive of evaporation of
the moisture from soils, and may be regarded as the natural and most
available means to be used by the farmer, both for the prevention of
evaporation and to moderate the access of excessive heat and dryness to
the active roots.
As regards the desirable thickness of this protective layer of tilled
surface-soil, it should be kept in mind that in the humid region, where
rain can be expected at intervals of from one to three weeks, the
feeding roots may usually be found within a few inches of the surface;
while in the arid region, where irrigation is practiced at long
intervals or sometimes not at all, so that no water enters the soil
oftener than from two to six months, the roots necessarily vegetate at
lower depths, and hence the protective surface-layer can, and should
be, of greater thickness, to prevent the penetration of excessive heat
and dryness during the long interval.
The failure to appreciate this necessary difference often leads to
heavy losses on the part of newcomers to the arid region, who in this
as in other respects are apt to follow blindly the precepts familiar to
them in the East, until taught better by sore experience. In the East
and Middle West a depth of three inches is considered the proper one
for the protective surface-layer; and in the case of maize even this
is considered excessive in many cases. In the arid region this depth
should be at least doubled where irrigation is not practiced at least
every four to six weeks; and in some sandy soils even seven and eight
inches is not too much for effective protection.
_Illustrations of Effects of Surface Tillage._—The efficacy of loose
surface tilth in preventing evaporation, as compared with mere
superficial scratching or with the total omission of cultivation,
is well exemplified in a series of investigations conducted on this
subject during the extremely dry season of 1898, by the California
Experiment Station; the seasonal rainfall having during that year been
on an average from one-third to one-half only of the usual amount, so
as to test to the utmost the endurance of all growing plants. Some
of the details of this investigation have been given above (p. 214)
in connection with the question of moisture requirements of crops.
Loughridge[97] also investigated the moisture conditions in adjacent
orchards differently treated in cultivation. In one of these cases
two orchards of apricots were separated only by a lane, and the soil
identical; but one owner had omitted cultivation, while the other had
cultivated to an extra depth in view of the dry season apparently
impending. The results are best shown by the plates below, showing
representative trees and the annual growth made by each. The table
annexed shows the differences in the moisture-content of the two fields
to the depth of six feet, in July:
MOISTURE IN CULTIVATED AND UNCULTIVATED LAND.
===================+=====================+==================
| Cultivated. | Uncultivated.
Depth in soil. +------------+--------+---------+--------
| Per cent. |Tons per|Per cent.|Tons per
| | acre. | | acre.
-------------------+------------+--------+---------+--------
First foot | 6.4 | 128 | 4.3 | 86
Second foot | 5.8 | 116 | 4.4 | 88
Third foot | 6.4 | 128 | 3.9 | 78
Fourth foot | 6.5 | 130 | 5.1 | 100
Fifth foot | 6.7 | 134 | 3.4 | 68
Sixth foot | 6.0 | 120 | 4.5 | 90
| --- | --- | --- | ---
Total for six foot | 6.3 | 756 | 4.2 | 512
-------------------+------------+--------+---------+--------
[97] Rep. Calif. Expt. Sta. for 1897-98, p. 65.
The difference of 244 tons per acre of ground shown by the analyses is
quite sufficient to account for the observed difference in the cultural
result. The cause of this difference was that in the _uncultivated_
field there was a compacted surface-layer of several inches in
thickness, which forcibly abstracted the moisture from the substrata
and evaporated it from its surface; while the loose surface soil on the
_cultivated_ ground was unable to take any moisture from the denser
subsoil.
[Illustration: Cultivated.]
[Illustration: Uncultivated.
FIGS. 48, 49.—Apricot Trees, Creek Bench Land, at Niles, Cal.]
[Illustration: FIG. 50.—New Growth and Fruit on Trees, Cultivated and
Uncultivated. Creek Bench Land at Niles, Cal.]
The cultural results were that on the cultivated ground the trees made
about three feet of annual growth, and the fruit was of good, normal
size; while the trees in the uncultivated ground made barely three
inches of growth, and the fruit was stunted and wholly unsaleable. It
may be added that when, instructed by the season’s experience, the
owner of the “uncultivated” orchard cultivated deeply the following
season, his trees showed as good growth and fruit as his neighbor’s.
EVAPORATION THROUGH THE ROOTS AND LEAVES OF PLANTS.
Undesirable as is the evaporation from the surface of the soil, under
all but exceptional conditions the evaporation from the leaves of
plants is one of the essential functions of vegetable development. Not
only because water serves as the vehicle of the plant-food absorbed by
the roots and to be organized by and redistributed from the leaves,
and the aeration occurring in the latter must of necessity result in
a certain degree of evaporation; but largely because the conversion
of liquid water into vapor serves to prevent an injurious rise of
temperature in the leaves under the influence of hot sunshine and dry
air. It is undoubtedly for the latter purpose that the greater part of
the enormous amount of water required, as above stated (chap. 11) for
the production of one part of dry substance, is actually used. When
sufficient water to supply the required evaporation through the leaves
cannot be brought up from the soil, the plant begins to wilt; or in the
case of some plants with very thin and soft leaves the blade normally
begins to droop during the hottest hours of the day; thus escaping
excessive exposure to the sun’s rays, and recovering their turgor later
in the afternoon.
The amount of water actually evaporated from orchard
trees has unfortunately not been directly determined, the
investigations made in this respect having borne mainly upon
forest trees. The Austrian Forest Experiment Station made a
series of elaborate investigations on this subject in 1878,
and the following data (quoted from the Report of the U.
S. Dep’t of Agriculture for 1889) convey some idea of the
results.
It was found that the surface-areas of the leaves do not
give reliable results, but that these depend very largely
upon the thickness (mass) of the leaves. The dry weight of
the latter was found, as in the case of field crops, to
correspond most nearly to the observations made directly.
It was thus found that _e. g._ birch and linden
transpired during their annual period of vegetation from
600 to 700 pounds of water per pound of dry leaves; oaks
200 to 300, while the figures for ash, beech and maple
were in between. On the other hand the conifers—spruce,
fir and pine—ranged, under the same conditions, from 30
to 70 pounds of water only. In another year, these figures
were increased for deciduous trees to from 500 to 1000,
the conifers, 75 to 200 pounds. This great variability
in different seasons, together with other elements of
uncertainty, render these figures only roughly approximate;
but it will be noted that the figures for deciduous trees
are in general of the same order as those given above for
field crops. Assuming the evaporation for citrus trees to
be approximately the same as for the European evergreen oak
(_Q. cerris_) viz. 500 pounds per pound of dry matter,
and taking the weighings made by Loughridge of the leaves of
a 15-year-old orange tree at Riverside as a basis (40 pounds
of dry leaves), the water evaporated by each such tree would
be about 20,000 pounds per year, or about 1000 tons per acre
of 100 trees. This is equivalent to about 9 acre-inches of
rainfall, out of the 35 inches commonly given.
Since different plants evaporate very different amounts of water
during a given time, according to their leaf-surface and the number
and size of their stomates, the maintenance of the equilibrium between
the soil-supply and the evaporation of the leaf-surface requires
correspondingly varying moisture-conditions in the soil. Therefore
desert plants, with their elaborate structural provisions against
leaf-evaporation, will develop normally, and without wilting, under
conditions which in the case of most culture plants would result in
severe injury or death. Since diminution of leaf-surface will in all
cases diminish evaporation, the heroic measure of cutting back the
twigs and branches of shrubs and trees in seasons of severe drought
is sometimes resorted to in order to save their life. In Nature this
diminution of leaf-surface may be observed in many cases of desert
plants, whose “fugacious” leaves are developed during the rainy season,
in winter and early spring; dropping off so soon as the dry season
begins, and leaving only the green surface of twigs, stems or spines to
perform the functions of the leaves.
The shading of the ground by leafy vegetation will, of course, greatly
diminish and sometimes suppress evaporation from the soil-surface; thus
very nearly fulfilling the same conditions referred to above (chap. 7,
page 111) in discussing the effect of natural vegetation in rendering
tillage unnecessary; the beating of rains, and the formation of surface
crusts, being alike prevented. This fact is of essential importance in
contributing to the welfare of crops sown broadcast, where subsequent
cultivation is impracticable.
_Weeds Waste Moisture._—The injurious effects of weedy growth among
culture plants are in most cases due quite as much to the appropriation
of moisture that should have gone to the crop, as to the abstraction
of plant-food, to which the injury is generally attributed. This is
much more obvious in the arid region, where during the dry summers
every pound of moisture counts, than where summer rains obscure this
influence. It has led orchardists in California almost to an excess of
clean culture, resulting in the burning-out of the humus from the bare
surface-soil during the long, hot summers, and an injurious compacting
impossible to remedy by the most careful tillage. It thus happens
that green-manuring, the natural remedy for this evil, cannot safely
be done there with summer crops, but must be accomplished with winter
crops, such as can be turned under before the dry season begins. The
same objection holds against the growing of summer crops between the
orchard-rows.
DISTRIBUTION OF MOISTURE IN THE SOIL AS AFFECTED BY VEGETATION.
The investigations of Wollny and others have long shown quantitatively
what common experience has taught the farmer, viz., that a field in
crops or grass is always drier within the soil-mass penetrated by the
roots than is a cultivated field bare of crops, unless perhaps when
heavily crusted on the surface. The depletion of moisture caused by
grass sward is the most easily observed because of the shallowness
of the root-system; and this is one cause at least why grass sward
does not occur naturally in the arid region, and when planted cannot
be maintained without irrigation repeated at short intervals.
Deeper-rooted plants of course deplete the soil at different and
varying levels; and where surface roots are few or absent it may
readily happen that the surface soil is moister than the subsoil.
This was very strikingly shown by the investigations of
Ototzky in the South-Russian steppes, in comparing both the
moisture contents and the depth of bottom water as between
forest land and the open plains. On the steppe near Chipoff,
Government of Voronej, he found the ground water at from 3
to 5 meters (10-16 feet) depth; under the forest in the same
region and in identical underground formations, the water
level stood at 15 meters. In the Black Forest near Cherson,
the water is found at about 15 feet beneath the surface;
under the steppe and in cultivated ground it stood at 10
feet. At the same time the forest soil was moister in the
upper two feet than the soil of the steppe, where surface
evaporation (partly through shallow plant-roots, partly
direct) was greater than under the shadow of the forest;
under which, moreover, there were few shallow rooted plants
to draw upon the moisture of the surface soil.
The great evaporation from forests is a matter demonstrated by actual
measurement; hence it is not surprising that certain shallow-rooted
trees should serve for the reclamation of wet ground, as has been
demonstrated on the large scale, _e. g._, in the use of the eucalyptus
in the Pontine Marshes of Italy, and of the maritime pine in the Landes
of western France. Thus the sanitation of swampy districts through
tree-planting has become one of the established measures in their
settlement. But this refers only to the evaporation from the trees
themselves; for in the shade of the forest, a free water-surface is
found to evaporate on the average only one-third as much as in open
ground. Of course there must be a correspondingly great difference in
the amounts of evaporation from the soil-surfaces in the respective
areas.
The great draft made by the _Eucalyptus globulus_ upon soil-moisture
has been also abundantly shown in California, where on account of its
rapid growth this tree has been largely used for windbreaks. It was
found that the trees deplete the fields of moisture for from twenty to
thirty feet on either side, so as to materially reduce crops within
that limit. For this reason the pine and cypress has of late found
greater acceptance for this purpose.
_Mulching._—Covering the soil with straw or similar loose materials
to prevent waste of moisture is a common garden practice everywhere,
although not usually applicable on the large scale. It may readily
however, be carried to excess, in preventing not only evaporation but
also the warming of the soil which is so needful to the thrifty growth
of plants. It must not therefore be done too early in the season; and
after cold rains it sometimes becomes necessary to remove the mulch in
order to allow the ground to become properly warmed. Mulching in early
spring is often used to retard blooming of trees where spring frosts
are feared.
In the arid region, sanding of the surface is sometimes resorted to
for the prevention of the evaporation which brings alkali salts to the
surface. But the necessity of repeating this dressing annually unless
cultivation can be omitted, restricts the use of this expedient to
narrow limits.
The sanding of the surface of cranberry plantations in swamps or bogs
in the northern parts of the humid region doubtless owes its efficacy
largely, if not chiefly, to the retention of moisture, while at the
same time it prevents the consolidation of the surface, so as to render
tillage unnecessary.