True, the localities included in the following table are extreme, but they ill.u.s.trate the large possible evaporation, ranging from about six to thirty-five times the precipitation. At the same time it must be borne in mind that while such rates of evaporation may occur from free water surfaces, the evaporation from agricultural soils under like conditions is very much smaller.
Place Annual Precipitation Annual Evaporation Ratio (In Inches) (In Inches) El Paso, Texas 9.23 80 8.7 Fort Wingate, New Mexico 14.00 80 5.7 Fort Yuma, Arizona 2.84 100 35.2 Tucson, AZ 11.74 90 7.7 Mohave, CA 4.97 95 19.1 Hawthorne, Nevada 4.50 80 17.5 Winnemucca, Nevada 9.51 80 9.6 St. George, Utah 6.46 90 13.9 Fort d.u.c.h.esne, Utah 6.49 75 11.6 Pineville, Oregon 9.01 70 7.8 Lost River, Idaho 8.47 70 8.3 Laramie, Wyoming 9.81 70 7.1 Torres, Mexico 16.97 100 6.0
To understand the methods employed for checking evaporation from the soil, it is necessary to review briefly the conditions that determine the evaporation of water into the air, and the manner in which water moves in the soil.
The formation of water vapor
Whenever water is left freely exposed to the air, it evaporates; that is, it pa.s.ses into the gaseous state and mixes with the gases of the air. Even snow and ice give off water vapor, though in very small quant.i.ties. The quant.i.ty of water vapor which can enter a given volume of air is definitely limited. For instance, at the temperature of freezing water 2.126 grains of water vapor can enter one cubic foot of air, but no more. When air contains all the water possible, it is said to be saturated, and evaporation then ceases.
The practical effect of this is the well-known experience that on the seash.o.r.e, where the air is often very nearly fully saturated with water vapor, the drying of clothes goes on very slowly, whereas in the interior, like the dry-farming territory, away from the ocean, where the air is far from being saturated, drying goes on very rapidly.
The amount of water necessary to saturate air varies greatly with the temperature. It is to be noted that as the temperature increases, the amount of water that may be held by the air also increases; and proportionately more rapidly than the increase in temperature. This is generally well understood in common experience, as in drying clothes rapidly by hanging them before a hot fire. At a temperature of 100 deg F., which is often reached in portions of the dry-farm territory during the growing season, a given volume of air can hold more than nine times as much water vapor as at the temperature of freezing water. This is an exceedingly important principle in dry-farm practices, for it explains the relatively easy possibility of storing water during the fall and winter when the temperature is low and the moisture usually abundant, and the greater difficulty of storing the rain that falls largely, as in the Great Plains area, in the summer when water-dissipating forces are very active. This law also emphasizes the truth that it is in times of warm weather that every precaution must be taken to prevent the evaporation of water from the soil surface.
Temperature Grains of Water held in in Degrees F. One Cubic Foot of Air 32 2.126 40 2.862 50 4.089 60 5.756 70 7.992 80 10.949 90 14.810 100 19.790
It is of course well understood that the atmosphere as a whole is never saturated with water vapor. Such saturation is at the best only local, as, for instance, on the seash.o.r.e during quiet days, when the layer of air over the water may be fully saturated, or in a field containing much water from which, on quiet warm days, enough water may evaporate to saturate the layer of air immediately upon the soil and around the plants. Whenever, in such cases, the air begins to move and the wind blows, the saturated air is mixed with the larger portion of unsaturated air, and evaporation is again increased. Meanwhile, it must be borne in mind that into a layer of saturated air resting upon a field of growing plants very little water evaporates, and that the chief water-dissipating power of winds lies in the removal of this saturated layer. Winds or air movements of any kind, therefore, become enemies of the farmer who depends upon a limited rainfall.
The amount of water actually found in a given volume of air at a certain temperature, compared with the largest amount it can hold, is called the relative humidity of the air. As shown in Chapter IV, the relative humidity becomes smaller as the rainfall decreases. The lower the relative humidity is at a given temperature, the more rapidly will water evaporate into the air. There is no more striking confirmation of this law than the fact that at a temperature of 90 deg sunstrokes and similar ailments are reported in great number from New York, while the people of Salt Lake City are perfectly comfortable. In New York the relative humidity in summer is about 73 per cent; in Salt Lake City, about 35 per cent. At a high summer temperature evaporation from the skin goes on slowly in New York and rapidly in Salt Lake City, with the resulting discomfort or comfort.
Similarly, evaporation from soils goes on rapidly under a low and slowly under a high percentage of relative humidity.
Evaporation from water surfaces is hastened, therefore, by (1) an increase in the temperature, (2) an increase in the air movements or winds, and (3) a decrease in the relative humidity. The temperature is higher; the relative humidity lower, and the winds usually more abundant in arid than in humid regions. The dry-farmer must consequently use all possible precautions to prevent evaporation from the soil.
Conditions of evaporation from from soils
Evaporation does not alone occur from a surface of free water. All wet or moist substances lose by evaporation most of the water that they hold, providing the conditions of temperature and relative humidity are favorable. Thus, from a wet soil, evaporation is continually removing water. Yet, under ordinary conditions, it is impossible to remove all the water, for a small quant.i.ty is attracted so strongly by the soil particles that only a temperature above the boiling point of water will drive it out. This part of the soil is the hygroscopic moisture spoken of in the last chapter.
Moreover, it must be kept in mind that evaporation does not occur as rapidly from wet soil as from a water surface, unless all the soil pores are so completely filled with water that the soil surface is practically a water surface. The reason for this reduced evaporation from a wet soil is almost self-evident. There is a comparatively strong attraction between soil and water, which enables the moisture to cling as a thin capillary film around the soil particles, against the force of gravity. Ordinarily, only capillary water is found in well-tilled soil, and the force causing evaporation must be strong enough to overcome this attraction besides changing the water into vapor.
The less water there is in a soil, the thinner the water film, and the more firmly is the water held. Hence, the rate of evaporation decreases with the decrease in soil-moisture. This law is confirmed by actual field tests. For instance, as an average of 274 trials made at the Utah Station, it was found that three soils, otherwise alike, that contained, respectively, 22.63 per cent, 17.14 per cent, and 12.75 per cent of water lost in two weeks, to a depth of eight feet, respectively 21.0, 17. 1, and 10.0 pounds of water per square foot. Similar experiments conducted elsewhere also furnish proof of the correctness of this principle. From this point of view the dry-farmer does not want his soils to be unnecessarily moist. The dry-farmer can reduce the per cent of water in the soil without diminishing the total amount of water by so treating the soil that the water will distribute itself to considerable depths. This brings into prominence again the practices of fall plowing, deep plowing, subsoiling, and the choice of deep soils for dry-farming.
Very much for the same reasons, evaporation goes on more slowly from water in which salt or other substances have been dissolved. The attraction between the water and the dissolved salt seems to be strong enough to resist partially the force causing evaporation.
Soil-water always contains some of the soil ingredients in solution, and consequently under the given conditions evaporation occurs more slowly from soil-water than from pure water. Now, the more fertile a soil is, that is, the more soluble plant-food it contains, the more material will be dissolved in the soil-water, and as a result the more slowly will evaporation take place. Fallowing, cultivation, thorough plowing and manuring, which increase the store of soluble plant-food, all tend to diminish evaporation. While these conditions may have little value in the eyes of the farmer who is under an abundant rainfall, they are of great importance to the dry-farmer.
It is only by utilizing every possibility of conserving water and fertility that dry-farming may be made a perfectly safe practice.
Loss by evaporation chiefly at the surface
Evaporation goes on from every wet substance. Water evaporates therefore from the wet soil grains under the surface as well as from those at the surface. In developing a system of practice which will reduce evaporation to a minimum it must be learned whether the water which evaporates from the soil particles far below the surface is carried in large quant.i.ties into the atmosphere and thus lost to plant use. Over forty years ago, Nessler subjected this question to experiment and found that the loss by evaporation occurs almost wholly at the soil surface, and that very little if any is lost directly by evaporation from the lower soil layers. Other experimenters have confirmed this conclusion, and very recently Buckingham, examining the same subject, found that while there is a very slow upward movement of the soil gases into the atmosphere, the total quant.i.ty of the water thus lost by direct evaporation from soil, a foot below the surface, amounted at most to one inch of rainfall in six years. This is insignificant even under semiarid and arid conditions. However, the rate of loss of water by direct evaporation from the lower soil layers increases with the porosity of the soil, that is, with the s.p.a.ce not filled with soil particles or water. Fine-grained soils, therefore, lose the least water in this manner. Nevertheless, if coa.r.s.e-grained soils are well filled with water, by deep fall plowing and by proper summer fallowing for the conservation of moisture, the loss of moisture by direct evaporation from the lower soil layers need not be larger than from finer grained soils
Thus again are emphasized the principles previously laid down that, for the most successful dry-farming, the soil should always be kept well filled with moisture, even if it means that the land, after being broken, must lie fallow for one or two seasons, until a sufficient amount of moisture has acc.u.mulated. Further, the correlative principle is emphasized that the moisture in dry-farm lands should be stored deeply, away from the immediate action of the sun"s rays upon the land surface. The necessity for deep soils is thus again brought out.
The great loss of soil moisture due to an acc.u.mulation of water in the upper twelve inches is well brought out in the experiments conducted by the Utah Station. The following is selected from the numerous data on the subject. Two soils, almost identical in character, contained respectively 17.57 per cent and 16.55 per cent of water on an average to a depth of eight feet; that is, the total amount of water held by the two soils was practically identical.
Owing to varying cultural treatment, the distribution of the water in the soil was not uniform; one contained 23.22 per cent and the other 16.64 per cent of water in the first twelve inches. During the first seven days the soil that contained the highest percentage of water in the first foot lost 13.30 pounds of water, while the other lost only 8.48 pounds per square foot. This great difference was due no doubt to the fact that direct evaporation takes place in considerable quant.i.ty only in the upper twelve inches of soil, where the sun"s heat has a full chance to act.
Any practice which enables the rains to sink quickly to considerable depths should be adopted by the dry-farmer. This is perhaps one of the great reasons for advocating the expensive but usually effective subsoil plowing on dry-farms. It is a very common experience, in the arid region, that great, deep cracks form during hot weather. From the walls of these cracks evaporation goes on, as from the topsoil, and the pa.s.sing winds renew the air so that the evaporation may go on rapidly. The dry-farmer must go over the land as often as needs be with some implement that will destroy and fill up the cracks that may have been formed. In a field of growing crops this is often difficult to do; but it is not impossible that hand hoeing, expensive as it is, would pay well in the saving of soil moisture and the consequent increase in crop yield.
How soil water reaches the surface
It may be accepted as an established truth that the direct evaporation of water from wet soils occurs almost wholly at the surface. Yet it is well known that evaporation from the soil surface may continue until the soil-moisture to a depth of eight or ten feet or more is depleted. This is shown by the following a.n.a.lyses of dry-farm soil in early spring and midsummer. No attempt was made to conserve the moisture in the soil:--
Per cent of water in Early spring Midsummer 1st foot 20.84 8.83 2nd foot 20.06 8.87 3rd foot 19.62 11.03 4th foot 18.28 9.59 5th foot 18.70 11.27 6th foot 14.29 11.03 7th foot 14.48 8.95 8th foot 13.83 9.47 Avg 17.51 9.88
In this case water had undoubtedly pa.s.sed by capillary movement from the depth of eight feet to a point near the surface where direct evaporation could occur. As explained in the last chapter, water which is held as a film around the soil particles is called capillary water; and it is in the capillary form that water may be stored in dry-farm soils. Moreover, it is the capillary soil-moisture alone which is of real value in crop production. This capillary water tends to distribute itself uniformly throughout the soil, in accordance with the prevailing conditions and forces. If no water is removed from the soil, in course of time the distribution of the soil-water will be such that the thickness of the film at any point in the soil ma.s.s is a direct resultant of the various forces acting at that particular point. There will then be no appreciable movement of the soil-moisture. Such a condition is approximated in late winter or early spring before planting begins. During the greater part of the year, however, no such quiescent state can occur, for there are numerous disturbing elements that normally are active, among which the three most effective are (l) the addition of water to the soil by rains; (2) the evaporation of water from the topsoil, due to the more active meteorological factors during spring, summer, and fall; and (3) the abstraction of water from the soil by plant roots.
Water, entering the soil, moves downward under the influence of gravity as gravitational water, until under the attractive influence of the soil it has been converted into capillary water and adheres to the soil particles as a film. If the soil were dry, and the film therefore thin, the rain water would move downward only a short distance as gravitational water; if the soil were wet, and the film therefore thick, the water would move down to a greater distance before being exhausted. If, as is often the case in humid districts, the soil is saturated, that is, the film is as thick as the particles can hold, the water would pa.s.s right through the soil and connect with the standing water below. This, of course, is seldom the case in dry-farm districts. In any soil, excepting one already saturated, the addition of water will produce a thickening of the soil-water film to the full descent of the water. This immediately destroys the conditions of equilibrium formerly existing, for the moisture is not now uniformly distributed. Consequently a process of redistribution begins which continues until the nearest approach to equilibrium is restored. In this process water will pa.s.s in every direction from the wet portion of the soil to the drier; it does not necessarily mean that water will actually pa.s.s from the wet portion to the drier portion; usually, at the driest point a little water is drawn from the adjoining point, which in turn draws from the next, and that from the next, until the redistribution is complete. The process is very much like stuffing wool into a sack which already is loosely filled. The new wool does not reach the bottom of the sack, yet there is more wool in the bottom than there was before.
If a plant-root is actively feeding some distance under the soil surface, the reverse process occurs. At the feeding point the root continually abstracts water from the soil grains and thus makes the film thinner in that locality. This causes a movement of moisture similar to the one above described, from the wetter portions of the soil to the portion being dried out by the action of the plant-root.
Soil many feet or even rods distant may a.s.sist in supplying such an active root with moisture. When the thousands of tiny roots sent out by each plant are recalled. it may well be understood what a confusion of pulls and counter-pulls upon the soil-moisture exists in any cultivated soil. In fact, the soil-water film may be viewed as being in a state of trembling activity, tending to place itself in full equilibrium with the surrounding contending forces which, themselves, constantly change. Were it not that the water film held closely around the soil particles is possessed of extreme mobility, it would not be possible to meet the demands of the plants upon the water at comparatively great distances. Even as it is, it frequently happens that when crops are planted too thickly on dry-farms, the soil-moisture cannot move quickly enough to the absorbing roots to maintain plant growth, and crop failure results. Incidentally, this points to planting that shall be proportional to the moisture contained by the soil. See Chapter XI.
As the temperature rises in spring, with a decrease in the relative humidity, and an increase in direct sunshine, evaporation from the soil surface increases greatly. However, as the topsoil becomes drier, that is, as the water fihn becomes thinner, there is an attempt at readjustment, and water moves upward to take the place of that lost by evaporation. As this continues throughout the season, the moisture stored eight or ten feet or more below the surface is gradually brought to the top and evaporated, and thus lost to plant use.
The effect of rapid top drying of soils
As the water held by soils diminishes, and the water film around the soil grains becomes thinner, the capillary movement of the soil-water is r.e.t.a.r.ded. This is easily understood by recalling that the soil particles have an attraction for water, which is of definite value, and may be measured by the thickest film that may be held against gravity. When the film is thinned, it does not diminish the attraction of the soil for water; it simply results in a stronger pull upon the water and a firmer holding of the film against the surfaces of the soil grains. To move soil-water under such conditions requires the expenditure of more energy than is necessary for moving water in a saturated or nearly saturated soil.
Under like conditions, therefore, the thinner the soil-water film the more difficult will be the upward movement of the soil-water and the slower the evaporation from the topsoil.
As drying goes on, a point is reached at which the capillary movement of the water wholly ceases. This is probably when little more than the hygroscopic moisture remains. In fact, very dry soil and water repel each other. This is shown in the common experience of driving along a road in summer, immediately after a light shower.
The ma.s.ses of dust are wetted only on the outside, and as the wheels pa.s.s through them the dry dust is revealed. It is an important fact that very dry soil furnishes a very effective protection against the capillary movement of water.
In accordance with the principle above established if the surface soil could be dried to the point where capillarity is very slow, the evaporation would be diminished or almost wholly stopped. More than a quarter of a century ago, Eser showed experimentally that soil-water may be saved by drying the surface soil rapidly. Under dry-farm conditions it frequently occurs that the draft upon the water of the soil is so great that nearly all the water is quickly and so completely abstracted from the upper few inches of soil that they are left as an effective protection against further evaporation. For instance, in localities where hot dry winds are of common occurrence, the upper layer of soil is sometimes completely dried before the water in the lower layers can by slow capillary movement reach the top. The dry soil layer then prevents further loss of water, and the wind because of its intensity has helped to conserve the soil-moisture. Similarly in localities where the relative humidity is low, the sunshine abundant, and the temperature high, evaporation may go on so rapidly that the lower soil layers cannot supply the demands made, and the topsoil then dries out so completely as to form a protective covering against further evaporation. It is on this principle that the native desert soils of the United States, untouched by the plow, and the surfaces of which are sun-baked, are often found to possess large percentages of water at lower depths. Whitney recorded this observation with considerable surprise, many years ago, and other observers have found the same conditions at nearly all points of the arid region. This matter has been subjected to further study by Buckingham, who placed a variety of soils under artificially arid and humid conditions. It was found in every case that, the initial evaporation was greater under arid conditions, but as the process went on and the topsoil of the arid soil became dry, more water was lost under humid conditions. For the whole experimental period, also, more water was lost under humid conditions. It was notable that the dry protective layer was formed more slowly on alkali soils, which would point to the inadvisability of using alkali lands for dry-farm purposes. All in all, however, it appears "that under very arid conditions a soil automatically protects itself from drying by the formation of a natural mulch on the surface."
Naturally, dry-farm soils differ greatly in their power of forming such a mulch. A heavy clay or a light sandy soil appears to have less power of such automatic protection than a loamy soil. An admixture of limestone seems to favor the formation of such a natural protective mulch. Ordinarily, the farmer can further the formation of a dry topsoil layer by stirring the soil thoroughly.
This a.s.sists the sunshine and the air to evaporate the water very quickly. Such cultivation is very desirable for other reasons also, as will soon be discussed. Meanwhile, the water-dissipating forces of the dry-farm section are not wholly objectionable, for whether the land be cultivated or not, they tend to hasten the formation of dry surface layers of soil which guard against excessive evaporation. It is in moist cloudy weather, when the drying process is slow, that evaporation causes the greatest losses of soil-moisture.
The effect of shading
Direct sunshine is, next to temperature, the most active cause of rapid evaporation from moist soil surfaces. Whenever, therefore, evaporation is not rapid enough to form a dry protective layer of topsoil, shading helps materially in reducing surface losses of soil-water. Under very arid conditions, however, it is questionable whether in all cases shading has a really beneficial effect, though under semiarid or sub-humid conditions the benefits derived from shading are increased largely. Ebermayer showed in 1873 that the shading due to the forest cover reduced evaporation 62 per cent, and many experiments since that day have confirmed this conclusion. At the Utah Station, under arid conditions, it was found that shading a pot of soil, which otherwise was subjected to water-dissipating influences, saved 29 per cent of the loss due to evaporation from a pot which was not shaded. This principle cannot be applied very greatly in practice, but it points to a somewhat thick planting, proportioned to the water held by the soil. It also shows a possible benefit to be derived from the high header straw which is allowed to stand for several weeks in dry-farm sections where the harvest comes early and the fall plowing is done late, as in the mountain states.
The high header stubble shades the ground very thoroughly. Thus the stubble may be made to conserve the soil-moisture in dry-farm sections, where grain is harvested by the "header" method.
A special case of shading is the mulching of land with straw or other barnyard litter, or with leaves, as in the forest. Such mulching reduces evaporation, but only in part, because of its shading action, since it acts also as a loose top layer of soil matter breaking communication with the lower soil layers.
Whenever the soil is carefully stirred, as will be described, the value of shading as a means or checking evaporation disappears almost entirely. It is only with soils which are tolerably moist at the surface that shading acts beneficially.
Alfalfa in cultivated rows. This practice is employed to make possible the growth of alfalfa and other perennial crops on arid lands without irrigation.
The effect of tillage
Capillary soil-moisture moves from particle to particle until the surface is reached. The closer the soil grains are packed together, the greater the number of points or contact, and the more easily will the movement of the soil-moisture proceed. If by any means a layer of the soil is so loosened as to reduce the number of points of contact, the movement of the soil-moisture is correspondingly hindered. The process is somewhat similar to the experience in large r airway stations. Just before train time a great crowd of people is gathered outside or the gates ready to show their tickets. If one gate is opened, a certain number of pa.s.sengers can pa.s.s through each minute; if two are opened, nearly twice as many may be admitted in the same time; if more gates are opened, the pa.s.sengers will be able to enter the train more rapidly. The water in the lower layers of the soil is ready to move upward whenever a call is made upon it. To reach the surface it must pa.s.s from soil grain to soil grain, and the larger the number of grains that touch, the more quickly and easily will the water reach the surface, for the points of contact of the soil particles may be likened to the gates of the railway station. Now if, by a thorough stirring and loosening of the topsoil, the number of points of contact between the top and subsoil is greatly reduced, the upward flow of water is thereby largely checked. Such a loosening of the topsoil for the purpose of reducing evaporation from the topsoil has come to be called cultivation, and includes plowing, harrowing, disking, hoeing, and other cultural operations by which the topsoil is stirred. The breaking of the points of contact between the top and subsoil is undoubtedly the main reason for the efficiency of cultivation, but it is also to be remembered that such stirring helps to dry the top soil very thoroughly, and as has been explained a layer of dry soil of itself is a very effective check upon surface evaporation.
That the stirring or cultivation of the topsoil really does diminish evaporation of water from the soil has been shown by numerous investigations. In 1868, Nessler found that during six weeks of an ordinary German summer a stirred soil lost 510 grams of water per square foot, while the adjoining compacted soil lost 1680 grams,--a saving due to cultivation of nearly 60 per cent. Wagner, testing the correctness of Nessler"s work, found, in 1874, that cultivation reduced the evaporation a little more than 60 per cent; Johnson, in 1878, confirmed the truth of the principle on American soils, and Levi Stockbridge, working about the same time, also on American soils, found that cultivation diminished evaporation on a clay soil about 23 per cent, on a sandy loam 55 per cent, and on a heavy loam nearly 13 per cent. All the early work done on this subject was done under humid conditions, and it is only in recent years that confirmation of this important principle has been obtained for the soils of the dry-farm region. Fortier, working under California conditions, determined that cultivation reduced the evaporation from the soil surface over 55 per cent. At the Utah Station similar experiments have shown that the saving of soil-moisture by cultivation was 63 per cent for a clay soil, 34 per cent for a coa.r.s.e sand, and 13 per cent for a clay loam. Further, practical experience has demonstrated time and time again that in cultivation the dry-farmer has a powerful means of preventing evaporation from agricultural soils.
Closely connected with cultivation is the practice of scattering straw or other litter over the ground. Such artificial mulches are very effective in reducing evaporation. Ebermayer found that by spreading straw on the land, the evaporation was reduced 22 per cent; Wagner found under similar conditions a saving of 38 per cent, and these results have been confirmed by many other investigators.
On the modern dry-farms, which are large in area, the artificial mulching of soils cannot become a very extensive practice, yet it is well to bear the principle in mind. The practice of harvesting dry-farm grain with the header and plowing under the high stubble in the fall is a phase of cultivation for water conservation that deserves special notice. The straw, thus incorporated into the soil, decomposes quite readily in spite of the popular notion to the contrary, and makes the soil more porous, and, therefore, more effectively worked for the prevention of evaporation. When this practice is continued for considerable periods, the topsoil becomes rich in organic matter, which a.s.sists in r.e.t.a.r.ding evaporation, besides increasing the fertility of the land. When straw cannot be fed to advantage, as is yet the case on many of the western dry-farms, it would be better to scatter it over the land than to burn it, as is often done. Anything that covers the ground or loosens the topsoil prevents in a measure the evaporation of the water stored in lower soil depths for the use of crops.