Chapter VI.

MOISTURE OF THE SOIL.

IN GENERAL.

The soil receives its water supply either by natural rainfall or by irrigation. The plant in successive generations of cultivation adapts itself to the ordinary supply of water, but in order to perpetuate its kind it must have sufficient during the growing season to serve it as a medium for extracting from the soil and air the nutritious substances needed by it for its own development. The water really available to the plant is principally that which is left in the soil close to the roots after the surface drainage has carried off a large per cent of the original rainfall and after the evaporation by the dry wind has taken 20 per cent of the remainder from the surface soil and after a further large per cent of the remainder has by percolation or seepage slowly settled down beyond the reach of the roots of the plant. Thus it happens that the roots rarely have left for their use 20 per cent of the original rainfall, and this is the so-called "useful remainder." Generally this remainder is best expressed as a percentage of what the soil would hold were it completely saturated. Therefore its absolute quantity will vary with the character of different soils

EVAPORATION FROM THE SURFACE OF FRESH WATER.

MONTSOURIS DATA FROM DESCROIX.

An approximate idea of the relation between the velocity of the wind, its temperature, and its dryness, on the one hand, and its power to evaporate water on the other, may be obtained by collating the data given by Descroix in his article on "The climatology of Paris," in the Montsouris Annuaire, 1890, page 121. From the mass of data given by him I select the averages taken according to the direction of the wind, or wind roses, for the three summer months June, July, and August, 1889, as these are the months during which crops are liable to suffer the most severely from droughts and dry winds. I give them in the following table:

We see that the driest winds, or those whose relative humidity is small, such as the north and east winds, give a large evaporation, and that the velocity and temperature of the west winds, which are a little less than those of the southwest winds, does not compensate for the dryness, which latter enables them to evaporate a little less than the southwest winds.

By multiplying the average daily evaporation by the number of days we obtain the total evaporation from the saturated paper of the Piche instrument. This exceeds the total rainfall, but we are not to infer that the evaporation from ground and leaves must also necessarily exceed the rainfall, although this is generally true for the sum

mer season.

BOSTON DATA FROM E. J. FITZGERALD.

The evaporation of the water from leaves and from the ground depends upon the temperature, wind, and humidity of the air. It is a rather complex result; if the above-mentioned elements remain constant for any time at the surface of the mass of water the evaporation from that surface will be closely represented by the following formula which is due to Fitzgerald, of Boston,

E=0.0166 (P-p) (1+1 W),

where W is the velocity of the wind in miles per hour; P the tension of vapor in inches of mercury corresponding to the temperature of the water; p is the tension of vapor corresponding to the dew point in the free air; E is the evaporation expressed in inches of depth of water evaporated per hour under atmospheric pressure between 29 and 31 inches of the barometer.

The evaporation from leaves and soils is usually less than that from water about in the proportion in which the soil approximates its

state of maximum saturation, or in proportion as the leaf can transpire moisture through its cell walls.

Therefore any observations of evaporation that we may make for comparative purposes can give us only the relative evaporating power of the wind and not the absolute evaporation from plants and soils.

THE PICHE EVAPORIMETER.

The simplest apparatus for observing evaporation is that known as the Piche evaporimeter. This consists of a glass tube closed at the top and hung in a free exposure; the tube is less than half an inch in diameter and filled with water; its lower open end is closed by a horizontal disk of bibulous paper about twice the diameter of the tube; the water evaporated from this paper is supplied from within the tube. The observer has simply to read the height of the water in the tube as it slowly descends hour by hour. The number so read off is easily converted into one that expresses the depth of water evaporated per hour from unit surface.

The following table from Montsouris Annuaire, 1888, page 254, shows the average evaporation thus determined by an instrument placed in the shade, also the corresponding temperatures and other data, as observed at Montsouris during thirteen years.

Prof. Thomas Russell, of the Signal Office, has published results of some observations on the effect of the wind on the evaporation from the disks of the Piche evaporimeter. (See Annual Report Chief Signal Officer, 1888, p. 176, or Monthly Weather Review, 1888,

p. 235.). He finds that with the temperature of the air 84° F. and relative humidity 50 per cent the evaporation varies with the velocity of the wind at the surface of the moist disk as in the following paragraph:

INFLUENCE OF THE WIND ON EVAPORATION.

At 5 miles an hour the evaporation is 2.2 times that in a calm; at 10 miles, 3.8 times; at 15 miles, 4.9 times; at 20 miles, 5.7 times; at 25 miles, 6.1 times; at 30 miles, 6.3 times.

The observations of the Piche instruments, as exposed in Signal Service shelters at 18 different stations, gave the results in the table following. (See Monthly Weather Review, September, 1888, p. 236.) The readings on the scale of the Piche instrument have been converted into depths of water that would be evaporated from a free surface of water within the same instrument shelter during the respective months by multiplying them by the constant coefficient 1.33, so that the evaporations here given in inches of depth of water correspond entirely to the ordinary methods of measuring rainfall.

Evaporation, in inches, observed with Piche instruments within the Signal Service thermometer shelters in 1888.

" In October at Boise City the evaporation was 7.60 inches.

Profesor Russell has also devised the following very satisfactory formula connecting the total daily evaporation in inches with the meteorological elements on which it depends, viz, the vapor tensions, Pw for mean wet bulb and pa for mean dew-point temperatures, (b) barometric pressure, by means of which he has been able to compute

the possibilities of evaporation within Signal Service shelters over the whole country for an average wind velocity.

Daily evaporation=30[ 1.98 pw+43.9 (pw—på)]

His results in this respect are platted on chart No. VI of the Monthly Weather Review, September, 1888, and show that the total annual depth of evaporation has its maximum of over 90 inches in southern Arizona, California, and New Mexico, whence it diminishes to a minimum of 20 inches annually in the northwest corner of the State of Washington and thence eastward to Maine. These figures, like his formula, take no account of the wind, because within the Signal Service shelters the wind is reduced to a velocity far less than that in the open air. These figures, therefore, represent the evaporation in open air only when there is no wind above some small limit-say 6 miles per hour but may be adapted to strong winds by the use of the figures given in the first paragraph of this section.

CULTIVATION DIMINISHES SURFACE-SOIL EVAPORATION.

The general effect of cultivation is to pulverize the upper soil; this protects the capillary roots from surface exposure, it breaks up the capillary outlets of the moisture in the soil, checks the natural evaporation that goes on at the surface, and thus preserves the water within the soil for the use of the plants. Dr. E. L. Sturtevant's observations show that the extent to which the water is thus conserved by cultivation during the months from May to September, 1885, at Geneva, N. Y., may be thus expressed: With a rainfall of 14.42 inches the cultivated soil evaporated 1.4 inches less than the uncultivated naked soil and 2.25 inches less than the soil covered with sod. In other words, the efficiency of the soil to retain useful water is increased by cultivation to an extent equivalent to 10 per cent of the rainfall. If the capillary connections between the soil in the neighborhood of the roots and the supply of moisture lower down be broken no supply of moisture can come up from below, but if the soil be well rolled the compacting will aid the capillary attraction and the plants will secure moisture from below. Again, when weeds are allowed to grow freely the injury to the crops is not due to robbing the soil of nutrition nor to their shading the ground, but principally to their robbing the soil of its moisture. Those who can with impunity allow weeds to grow must have soils containing an excessive moisture, which they thus get rid of, while those who have a comparatively dry soil must destroy the weeds in order to reserve moisture for the use of their crops. (Agr. Sci., Vol. I, p. 216.)