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 |
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 year | 66.92 | 74.68 | 108.23 | |||
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.