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.
| Depth in soil. | Cultivated. | Uncultivated. | ||
|---|---|---|---|---|
| Per cent. | Tons per acre. | Per cent. | Tons per 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 |