Under the operation of natural laws only a part of rainfall can be collected, conserved and made available for human needs. As an nounced under the article on RAIN/AU, com munities must also consider its disposal. That will appear from what follows.
Disposal of Water from rain or melting snows disappears from the catchment areas. (I) by evaporation, (2) by transpiration, (3) by runoff and (4) by perco lation. These methods dispose of varying amounts dependent somewhat upon the sur faces, whether land, or water, the seasons, climate, temperature, locality, altitude. vegeta tion, character of the soil and other physical conditions. They may be briefly stated as follows : (I) Evaporation to some extent already considered under the article on Raitsrau. may dispose of any part or all of the rainfall over a limited area and in some instances evapora tion may exceed rainfall, as it did in Massachu setts in 1883. Evaporation goes on from ice and snow less than from land or water surfaces. Vegetation and trees intercept precipitation and also retard evaporation. They tend to increase percolation for the moisture penetrates the soil in and about the roots and is held there until it percolates into the deep strata of the earth. Various formula have been deduced to determine evaporation, as stated in the article on %comm. These, how ever, are not conclusive.
Capillary attraction acts upon water 30 or more inches below the surface and occasionally lifts a film of water 6 to 30 inches above the ground-water level and so aids evaporation. Wilton Whitney, in 1897, reported culture Year Book') that capillarity drew moisture up 20 feet or more to nourish crops in the soils of California. Prof. J. B. Stew art of the Agricultural College of Michi gan reported that capillarity operated upon water from 45 to 70 inches below the surface. The depth of the water-table below the surface is not uniform and varies in different localities. In the Central States it was found by W. J. McGee to be about 22 inches below the surface. Charles H. Lee of the Geological Survey was of the opinion that the capillarity lift is limited to four feet in coarse sandy soil and to eight feet in fine sandy and clay soils. Thus by the operation of natural laws is the verdure of the earth nourished and sustained from the waters below, though there be insufficient pre cipitation from above. Warm sunshine and
gentle winds increase evaporation.
From some soils evaporation averages 16.68 inches annually, where the rainfall is 3029 inches and the percolation 13.61 inches. Over th.: Ohio River watersheds, where the rainfall averages 41.1 inches, the evaporation averaged 14.8 inches; over the James River watersheds, where the rainfall averaged 42.1 inches, the evaporation averaged 16.3 inches; and over the Sacramento River watershed, where the rain fall averaged 322 inches, the evaporation averaged 8.5 inches. Evaporation from the Croton watershed was computed by John R. Freeman for a period of 32 years at 24.74 inches. Evaporation has been computed over the Sudbury watershed at 23.63 inches and at Nashua at 23.76 inches. From water surfaces evaporation is much greater than from land surfaces. From the Chestnut Hill reservoir near Boston for a period of years it averaged 39.2 inches and from a water surface at Croton, N. Y., it averaged 3968 inches. From Mount Hope reservoir at Rochester for 10 years, it averaged 44.45 inches, trom the Nfuskingum River 40 inches. from Owens Lake. California, 80 inches, from Yakima River, Washington, inches, from East Lake. Birmingham, Ala., it ranged from 511 inches to 69.4 inches.
The United States Weather Bureau and United States Department of Agriculture for some years have compiled a record of evaporation from the principal watersheds of the United States. These show that about two-thirds of rainfall or precipitation over the United States is disposed of by evaporation In Massachusetts in 1883, evaporation was 39.12 inches and rainfall was only 32.78 inches.
From a water surface at Lea Bridge, Eng land, it averaged 20.6 inches, whereas for 14 years it averaged 18.14 inches from land sur faces, where the rainfall was 2.5.72 inches. At Rothamsted, England, evaporation 10.68 inches, where rainfall averaged inches. From Tails reservoir, Edinburgh, it averaged 15 inches. The characteristics of land areas and their general physical condi tions, together with atmospheric influences, at.ect evaporation.