The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

Usage license:: Get license information via the feedback formular.

Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

WATER SUPPLY 223 practically 100 per cent. on a small rocky islet, to nothing in some desert regions. The amount varies with climate, vegetation, the geological and geographical character of the country, and the nature of the rainfall. The average rain- fall in the Thames valley for forty years was 28% inches, and during that period the run-off disposed of 8 inches of rainfall p.a., or 28% per cent. For the earth as a whole the run-off is estimated at about one-fifth of the rainfall. Evaroration—Evaporation is measured by evaporation gauges, of which the most reliable are tanks usually a square yard in area, floating in large sheets of water, as at Croton Reservoir near Boston and the Lea Valley reservoirs in Lon- don. The annual evaporation around London, as at Hemel Hempstead, is 21-24 inches, out of 28-18 inches of rain, or over 75 per cent. In Britain the average evaporation from lakes and reservoirs is estimated at about 20 inches a year; in Egypt (B. F. E. Keeling, Nature, Ixxxi, 1909, p. 403) } inch a day ; in India 60 inches a year ; in Central Australia over 100 inches a year; in many areas evaporation removes the whole of the rainfall. The rate varies less from year to year than might be expected; for in a wet year, though evaporation is less from an equal surface of water, the evapo- rating surfaces are wider and counterbalance the slower rate. In still weather the evaporation from a large body of water is nearly the same for each hour of day and night; on a dry day in winter the evaporation may be greater than on a still day in summer. PercoraTioN—The water that percolates underground is protected from organic pollution and from loss by evapora- tion, and as it may flow to arid districts it is offen of high importance. The proportion of rainfall available for percola- tion is however often small, and may be nothing. The amount is measured by percolation or Dalton Gauges. They consist of a block of undisturbed ground, usually a square yard in area, around which are placed watertight walls and floor. The water that percolates to the bottom drains to a measuring vessel. The amount of percolation to different depths is determined by adjacent gauges of the depths required. Much water percolates underground from rivers, by seepage through the sands on the bed; the loss depends

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