Result of inquiries as to quantities of water required per acre for irrigated lands : and the supply of sewage obtainable from the metropolis. May, 1849.

  • London (England). Metropolitan Commission of Sewers.
Date:
[1849]
Catalogue details

Licence: Public Domain Mark

Credit: Result of inquiries as to quantities of water required per acre for irrigated lands : and the supply of sewage obtainable from the metropolis. May, 1849. Source: Wellcome Collection.

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    If we calculate the mean amount of water in these thirteen varieties of saturated soils to be equal to 42lbs. per cubic foot of earth, or 378lbs. per square yard of soil a foot deep, then 378 x 4,840 (the number of square yards in an acre) gives l,8*29,520lbs. of water needed to saturate an acre of perfectly dry land to the depth of one foot or about 816 tons. If we suppose that the saturation of the soil need only extend to a depth of nine inches, then about 543 tons of water would be sufficient; if a depth of only 6 inches, then 408 tons; and if to only a depth of 4 inches, then 273 tons would suffice. It appears from the recent report of Dr J. Stark (Statement on Sewer Manure by Mr E. Chadwick, p. 49), that the celebrated Craigentinny Meads near Edinburgh are annually irrigated by the City Sewage about eighteen times. For instance, a certain plot was watered in its turn, in 1845, May 3 and 14, June 3 and 20, August 15 and 31, Oct. 8 and 29, Nov. 24, Dec. 31; in 1846, Jan. 30, Feb. 18, March 5 and 22, April 2 and 13. Now, if we calculate that, on an average of months, 250 tons of irrigation water would suffice to saturate the soil (always partially and sometimes thoroughly previously moistened), then it would require 4,700 tons of water to give these eighteen irrigations to an acre of land. We deem the quantities we have stated as likely to be an average amount of sewage fluid required for the irrigation of an acre of grass land to be nearly correct. In the recent report of Mr George Buchanan, Engineer for the Irrigated Meads of Craigentinny, near Edinburgh, to the Commissioners of Metropolitan Sewers, he states the quantity of water necessary per acre (p. 6) to be equal to a stream of sewer fluid of 3j cubic feet per minute for 12 days of 10 hours each. Now this we calculate to be equal to x 60 x 10 x 12 = 23400 cubic feet, weighing 62Jibs, per cubic foot, or 62J x 23400 = 1.462.500lbs., or about 650 tons. It will be remarked, however, that the sewage fluid in this case had to sustain the soakage, leakage, and evaporation during thirteen days of the month of May. This slow irrigation is far from being the most economical mode of applying the sewage—the more rapid and copious application being to be pre¬ ferred, as affording little time for the soakage of the substratum, and the leakage from the soil into the drains, and evaporation from the surface. Mr Buchanan also adds (see post p. 7), “ In some parts the soil consists of very stiff clay, resting on a similar substratum, and other parts of a red sand, and the sand requires nearly twice as much water for saturation as the clay.” We see, then, that the amount stated by Mr Buchanan of about 700 tons per acre having been used at Edinburgh upon a mead, and subject to many deductions from its saturating power, is not very widely different from the average amount of about 800 tons required, according to Professor Schubler, to saturate the same extent of nearly chemically dry earths. Then, again, by avoiding the soakage and drainage waste of twelve days, and taking the average of moderately retentive soils, a very material saving would undoubtedly be accomplished; and, moreover, by raising the sewage fluid to the contemplated elevations, it would be available tor the use ot the irrigator a second time (see Mr Buchanan's Report, p. 6). Upon the whole,