The Thirlmere water scheme of the Manchester Corporation : with a few remarks on the Longdendale Works, and water-supply generally / [James Mansergh].

  • Mansergh, James
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: The Thirlmere water scheme of the Manchester Corporation : with a few remarks on the Longdendale Works, and water-supply generally / [James Mansergh]. Source: Wellcome Collection.

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    Results of Rainfall. One inch of rainfall upon one acre is equivalent to 22,622 gallons. The average annual rainfall at Woodhead of 50 inches would therefore be 1,131,000 gallons per acre; and if this average applied to the whole 19,000 acres of watershed in the Longdendale valley, as it probably very nearly would, the total quantity of rain falling upon this area in a year would amount to 21,490,000,000 gallons, or nearly 60,000,000 gallons per day. This must not, however, be taken as the quantity which runs off the ground in the shape of streams, and which would therefore be collectable in a lake or artificial reservoir. A certain proportion of the rainfall is either re-evaporat£d from the surface, absorbed by vegetation, or percolates into the ground. The “ evaporation ” and “ absorption ” are, of course, so much absolute loss for waterworks purposes ; the water which percolates into the ground may possibly re-appear in the shape of springs within the scope of the works, and so become available. The amount of loss by evaporation and absorption depends upon the nature of the ground the rain falls upon. On flat, peaty moor- lands it is very considerable. On steep, rocky ground, especially where the rocks are of close texture and impervious, it is com- paratively small. The amount depends also upon the character of the rainfall. Light summer showers, in otherwise dry weather, influence in a very slight degree the run of the streams, because the moisture is greedily taken up by plants or readily evaporated. On the Manchester Works this “loss” has been measured for fourteen years, and has been found to average very nearly 10 inches, and to vary from 5 • 16 inches to 13'83 inches. From the clean, steep, and rocky watershed of Thirlmere, with the atmosphere frequently saturated with moisture, the loss will doubtless be less than in the Longdendale valley, and it will probably be sufficient to estimate it at 8 inches on the average of years.