32 HISTORY OF COLCHESTER CORPORATION WATER WORKS.
can be isolated, and regulated, to discharge its natural yield;
and in event of necessity arising the underground reservoir can
be drawn upon (by virtue of the collecting pipes being below the
natural lips) for a time thereby providing a quantity in excess of
the natural yield.
The natural yield of the springs varies slightly, as before
stated, with wet and dry periods. So far as I am able to ascer-
tain, there are no records of their yield previous to 1888, when I
had them gauged during the months of September and October;
they were then found to yield about 416,000 gallons per day,
excluding the two eastern springs, which have not been utilized.
These, however, were approximately gauged and found to yield
about 97,000 gallons. No further gaugings were taken until
November 1902, which was after a series of three dry years,
when they were found to yield about 300,000 gallons per day,
excluding the two eastern springs. On the 31st October 1903,
they yielded about 319,000 gallons. After the works were
completed in 1905, they were found to yield nearly 350,000
gallons, but as much as 500,000 gallons per day was drawn from
them for a short period in 1906.
The average annual rainfall from records taken at the Home
Farm, Lexden, at an elevation of 125 feet above Ordnance Datum,
by Mr. Bird from 1893 to 1909, a period of 17 years, amounts to
20.15 inches, and for the same period at Colchester, at an elevation
of 82 feet above Ordnance Datum, from records taken by Mr. H.
Goodyear, Borough Engineer, amounts to 19.66 inches
(see British Rainfall). For the purpse of my argument, I have
taken the average annual rainfall throughout this Lexden
gravel area or gathering ground as 20.15 inches, and assuming
37 % of this rainfall, equal to 7.45 inches, percolates into the
formation, then each square mile will absorb, say, 107,864,229
gallons of water, equal to a daily yield therefrom, if conditions
are favourable, of some 295,518 gallons throughout the year.1
If the gravel bed, as before stated, is 12 square miles in extent,
then the quantity contained therein will be twelve times the
above-mentioned figure, or 1,294,370,748 gallons, equal to a daily
supply therefrom of 3,546,216 gallons throughout the year.
The quantity of water actually extracted by the Corporation
from all their springs, viz., Lexden, Sheepen and at the water-
1 An inch of rainfall over a square mile equals 14.48 million gallons (14,478,420).