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