PERCOLATION, AND EVAPORATION. 47 



It has been estimated by Mr. W. Whitakcr, F.R.S., that in the 

 Geological Survey Maps, sheets 7 and 46 S.E., in which much 

 the greater portion of the above area is represented, out of nearly 

 600 miles square coloured as bare Chalk, the Chalk which would be 

 coloured as bare on Maps showing the Drift beds, occupies only 

 about 34 per cent, of the area, 21| per cent, of the Chalk area 

 being covered by permeable beds, 41 per cent, by mixed beds 

 (partly permeable and partly impermeable), and 3-^ per cent, by 

 impermeable beds. I have converted the square miles given 

 by Mr. AYhitaker into percentages of the whole area. If we 

 consider the mixed beds as half permeable and half impermeable, 

 it follows that 24 per cent. (20+ + 3^) should be deducted from 

 the area of Chalk, that is Chalk not covered by Tertiary beds, 

 to arrive at the area of permeable strata. The Chalk area of the 

 Colne must therefore be reduced from 195 square miles to 148, and 

 the Chalk area of the Lea from 295 square miles to 224. 



An inch of rain per annum percolating throughout an area 

 of a square mile will yield 39,623 gallons of water per diem. 

 Therefore 7 "72 ins. percolating per annum through the 148 square 

 miles of permeable strata in the catchment-basin of the Colne and 

 its tributaries above Harefield will yield about 45 million gallons 

 per diem, and 6'08ins. percolating per annum through the 224 

 square miles of permeable strata in the catchment-basin of the Lea 

 and its tributaries above Feilde's Weir will yield about 54 million 

 gallons per diem. The total yield from percolation through the 

 Chalk is therefore 99 million gallons per diem. 



To the amount of rain which percolates through the Chalk 

 should be added that which runs off the surface of the impermeable 

 strata. It is very difficult to form any estimate of this. There 

 must be much more evaporation from the surface of impermeable 

 beds than from the surface of permeable beds, for wherever water 

 stands it must be exposed much longer to evaporating influences 

 (the heat of the sun, drying winds, etc.) than when it sinks 

 beneath the surface. If we assume that impermeable beds yield 

 with ordinary, or not very heavy, rainfall, half the amount of water 

 that permeable beds do, we shall probably be very near the mark. 

 On this assumption the 87 square miles (40 + 47) of impermeable 

 beds in the Colne area will yield about 12^ million gallons per 

 diem, and the 186 square miles (115 -|- 71) of impermeable beds 

 in the Lea area will yield about 20^ million gallons per diem. 

 This brings the yield of the Colne area above Harefield to 

 572" million gallons, and that of the Lea area above Feilde's "Weir 

 to 7H million gallons. The total yield of the two areas with 

 ordinary rainfall should therefore be about 132 million gallons 

 per diem. 



But occasionally we have very heavy rain, which runs rapidly 

 off impermeable beds ; when excessively heavy it may even run off 

 permeable beds, but that is rarely the case. It is as impossible 

 to estimate the yield of these heavy falls, as it would be futile 

 to rely upon them for our water-supply. They temporarily 



