A HISTORY OF HERTFORDSHIRE 



about 46 per cent., these figures being the average results of a soil- and 

 a chalk-gauge at Nash Mills and a soil-gauge at Lea Bridge, and the 

 greatest difference between the average values given by either of the 

 three gauges being 2 per cent, in the summer and 3 per cent, in the 

 winter. The average annual percolation is therefore 26 per cent, of the 

 rainfall. Assuming the average annual rainfall in Hertfordshire to be 26 

 inches, this being the average for half a century ending 1892, and also 

 that this is equally divided between summer and winter, which it is very 

 nearly, we have 0-78 inch percolating in the summer and 5-98 inches in 

 the winter, giving an annual percolation of 6*76 inches. The difference 

 between the summer and the winter percolation is due to so much of the 

 rain being evaporated and absorbed by vegetation in the summer. It 

 cannot be said that the whole of the water which goes down three feet 

 into the soil reaches the plane of saturation, but the moisture which is 

 brought up from a greater depth by absorption into the roots of trees or 

 by capillary action cannot be so great as to materially affect these figures. 



It might be thought that our rivers would be highest in the winter 

 and lowest in the summer, but such is not the case. Owing to the 

 slowness of the percolation the surface of the plane of saturation rises 

 for a considerable time after the rain has fallen, and consequently our 

 rivers have in them the greatest volume of water in the spring and the 

 least in the autumn. 



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 than when it sinks beneath the surface. 

 If it be assumed 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. 



The yield of the catchment-basins of the two principal rivers of 

 Hertfordshire, the Colne and the Lea, is a question of much importance 

 in connection with the water-supply of London. It would occupy too 

 much space to go fully into this matter here, and for a detailed exami- 

 nation of it reference should be made to a paper by the present writer. 1 

 It has there been shown that, irrespective of our county boundary, the 

 area of permeable strata in the basin of the Colne above Harefield is 

 about 148 square miles and of impermeable strata about 87 square miles, 

 and that the area of permeable strata in the basin of the Lea above 

 Feilde's Weir is about 224 square miles and of impermeable strata about 

 1 86 square miles ; also that the probable yield from percolation through 

 the Chalk is about 45 million gallons per diem in the Colne basin and 

 54 million in the Lea basin, and from water running off the surface of 

 impermeable beds about 1 2^ million gallons per diem in the Colne basin 



1 Hopkinson, ' Hertfordshire Rainfall, Percolation, and Evaporation,' Trans. Herts Nat, 

 Hist. Soc., vol. ix. pp. 33-72, pi. i. (1896). 



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