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). 28
