Over Saale River. in Germany, where for 14 years precipitation averaged 23.78 inches, the runoff averaged 7.17 inches. Over Remsehed Dam in Germany, where for nine years pre cipitation averaged 45.62 inches, the runoff aver aged 30.78 inches. Over the Woodburn River in Ireland, where precipitation was 36.29 inches, the runoff was 23.04 inches. Over the Buffalo River in South Africa, where precipitation was 29.52 inches, the runoff was 5.30 inches.
The mean runoff from 2D watersheds in France is nearly 50 per cent of the precipi tation, while in Germany the mean runoff from nine watersheds does not, excepting in three in stances, exceed one-third the precipitation.
The mean runoff from the watersheds of Great Britain ranges from 50 per cent to 75 per cent of the precipitation. The foregoing data, largely from approved reports, indicate observed runoffs. They are not merely esti mates from curves which are fraught with more or less error, owing to the failure in sonic instances to take into consideration all the necessary physical elements of a given water shed to determine its actual runoff.
The French physicist, G Lidv, proposed the equation of P E + R -R", wherein P stands for percolation,. F. for evaporation, R for runoff and R" for rainfall, hut that does not always accord with actual measurements. Atmospheric and material conditions may so affect the fac tors of the equation as to make an un balanced equation.
T. U. Taylor, of the Society of Civil Engi neers, well says: •Runoff is a complex factor depending on rainfall topography, vegetation, kinds and condition of the soil at the time of the rains.' (Proceedings of the Society of Civil Engineers. Vol. XL, p. 166).
(4) Percolation is the descent of water from rain or snow, or from other sources into the porous strata of the earth due to gravity. ‘Vater thus descends to the saturated horizon usually a little above the water-bearing level of the ground water. As the latter is drawn upon by capillarity and ceaselessly flows away between the strata of the earth, it is depleted and the percolating waters replenish the fosses. Such (gravity waters' descend, where the soils do not admit of capillarity. Ground waters fill the subterranean channels, supply springs and wells and descend caverns measureless to mart Down to a sonless sea" The subterranean Rubicon in Belgium is a river of ground waters.
Ground waters are well nigh unfailing sources of water supplies as we shall see from what follows. As already stated, percolation disposes of what is left after evaporation, transpiration and runoff have eliminated a large part of the rainfall. It is conditioned some
what upon the physical formation of the ter ritory and also upon climate, temperature, eleva tion and slope of watershed. On mountain slopes and hillsides, where the surface and strata are tilted there is little percolation, but excessive runoff. Permeability of the strata determines their storage capacity. Soils, un consolidated deposits, sands, gravels, sand stones, porous limestones, slate, till, conglom erate quartzite and other rocks absorb quan tities of water dependent upon their porosity. The denser rocks, such as granites, gneisses and schists are relatively impervious to satur ation and in such geological formations only in joints, faults, bedding planes, caverns and sub surface basins and channels is water collected. The porosity of all these different strata and the physical conditions of the earth's crust largely control the amount and depth of per colation, which ranges in different locations from 10 per cent to 50 per cent of the pre cipitation.
Into some soils over which the rainfall is 30.29 inches the percolation averages 13.61 inches thereof. On Long Island it was esti mated by the Burr-Hering-Freeman Commis sion to be from 30 per cent to 50 per cent of the rainfall and to range from 15 to 25 inches, conditioned upon the dry and wet years. In Muhlthal, Germany, where the rainfall was 47.1 inches, the percolation was found by Walter E. Spear to equal 30.42 inches. He estimated the percolation in Germany to equal 50 per cent of the rainfall, in Holland to range from 11.1 inches to 153 inches of the rainfall and in Belgium to range from 6 to 9.7 inches of the rainfall. Herbert E. Gregory estimated that 25 per cent of the 46.89 inches of rainfall over Connecticut is absorbed in the ground, while in some sections of the United States such absorption is greatest during the period of heaviest rainfall and then it ranges from 80 per cent to 95 per cent of the precipitation. In this manner water enters the strata of the earth and forms underground streams or is collected in springs and in subsurface basins. Such waters are known as ground waters and supply wells and also flow toward river beds and to adjacent waters, such as lakes, seas and oceans. Ground waters are also supplied from streams flowing over the surface. Large habitable areas are supplied from underground flow by means of innumerable wells, tapping that flow, or by means of springs or by means of conduits, as on Long Island, or by means of infiltration gal leries, as in some parts of Belgium, Holland and Germany.