CHAPTER III 

 THE WORK OF GROUND (UNDERGROUND) WATER 



The average amount of precipitation on the land is estimated 

 at about 40 inches per year. A part of this water sinks beneath 

 the surface, a part forms pools or lakes, a part runs off at once, and 

 a part of it is evaporated. The proportion of the rainfall which 

 follows each of these courses depends on several conditions, among 

 which are (i) the topography of the surface, (2) the rate of rainfall 

 (or the rate at which snow melts), (3) the porosity of the soil or 

 rock, (4) the amount of water which the soil contains when the rain 

 falls or the snow melts, (5) the amount of vegetation on the surface, 

 and (6) the dryness of the atmosphere. The steeper the slopes, 

 the more rapid the rainfall, the less porous the soil, the wetter it is, 

 and the less the vegetation, the more water will run off without 

 sinking beneath the surface. 



The water which sinks into the ground becomes ground-water. 

 The thousands of wells in lands peopled by civilized man, and the 

 many springs which issue from the slopes of mountains and valleys 

 prove that it is abundant and widely distributed. 



That ground-water is connected intimately with rainfall is 

 shown by the following facts: (i) The level of water in wells com- 

 monly sinks during droughts, and rises after rains; and the sinking 

 is greater when the drought is long, and the rise greater when the 

 rainfall is heavy. (2) Many springs discharge less water in times 

 of drought, and others cease to flow altogether. (3) Rain-water is 

 seen to sink beneath the surface, wherever the soil is porous. Sink- 

 ing through the soil to the solid rock, it finds cracks and pores, and 

 through them it descends to greater depths. Nowhere are the rocks 

 which we see so compact and so free from cracks, when any con- 

 siderable area is considered, as to prevent the sinking of water 

 through them. 



The amount of ground-water in a given region does not depend 

 entirely on the local rainfall. Ground-water is constantly moving, 

 and some of it flows far from the place where it entered. Thus 



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