CHAPTER III 

 GROUND-WATER 



GENERAL FACTS ABOUT LAND-WATER 



Water is one of the most vigorous agents working on the land. 

 Its activity is seen on every slope during heavy rains, in every 

 stream, and in the waves of lakes and seas. Even the water in the 

 soil and in the rocks beneath the soil is active, as we shall see. 



Source of land-water. The water on the land and in the soil 

 and rocks has fallen from the atmosphere, which always contains 

 some moisture in the form of water vapor. This vapor cannot be 

 seen, but it is constantly passing up into the air from all moist sur- 

 faces by evaporation. Under certain conditions, some of the water 

 vapor in the air is condensed into drops which fall as rain; or, if the 

 temperature at which the vapor condenses is below the freezing- 

 point, the moisture freezes as it condenses, forming snowflakes in- 

 stead of raindrops. The moisture which falls from the atmosphere, 

 whether in the form or rain or snow, is called precipitation. The 

 precipitation on the land each year would make a layer of water 

 something like three feet deep if it all fell at one time and were 

 equally distributed. In other words, the average amount of pre- 

 cipitation on the land is probably between 35 and 40 inches a year, 

 if we count a foot of snow as an inch of water. Forty inches of 

 water over the whole of the land would make about 35,000 cubic 

 miles of water. Since the rivers carry only about 6,500 cubic miles 

 of water to the sea each year, it is clear that the larger part of the 

 rainfall is not carried to the sea by rivers. 



The fate of rain-water. The water which falls as rain does not 

 remain long where it fell. Some of it sinks beneath the surface, 

 some of it forms pools or lakes, some of it runs off over the surface, 

 and some of it goes back into the air by evaporation. That which 



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