It is, as we shall discover, largely by acts which decrease 

 absorption, infiltration, and the ground-water store, and 

 which accelerate run-off, that Man has seriously disturbed 

 natural arrangements and harmed himself. 



e. Water exposed on the surface of the oceans, lakes, 

 rivers, and land, including that transpired by vegetative 

 cover, is again absorbed into the atmosphere and the natural 

 circulation is continued indefinitely. 



The relative quantities of water in these various sectors 

 of the hydrologic cycle may vary greatly, in a manner not 

 yet understood. Seasons follow one another with due reg- 



TEN YEAR MOVING AVERAGES OF ANNUAL PRECIPITATION 



70 80 90 1900 10 20 193O 



FOR TEN YEARS ENDING- 



Prepared from data by J. B. Kincer, U. S. Weather Bureau. 



Figure 5. Ten-year moving averages of annual precipitation. Note the alternating periods 

 of greater and lesser precipitation. 



ularity. Generally in each area there are periods of precipi- 

 tation, alternating with periods of relative drought; but 

 beyond these facts of common knowledge our information is 

 not yet sufficiently definite to permit long-range forecasting. 

 It may be known that in a particular region some snow is 

 certain to fall in the winter season, and that spring and fall 

 will be the rainy seasons, but science cannot predict how 

 much snow or how much rain will fall in any future season. 

 The laws of these relations are not yet known. 



A few data have been accumulated which suggest rather 

 than prove that there may be, in parts of the United States 



8 



