PART TL— WATER RESOURCES, 
INTRODUCTION. 
SURFACE AND SUBSURFACE WATER. 
The water supplied to land areas comes from the atmosphere and in 
part flows over the surface and in part sinks below the surface. What 
relation the surface " run-off" shall bear to the underflow depends on 
a variety of climatic, topographic, and geologic conditions. 
Of climatic conditions one of the most important is the rate at which 
water is supplied. If, for example, the same amount of water is pre- 
cipitated in an hour, as during a torrential rain, that at another time 
falls in several hours during a gentle, rain, the ratio of the run-off to 
the amount of water sinking into the earth is far greater in the first 
than in the second instance. The length of the periods between rains. 
rate of evaporation, etc., are also important factors in the process of 
dividing the water that reaches land areas into surface and subsurface 
portions, but need not be discussed at this time. 
Among topographic and geologic conditions of interest in this con- 
nection the principal ones are surface slope and the porosit}^ or As- 
sured condition of the surface rocks, including the sheet of rock waste 
which generally forms the surficial layer of the land. The degree of 
slope varies from vertical to horizontal. On cliffs and steep slopes 
generally the run-off is large in proportion to the amount of water 
that is absorbed, while on plains there may be no means of surface 
escape and all of the water precipitated may sink into the rocks or be 
evaporated. The degree of porosity varies from that of compact rocks, 
which are practically nonabsorbent, to that of loose soils, sands, etc., 
which drink in all the water that falls upon them. The more compact 
rocks are frequently fissured or jointed and thus afford an easy down- 
ward passage for all of the water reaching their surfaces. 
In southern Idaho the annual precipitation is small and occurs at in- 
tervals, and although in part descending as torrential rains, a much 
larger portion is furnished by gentle rains and by the melting of 
snow. The climatic conditions thus favor the absorption of the water 
by the rocks rather than surface run-off The same result is favored 
also by the presence generally of a thick sheet of porous rock waste, 
especially in the mountains and foothills, and by the porous or fissured 
condition of the rocks forming the surfaces of the plains. On the 
Snake River Plains there is practically no run-off, all the water 
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