CHAPTER XXX, 237 



Steep grade into the valleys and there delivers the same with suf- 

 ficient force to rise upon or above the surface of the ground after be- 

 ing allowed a free passageway by a pipe inserted through the cap- 

 ping material of the stratum into the water-flow itself. 



The amount available for the seepage into the underground chan- 

 nels cannot be measured, it can only be calculated as the remainder 

 of the rainfall after deducting the amount necessary for plant life and 

 evaporation, and also deducting the run-off which, under certain 

 conditions, can be measured. 



It is, however, very easy to see that the condition of the surface 

 of the ground governs largely the proportion of rainfall which runs 

 off quickly from the watershed and the proportion which is retained 

 to seep into the ground. The rougher the surface of the ground is 

 and the more porous we find the same, the more It will facilitate seep- 

 age into subterranean channels. 



THIRD— THE RUN-OFF. 



The run-off consists of the storm run-off and the summer run-off. 

 The storm run-off is that part of the run-off which flows from the 

 surface of the watershed after the upper crust of the same has been 

 suificiently saturated and no more water can be absorbed by it. This 

 saturation depends not only upon the amount of rainfall and the time 

 in which it fell, but also upon the character of the surface of the water- 

 shed. It is readily seen that if the moisture falls in torrents the 

 surface of the ground quickly becomes super-saturated ana the greater 

 part of the rain runs off rapidly without having time to enter the 

 subterranean reservoirs of the earth. It is also apparent that if the 

 surface of the watershed is denuded of timber and underbrush, the 

 storm run-off will be largely increased over the same from a well- 

 timbered watershed upon which the trees and undergrowth form a 

 resistance to the run-off and retain the water longer upon the surface, 

 giving it more time to penetrate Into the pores of the earth and sub- 

 sequently into the subterranean channels and reservoirs which retain 

 their water and give it up gradually during the summer into the 

 streams or artesian supplies of the valleys. Of course, the more water 

 we are able to save from the storm run-off and turn into the sub- 

 terranean channels, the more we increase the usefulness of the water- 

 shed for irrigation. 



The storm run-off from watersheds varies a great deal. Observa- 

 tion has shown that in Southern California it is as high as 95 per cent. 



