236 WATER SUPPLY. 



winds which have a tendency to shift the clouds before they have had 

 time to give up their saturation in form of rain or snow. 



The precipitation upon the surface of our watersheds is used in 

 three different ways, as it were, viz.: 1st, for evaporation and plant 

 life; 2d, for seepage into the earth, then collecting into surface, sub- 

 terranean streams and gravel beds; 3d, for the storm run-off and run- 

 off during the irrigating season. 



FIRST— EVAPORATION AND PLANT LIFE. 



There is a certain amount of the water precipitated in form of 

 rain, snow and fogs upon the watershed which is taken up by the 

 evaporation from the ground caused by atmospheric conditions, and 

 also as nutriment necessary to sustain the plant life covering said 

 watershed. This amount is varying with the conditions of the water- 

 shed, and also the atmosphere. Upon undulating surfaces in the val- 

 leys and foothills of Southern California we have learned by experience 

 that ten to twelve inches in depth are necessary to maintain the plants 

 and to supply the evaporation. However, on the mountains there Is 

 only from eight to ten inches required for this purpose. The waste 

 caused by evaporation is, of course, greatly increased by high winds, 

 also if the precipitation falls in form of snow upon an unprotected 

 surface exposed to the sun on winter days and the winds sweeping 

 over the open surface. In the latter case the evaporation becomes 

 greater, and the major part of the entire snow fall will be lost. 



SECOND— SEEPAGE INTO THE EARTH. 



That portion of the rainfall which seeps into the ground beneath 

 the capillary attraction finds its way into the underflow of the rivers 

 and into the underground channels, saturating the gravel beds of the 

 valleys and filling the strata carrying the artesian supply. 



The saturated gravel beds feed the surface springs and supply 

 the streams of Southern California, which after having disappeared 

 from the surface reappear again and form a valuable part of our ir- 

 rigation systems. The lower San Gabriel river is in summer entirely 

 dry from the mouth of the canyon down to El Monte. At this place it 

 reappears at the surface. Another instance of the same occurrence 

 we find in the Santa Ana river, and many others of Southern Cali- 

 fornia. The artesian supply is made up in the same manner, by the 

 seepage of part of the rainfall falling upon the mountain watershed. 

 This part finds its way through the strata which carries it on a 



