238 WATER SUPPLY. 



of the total run-off from the watershed, while from the eastern and 

 from the wooded watersheds of the middle and southern Europe it is 

 as low as 50 per cent. This is chiefly due to the condition of the sur- 

 face of the watersheds, if we assume tnat the precipitation has fallen 

 under normal conditions, excluding extremes, cloud-bursts, etc. 



In order to make this clear to you I will compare the San Gabriel 

 river with Lytle creek. The watershed of the San Gabriel river is 

 greatly denuded, having only a very spare growth of trees and under- 

 brush. The extent of this watershed is about 226 square miles; dur- 

 ing the rainy seasons there are frequent and enormous floods in this 

 river, while the normal summer run-off is only estimated to be from 

 sixteen to twenty-two feet. 



The watershed of Lytle creek is only about fifty-seven square 

 miles, and is fairly well timbered, but it shows about the same av- 

 erage summer run-off as the above mentioned, while its storm run-off 

 is considerably less. 



The run-off from the watersheds depends largely upan the rain- 

 fall, of course, but it is not proportionate to the same. 



Mr. F. H. Newell, of the geological service of the United States, 

 has successfully attempted to show the relation of the ra'nfall 

 to the run-off by platting the depths of the rainfall in inches, horizon- 

 tally, and the mean annual run-ou, vertically, which resulted in two 

 curves, one showing the run-off from undulating surface and the other 

 from the mountains. This is found in a pamphlet issued by the En- 

 gineers' Club, Philadelphia, Vol. 12, No. 2, July, 1895. 



By decreasing the run-off from the watersheds, we do not only 

 increase the direct supply in the rivers and artesian belts, but also 

 increase the facilities for irrigation upon lands situated on lower 

 altitudes in the drainage basins of the rivers. The water which 

 is used direct upon the higher levels of the drainage basins will be 

 detained by seepage into the ground and forced to the surface again at 

 points further below. The amount which has not been used for plant 

 lite upon the first irrigated lands can be made available for irrigation 

 at the lower levels. We see this demonstrated in the drainage basin 

 of the Santa Ana river, San Gabriel and others. 



I have tried to make clear to you how necessary it is to have tne 

 watersheds well timbered and covered with underbrush in order to 

 create a good water supply for irrigation. We have, however, not 

 alone to cause plantings of new forests, also it is oi the highest impor- 

 tance to protect the now existing ones against their enemies. 



