426 A NATIONAL PLAN FOR AMERICAN FORESTRY 



The metropolitan water district is actively proceeding with its Colorado River 

 project to bring in 1,500 second-feet. [Through an aqueduct more than 200 

 miles long] * * *. 



If importations for Los Angeles City from Owens Valley be neglected, about 

 90 percent of all water supplies are derived from underground reservoirs under- 

 lying the valley floors on which the major part of the cities and towns and 

 agriculture have been developed. These underground reservoirs or basins in 

 turn get their supply by retaining a part of the wild and sudden floods of the 

 region and a part of the rainfall which comes upon the valley floors overlying 

 them. They regulate by natural processes the surplus waters of the wet for use 

 in the dry years and have made the present economic development of the region 

 possible. All plans for additional water supply propose further utilization 

 of the underground reservoirs and control, insofar as possible, of the supplies 

 placed in them. 



* * * The water plane in practically all of these has been falling for many 

 years past. Into some, salt water is penetrating from the ocean. Water is 

 being pumped from below sea level in 162 square miles of the Coastal Plain, 

 according to recent surveys. * * * 



The run-off that supplies these underground reservoirs comes 

 principally from the forest and brush covered slopes of the mountains 

 on which annual precipitation averages about 23 inches. Coniferous 

 forest occupies less than 5 percent of the south coast drainage area, 

 occurring principally on the higher mountain slopes and plateaus. 

 Practically all the steep mountain slopes are covered by chaparral, 

 brush, or woodland, which together occupy more than 40 percent of 

 the 11,075 square miles included in the drainages, ordinarily in a 

 dense stand that forms a complete canopy. All the lands having such 

 cover are classed as exerting a major watershed-protection influence. 



In these drainages there are two types of vegetative cover: Dense 

 chaparral on the slopes, and hardwood trees along stream channels. 

 In experiments carried on by the California Forest Experiment 

 Station, surface run-off from slopes recently burned has amounted 

 to only 1 or 2 percent of the season's precipitation of 20 to 30 inches. 

 Even this small surface run-off is from 2 to 30 times that from adjacent 

 brush-covered slopes. They showed also that on a level bare surface 

 with no run-off 60 percent of a 23-inch seasonal precipitation was 

 evaporated, leaving about 40 percent to become a part of underground 

 supplies. In contrast with this, an average of only 30 percent of the 

 seasonal precipitation was evaporated from soil covered with forest 

 litter from which no surface run-off occurred. Shrub growth and 

 litter on slopes prevent abnormal erosion, which would otherwise 

 become destructive. The annual run-off in streams from chaparral- 

 covered watersheds, which stream-flow records indicate amounts on 

 the average to from about 10 to 20 percent of the annual precipitation, 

 in large part reaches the streams by underground seepage from 

 slopes. In the long, dry summer period run-off is normally low, 

 and in many streams surface flow sometimes ceases entirely. 



Canyon-bottom vegetation of alders, wiUow, and such water-loving 

 species, transpires very large quantities of water back into the atmos- 

 phere, and by that much reduces the surface and subsurface stream 

 flow from the watershed, during periods of highest demand for water 

 in the valleys. In the south coast drainages this canyon-bottom veg- 

 etation occupies not more than 5 percent of the total area, but loss 

 of water through transpiration during long dry summers by this sub- 

 irrigated vegetation is relatively very large. Much water that would 

 otherwise be lost in transpiration in mountain canyons can be saved by 

 piping the water through the canyons past the stream-side vegetation. 



