56 



of water from rain or irrigation). This condition seldom occurs, 

 so that most of the moisture received is absorbed by plant life. 

 Without replenishment during the summer, soil moisture is de- 

 pleted by plant use until there is a deficiency at the beginning 

 of each rainy season. 



In connection with rainfall penetration studies initiated 

 in southern California in 1927, various shafts, tunnels, and 

 special plots were prepared for soil-moisture investigations from 

 which it was possible to determine the water consumed by native 

 brush and by various weeds and grasses. Soil samples were taken 

 to depths below the limits of root activity, and the moisture 

 content was determined by standard practices. The limit of rain- 

 fall penetration, depth of root activity, amount of moisture con- 

 tribution to underground -water supplies and evaporation-transpi- 

 ration losses chargeable to consumptive use were determined for 

 various soil types. 



A 3-year investigation of natural brush plots on outwash 

 slopes indicated that about 19 inches of rain fell before any 

 material moisture passed beyond the limits of the root zone. The 

 brush varied with location but included chamiso, sage, squaw 

 berry, scrub oak, cactus, and yucca -- all capable of existing on 

 a small water supply. During the 3-year period rainfall at the 

 various plots ranged from 12.66 inches to 20.90 inches, practi- 

 cally all of which was used by the brush cover. During this 

 period soil moisture within the root zones was often below field 

 capacity. Under these conditions rainfall and consumptive use are 

 equal, assuming no run-off. In years of greater rainfall, however, 

 or of rains falling upon soil filled to field capacity, some water 

 passes beyond the roots to the underground basin. Under the 

 latter condition consumptive use might increase because of in- 

 creased transpiration opportunity. Results of brush-cover inves- 

 tigations, shown in Table 9, indicate that all precipitation in 

 this area is consiimed by the dry-land brush and that with this 

 amount of rainfall there is no penetration to the underground 



