70 DIVISION OF NYATER RESOURCES 



matter, while low values come from coarse materials of low water 

 holding capacity. 



Soil moisture samples at the Santa Ana station, outside of soil 

 tanks, show average moisture of 2 to 3 per cent in the upper soil after 

 a long dry period and about 12 per cent four days after a heavy rain. 

 The former percentage is the Avilting point for this soil, while the 

 latter is near field capacity. Samples taken from soil tanks show 

 moisture equivalents that approximately agree with field capacities 

 previously determined. This is shown in Table 24 of moisture equiva- 

 lents, as determined from samples taken from above the water tables in 

 tanks at both Santa Ana and San Bernardino. 



Moisture equivalents of Chino silt loam at San Bernardino are 

 higher than at Santa Ana because of fine soil particles, as evidenced 

 in Table 23, and a greater variation occurs at the different depths. 

 The San Bernardino top soil has a high moisture equivalent, while for 

 subsoil it is decreased one-half. 



Porosity, Specific Yield, and Specific Retention 



At the end of three years of investigation at the Santa Ana 

 station and previous to dismantling the soil tanks, tests were made of 

 the soils in various tanks to determine (1) porosity, (2) specific reten- 

 tion, and (3) specific yield. 



Porosity is a measure of the total voids in a soil and is represented 

 as a percentage of the total volume. It varies inversely with the size 

 of soil particles and is greater for clay soils than it is for sand or 

 gravel. 



Specific retention is a measure of the water holding capacity of a 

 soil and is recorded as a percentage of the total volume. In deter- 

 mining specific retention, it is necessary to consider the depth to water 

 table, as more water is held in a soil in close proximity to the water 

 table than at several feet above it. 



Specific yield is the amount of water which will drain from a soil 

 by gravity. It also is measured as a percentage of the total volume. It 

 is influenced by the size of soil particles and is greater for soils of coarse 

 material than for soils composed of finer grains. It depends also upon 

 the amount of capillary moisture resulting from a high water table. 

 It is evident that both specific retention and specific yield are entirely 

 relative and not altogether functions of the soil, as they depend on the 

 depth to ground water and are different with each change in depth 

 within the capillary fringe. Stearns* sa,ys, "Obviously, in any direct 

 test, whether made in the laboratory or in the field, the true specific 

 retention of the material can be ascertained only by using a high 

 column of the material and disregarding the lower part that lies 

 within the capillary fringe." In considering these characteristics, it 

 is obvious that the specific retention is the complement of the specific 

 yield and that the sum of the two is equal to the total porosity. 



Water tables in the tanks in which these tests were made were from 

 2 to 5 feet below the surface, or mostly within the limits of capillary 

 rise, and therefore the specific yield and specific retention as given in 

 this report refer only to the conditions nnder which the tests were 



* Laboratory Tests on Phv.sical Properties of Water Bearing Materials, by 

 Norah D. Stearns. (U. S. Geol. Sur. Water Supply Paper 596-F, p. 13S.) 1927. 



