72 



DIVISION OF WATER RESOURCES 



This accounts for the frequently moist surface in this tank and for the 

 high rate of soil evaporation from disturbed soil. One column of 

 Table 26 includes a check of porosity by computation using the formula 



As 



P^lOO (1 ), where P is porosity, As is apparent specific 



Sp. gr. 



gravity, and Sp. gr. is the real specific gravity, which has been assumed 

 to have a value of 2.65. 



Chino silt loam is composed of finer material than is Hanford 

 fine sandy loam and, therefore, the porosity is greater. The finer 

 material holds a larger proportion of soil moisture which results in an 



TABLE 26 



POROSITY, SPECIFIC YIELD AND SPECIFIC RETENTION OF SOIL IN TANKS 



HAVING HIGH WATER TABLES 



> Computed by formula: Porosity=100| 1 



;ity=100l ] 



Apparent specific gravity 



Real specific gravity 

 ' Tank Xo. 14 contained disturbed soil. In all other tanks, the original soil column was unbroken. 



) 



increased specific retention and a smaller specific yield. For tanks 

 having a water-table depth of 3 feet in fine sandy loam, the specific yield 

 averaged 23.75 per cent of the volume of soil tested, but for Chino silt 

 loam, the yield was but 9.9 per cent. The computed porosity of this soil 

 agrees verv closelv with that found bv actual test. 



Apparent Specific Gravity 



Apparent specific gravity is defined as the ratio of the weight of 

 a unit of dry soil to that of an equal volume of water. It is sometimes 

 called volume weight. It varies with the soil material and is highest 

 for soils having the lowest porosity. It is always less than the real 

 specific gravity. 



Apparent specific gravity of the soils in a majority of tanks at 

 both stations was determined for use in computing the equivalent 

 depth of water in inches above the water table in each tank. These 



