WATER LOSSES FROM WET AREAS 



71 



made. Tliey are not true results as would be found in the absence of a 

 water table. For example, the true speciiic yield of a soil is measured 

 by the quantity of water which it will yield after it lias been saturated 

 and allowed to drain. Where a high water table exists, there can not 

 be complete drainage. 



A measure of specific yield is approximated by the difference 

 between the porosity of a soil and its moisture equivalent by volunu". 

 This represents the pore space remaining in a soil sample after it has 



TABLE 25 



COMPARISON OF THE COMPUTED SPECIFIC YIELD OF SOILS IN THE ABSENCE OF A 



WATER TABLE WITH THE OBSERVED SPECIFIC YIELD OF THE 



SAME SOILS HAVING HIGH WATER TABLES 



' Computed specific yield equals porosity minus moisture equivalent by volume. 



= Observed specific yield equals porosity minus specific retention. 



' Tank No. 14 contained disturbed soil. In all other tanks the original soil column was ujibroken. 



been centrifuged. To show the difference between the computed spe- 

 cific yield and the observed specific yield as measured in the tank tests, 

 Table 25 has been prepared. Here the porosity as determined by meas- 

 urement minus the moisture equivalent equals the computed spe- 

 cific yield. In the adjoining column the observed porosity minus the 

 specific retention equals the measured specific yield. The variation in 

 the two values is due entirely to capillary moisture resulting from a 

 high water table. 



In making these tests, soil moisture was first determined in each 

 tank and the water content of the soil was computed. Measured quan- 

 tities of water were poured into the tanks, raising the water level until 

 the soil was saturated. The volume of water required for saturation 

 added to the capillary moisture was then equal to the total pore space, 

 and fi'om this the percentage of porosity was computed. The capillary 

 moisture above the water tables was incapable of further drainage and 

 was, therefore, equal to the specific retention. Specific yield is the 

 difference between total porosity and specific retention. These quan- 

 tities are given in Table 26 for soils in various tanks at both the Santa 

 Ana and San Bernardino stations. 



It is shown in this table that both specific yield and specific reten- 

 tion vary with depth to water. The higher yields occur in those tanks 

 having the sliallower water tables. It will be observed that porosity of 

 disturbed soil in Tank No. 14 was close to the average of all tanks, but 

 that the specific retention greatly exceeded that of undisturbed soil. 



