GRAVITATIONAL WATER AND DRAINAGE 163 



recorded in the last table. If this per cent be subtracted 

 from the per cent given in Column IV of the last table, 

 the per cent of actual gravitational water in those soils 

 may be determined. This is shown by the preceding 

 table. 



The amount in Column V represents the pounds 

 of water per cubic foot which would be lost by drainage 

 from each of the soils if their pores were all completely 

 filled with water. Such a soil is said to be saturated. 

 That plane in the soil to which level all of the pores 

 are filled with water saturated is known as the 

 water-table. This region of saturation is sometimes 

 known as the "ground water." 



It is possible to have such a structure in a fine clay 

 soil that all of its spaces are practically filled with water 

 held capillarily. It will be noted from the table that 

 the proportion of the total water capacity which is 

 permanently retained increases with the fineness of 

 the soil, and consequently with the decrease in the 

 size of the individual pores, as is shown in Column 

 VI. The clay in the above tables appears to be very 

 thoroughly granulated, which is responsible for the 

 similarity in the ratios for the silt and clay. 



Gravitational water is directly injurious to upland 

 crops, but when it exists at a depth of from four to six 

 feet below the surface, it may serve as a reservoir from 

 which moisture is withdrawn by capillarity, to offset 

 losses by evaporation. Water may be removed by 

 capillarity from the saturated zone to the point where 

 the loss is taking place, and under these conditions the 

 ground water which then becomes capillary water 



