THE WATER OF SOILS. 227 



Ground or Bottom Water, Water Table. During and after 

 long-continued and abundant rains, the zone of supersatura- 

 tion continues to descend until it finally reaches a more or less 

 permanent level, varying somewhat from season to season, but 

 on the whole usually definable for each region and locality; 

 being the depth to which wells must be sunk in order to secure 

 a fairly permanent water supply. This is called the water 

 table, ground water, bottom water, or " first water." 1 The 

 proportion of the rainfall that reaches the permanent water 

 level varies enormously, of course, in different soils and at 

 different times. With brief and moderate rains, in soils of 

 high water-holding power and slow percolation, it may never 

 reach the bottom-water level; this is very commonly the case 

 in the arid regions. Where, as in the humid regions, rains are 

 frequent or much prolonged, one half and even more may 

 finally reach the permanent level ; runoff and evaporation dis- 

 posing of the balance. 



Lysimeters. For the determination of the amount of water percolat- 

 ing to given depths, water-tight receptacles called lysimeters are usually 

 employed. The best way to establish such receptacles is to isolate a 

 unit-area (usually a square meter) by digging all around it to the depth 

 desired, then surrounding it with a metal sheet soldered tightly at the 

 cut edges, and finally driving in a sharp-edged, stiff metal sheet so as 

 to form the bottom when soldered to the upright walls ; leaving on one 

 side an outlet for the percolating water, which is then received into a 

 measuring receptacle somewhat like a rain gauge. 



Hall (The Soil, p. 75) states that at Rothamstead, where an 

 average rainfall of 31.3 inches is distributed rather uniformly 

 through the season, and where the soil is a moderately clayey 

 loam, a little less than half percolates through 20 inches of 

 soil, and about 4$ c /< through 60 inches. 



Surface of Ground ll'atcr; Variations. The surface of the 



1 In contradistinction to other levels or "streams " of water which may usually 

 be found lower down, separated from the first water by some impervious stratum 

 of clay, hardpan or rock, and very commonly under sufficient pressure to rise 

 somewhat higher than the point at which it was struck, owing to connection with 

 higher-lying sources of supply. When such pressure is sufficient to cause an 

 overflow at the surface of the ground, we have " Artesian " water as commonly 

 understood. 



