226 SOILS. 



cessive intervals, as derived from the observation of the water 

 levels in the several holes. 1 



FIG. 42. Percolation in clay land after heavyrain. 



It will be seen that while at first the upper surface of the zone of 

 saturation coincides with the surface of the ground, in falling it 

 descends most rapidly on the highest ground, while at the lower levels 

 the holes may remain full or overflowing ; the drainage taking place 

 sideways as well as vertically. The curved surface connecting the levels 

 in the several holes gradually flattens, rapidly at first, then progressively 

 more slowly ; the water disappearing entirely, first from the holes lying 

 highest, then successively from those at lower levels ; those located in 

 valleys or drainage channels remaining full until surface-water ceases to 

 run in such channels. But even after liquid water has ceased to be 

 visible in the holes, the descent of the water continues within that por- 

 tion of the soil, tending (unless more rain should come before that time), 

 to establish the condition of equilibrium as existing in the soil columns 

 shown in the diagram on p. 205, chapt. 1 1 ; such as results from the capil- 

 lary ascent of water from below, but having above it a column of soil 

 of minimum water-content, of greater or less height according to the 

 length of time allowed for the water to descend. This is a very common 

 state of things during the long summer droughts in the arid region, 

 when neither rain nor irrigation has added to the water supply in the 

 soil for many months, and yet ordinary deciduous fruit trees mature 

 their normal crops. Frequently, however, before this state of equilibrium 

 is reached, evaporation from the surface so draws upon the water supply 

 within the first few feet, as to reduce the soil to undersaturation at the 

 lowest point of the descending column, so stopping farther descent and 

 soon reversing the direction of the movement. The latter is the usual 

 condition of scantily irrigated ground. 



1 The exact record of these observations was unfortunately destroyed by fire 

 the soil was a heavy clay, and it took ten days before the water disappeared from 

 the lowest hole. 



