\ 



32 IRRIGATION PRACTICE 



the water flows, the more completely will the lower soil 

 layers be provided with the same percentage of water. 

 In all cases, the law of distribution will be the same, 

 except as modified by the full or partial help given by 

 gravity, according to the direction in which the move- 

 ment is taking place. (Fig. 8.) 



26. Effect of hardpan. A layer through which water 

 can pass with great difficulty, if at all, is often found a 

 short distance below the soil surface. Sometimes it is 

 merely the "plowsole," resulting from the repeated plow- 

 ings of a somewhat clayey soil to the same depth. More 

 often it results from the mutual relations of climate and 

 soil. For example, in a country where the rainfall is 

 heavy, the very fine particles of a heavy clay soil are 

 washed downward, until the whole subsoil becomes more 

 or less impervious to water. In regions of light rainfall, 

 that is, in true dry-farming and irrigation regions, this 

 washing down of fine material stops abruptly at a point 

 representing the depth of penetration of the rainfall. 

 Approximately the same quantity of rain falls from year 

 to year on a certain soil. In the course of time there is 

 formed at this point an accumulation of material com- 

 monly called hardpan. Under a light rainfall, on a clay 

 soil, the hardpan may be only a foot or two below the 

 surface; on a sandier soil, from 4 to 10 feet, or even more, 

 below the surface. Students of arid soils often estimate 

 the annual precipitation of rain and snow by the depth 

 of the hardpan. 



Not only are the fine clay and silt particles washed 

 downward by the rains. Lime and other similar sub- 

 stances are dissolved by the descending water, which 

 cement together firmly the materials of the hardpan. 

 Such calcareous hardpans are often so hard that they 



