34 IRRIGATION PRACTICE 



found profitable to blast occasional holes through the 

 hardpan to serve as outlets for the excess water that 

 stands on the hardpans. Such holes, to be effective, 

 should occur frequently, in which case the process 

 becomes very expensive. 



27. Effect of gravel. When the soil is underlaid 

 with gravel, or if gravel seams pass through it within 10 

 or 12 feet of the surface, the normal distribution of the 

 soil moisture is disturbed. By such gravel is meant 

 the loose, open gravel which makes the soil discontinuous. 

 If gravel is mixed uniformly with the soil from the sur- 

 face downward, or at varying depths, the soil may be 

 looked upon as being continuous so far as the distribu- 

 tion of water is concerned. 



When water, moving downward, reaches a layer of 

 loose gravel, the descent of the moisture film is first 

 arrested, then the film is thickened until .the lower soil 

 pores are filled, and, if irrigation is continued, gravita- 

 tional water drips from the soil into the gravel below. 

 The water which thus passes into the gravel can not move 

 back by capillary means, and usually drains away into 

 the subsoil and is lost to plant use. 



Soils, made faulty because of gravel seams, should 

 therefore be irrigated lightly. Not enough water should 

 be added to allow any part to move into the subsoil. 

 Under such conditions, more frequent applications of 

 water become necessary. On the benches and foothills, 

 such soils are of frequent occurrence. 



28. Water table near surface. When the standing- 

 water table is within reach of plant-roots, heavy irriga- 

 tion should be avoided. Just enough water should be 

 added to keep the soil moist without allowing any appre- 

 ciable quantity to drain into the ground water. 



