28 IRRIGATION PRACTICE 



layers has gone above the point of slow capillarity. 

 Whenever that has been done, the moisture moves down- 

 ward freely in obedience to the law already stated. This 

 is really a matter of common experience, in opening any 

 irrigation project in the arid region. The farmer finds 

 that the water does not penetrate the soil deeply the 

 first year of irrigation; but, as time goes on, the soil 

 becomes wetter to greater depths, and at the same time 

 less water is required to produce crops. The moisture 

 content of the native undisturbed soil in arid regions is 

 usually below the point of lento-capillarity. The first 

 water added is used to bring the moisture content up to 

 this point. As this is accomplished, water moves downward 

 freely; and plants, also, are enabled to secure their water 

 supply with a smaller expenditure of energy. 



The above law of distribution, which appears to hold 

 for all unsaturated soils, above lento-capillarity, is a 

 provision of nature of utmost importance in the economic 

 use of irrigation water. Though water moves steadily 

 downward after an irrigation, by far the largest propor- 

 tion is held near the surface where plants can use it. It 

 has been roughly estimated, on the basis of the law of 

 distribution, that on a deep soil with a moisture per- 

 centage at the point of lento-capillarity, 85 per cent of 

 a heavy irrigation will be held in the upper 10 feet, within 

 reach of plants. By reducing the irrigations properly, it 

 is possible to prevent practically any of the irrigation 

 water from descending below the zone of plant activity. 

 On the other hand, if water is applied to a soil too fre- 

 quently or in excessive quantities, the excess will slide 

 downward to great depths, to reappear somewhere as 

 seepage or drainage water. A good understanding of this 

 principle, properly applied in irrigated districts, will do 



