CHAPTER III 



THE SOIL AS WATER RESERVOIR 



IN an ideal system of irrigation, water would be 

 applied continuously to the soil, and at a v rate to meet 

 the actual requirements of the plants growing on it. 

 Except in a very few cases, this ideal method is impossi- 

 ble. In practice, lands are watered intermittently. 

 When the "turn to water" comes, the farmer applies to 

 the soil in a few hours, or in a few days at most, as much 

 water as he is allowed, or as he believes will supply the 

 crop until the next turn comes, which may be a few days 

 or several weeks hence. Even in cases where the farmer 

 has at his disposal a continuous flow of water, it is sel- 

 dom practicable or wise to attempt continuous irrigation 

 of any one field. Irrigation is essentially an intermit- 

 tent practice. 



Plants can not live long without water. When the 

 water in the soil is reduced to the definite limit known as 

 the wilting coefficient, plants may be seriously injured in 

 a few hours, unless more water is added to the soil. In 

 view of this fact and the common knowledge that crops 

 thrive under systems of irrigation-rotation, it is evi- 

 dent that water applied in irrigation is held by the soil 

 in quantities sufficient to maintain crops in a good con- 

 dition until the next watering occurs. That is, water 

 may be stored in the soil in considerable quantities. The 

 property of soils to act as storage reservoirs for water is 

 of the highest importance in the practice of irrigation. 



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