4 IRRIGATION PRACTICE 



is so small that only certain crops can be grown, and of 

 these crops maximum yields are not obtained. The annual 

 rainfall, below which special moisture-conserving methods 

 are necessary, is different at different places. Ordinarily, 

 where the rainfall is 20 inches or less, the special water- 

 conserving methods of dry-farming must be used. Where 

 the average temperature is high; where heavy winds 

 blow largely; where the soils are shallow or infertile, or 

 where other water-dissipating factors are especially 

 active, the methods of dry-farming must be employed, 

 even if the rainfall reaches 25 or 30 inches annually. On 

 the other hand, with our present experience, we are 

 obliged to admit that when the annual rainfall is less than 

 10 inches, our present methods of water-conservation 

 are not sufficient, except in a few special localities, to 

 make profitable crop-production possible. 



6. Conditions of irrigation. Wherever proper methods 

 of manuring and tillage for water-conservation are insuffi- 

 cient to produce uniformly profitable or maximum crops, 

 irrigation is employed. The first aim of the farmer should 

 always be to store as much as possible of the natural 

 precipitation in the soil, and to apply water artificially 

 only to make up the deficiency in the quantity of water 

 required by plants. The quantity of irrigation water, 

 then, needed on any farm, will depend on the care with 

 which the rainfall is conserved for the use of plants. The 

 more thoroughly water-conservation methods are prac- 

 tised, the smaller the quantity of irrigation water required; 

 the more carelessly the rain-water is conserved, the larger 

 the quantity of irrigation water required. Irrigation 

 should always be, and in a good system of agriculture 

 always is, supplementary to the natural precipitation. 



7. Irrigation defined. Irrigation is the artificial appli- 



