

IRRIGATION 



251 



Drouth. A drouth may be said to occur whenever the 

 precipitation in any fifteen-day period falls below 1 inch. 

 Milo Williams, in a report prepared from the Weather 

 Bureau records, gives the following data for the years 1900 

 to 1909 inclusive, showing the number of periods of fifteen 

 days or more with less than 1 inch of rainfall and the total 

 number of days that the drouth extended beyond the fifteen- 

 day period. 



From this table an estimate can be made of the occurrence 

 and length of drouths that may be expected in these regions. 

 From facts relating to conditions of rainfall, the effect of 

 drouths on growing plants, and the quantity of water re- 

 quired to produce each pound of dry matter, it can readily be 

 seen that either the conservation of the moisture in the soil 

 or the direct application of water is essential in agricultural 

 production if losses are not to be incurred from this source. 



Dry farming. The conservation of moisture by cultiva- 

 tion, known as dry farming, is a method of treating the sur- 

 face of the soil in such a manner that evaporation is reduced 

 to a minimum. In humid regions this is carried on only to 

 the extent of frequent shallow cultivations that cover the 

 surface with a dust mulch which prevents cracking of the 

 ground and keeps the water from being raised to the surface 

 by capillarity and taken up by the dry, warm air. The 

 same results may be secured if the soil is covered with some 

 thin covering such as straw or other light material. 



The dry-farming method, as practiced in semiarid regions, 

 consists in cultivating the soil in such a manner that a large 



