IRRIGATION OF CEREALS 249 



water to be used for wheat and the small grains depends 

 upon many factors. Less water is required on clayey 

 than on sandy or gravelly soils. Deep soils require less 

 water than shallow soils, or soils underlaid by gravel or 

 hardpan. Bark, working in Idaho, found that, in actual 

 practice, grains received on medium clays and sandy 

 loams about 18 inches of water, while on sands or gravelly 

 soil nearly 36 inches were used. More water is necessary 



FIG. 60. Irrigated wheat in Montana. 



on new than on old land. A high temperature, a low 

 relative humidity and a steady wind increase the water 

 requirements of crops. All these and others previously 

 discussed, must be considered in deciding on the quantity 

 of water to be used. 



The fundamental law to be considered in determining 

 the quantity of water used in the production of wheat and 

 of other crops is that, as more water is applied to a field 

 the smaller is the relative yield of grain and of straw. 

 Undoubtedly, as water is applied, the total yield increases 



