2 IRRIGATION PRACTICE 



exceeds 100 inches and occasionally reaches 600 inches or 

 more, as at Assam, India. From district to district, the 

 world over, the quantity of water that falls annually 

 upon the farmers' fields is different. 



2. Seasonal rainfall. Moreover, the time of the year 

 at which the rain comes is not everywhere the same. In 

 some localities the rain falls chiefly in summer, during 

 the season of crop-growth; in others, during the spring; 

 in others, during the fall, and in yet others, during the 

 winter. Going eastward from the Pacific seaboard, this 

 difference in seasonal rainfall is well brought out. In 

 California, the heaviest rainfall comes during midwinter; 

 in the Great Basin, in early spring; on the eastern slope of 

 the Rocky Mountains, in late spring, and on the Great 

 Plains, near midsummer. The annual rainfall may be 

 fairly large in a given locality, but it does not necessarily 

 fall at the time that plants are growing. 



3. Variations in rainfall. Added to these variations 

 in the quantity of total rainfall, and in the distribution 

 throughout the year, is still another: The same quantity 

 of rain does not fall in the same place from year to year. 

 In one year there may be a large precipitation, and in 

 another a very light one; and the time of the year of the 

 heaviest downfall may be shifted somewhat from year 

 to year. 



True, the variations are not large. The driest year 

 seldom receives less than two-thirds the rainfall of the 

 wettest year, and usually it receives more. True, also, so 

 far as our records show, the average rainfall over a certain 

 locality for ten or twenty years is practically constant. 

 The average rainfall at any one place is nearly invariable, 

 although distinct variations occur in successive years. 

 (Fig. 1.) 



