96 Irrigation and Drainage 



We have already referred at some length to the 

 seemingly small amounts of water used by the wheat 

 crop in coming to maturity in the San Joaquin valley, 

 in California, and to the long period of some 60 days 

 at the close of its growing season during which it 

 receives no water, either as rain or by irrigation. 

 What is the minimum amount of water which is capa- 

 ble of producing a yield of 15, 20, 30 or 40 bushels of 

 wheat per acre, and how does this compare with the 

 actual rainfall of the San Joaquin valley ? 



We have made no observations with wheat, like those 

 which have been recorded for oats, barley, maize, clover 

 and potatoes, but from similar observations made by 

 Hellriegel, in Germany, it is probable that the amount 

 of water necessary to produce a ton of dry matter with 

 wheat is not very far from 906,000 pounds or 453 tons, 

 equal to 3.998 acre -inches. How many bushels of 

 wheat should this give ? 



The ratio of the dry weight of the kernels to that 

 of the straw and chaff in a crop of wheat has been 

 found to be as 1 to 1.1 in a dry season, but to be as 

 high as 1 to 1.5 when there has not been an undesir- 

 able stimulation to the growth of straw. But where 

 wheat is irrigated in the southeast of France, Gasparin 

 states that a ratio of 1 of grain to 2 of straw is usual. 



If we take the ratio of 1 to 1.5, and allow 60 pounds 

 to the bushel of wheat, we may compute the least 

 amount of water which is likely to enable a crop of 

 varying yields per acre to be produced, and the re- 

 sults of such a computation are given in the following 

 table: 



