IRRIGATION OP CEREALS 



245 



soluble substances or are derived from volcanic ash. Even 

 a small stream of water, with a slight fall, running on 

 such soils, unless watched with extreme care, may quickly 

 cut deep ravines and destroy the field. On such soils, there- 

 fore, the furrow method, which permits of a better con- 



*** 





FIG. 59. Canvas dam to check water. 



trol of water, is gradually becoming the only method. 

 Limited supplies of irrigation water also demand the fur- 

 row method, for it is evident that, with a given quantity 

 of water, more land may be covered by the furrow than 

 by the flooding method. It has been explained that the 

 yield of a crop to a unit of water is greater when small 

 quantities are used and a larger total yield will be obtained 

 when the small available quantity of water is spread over 

 a large area. 



Finally, under the best flooding system it is difficult 

 to secure an even distribution of water over ordinary 

 lands, which are not absolutely level. The furrow method 

 permits of a more even, though not by any means per- 

 fectly even distribution. This has been another determi- 

 ning factor in the acceptance of furrow irrigation for small 

 grains. 



