218 Irrigation and Drainage 



the crop by insect enemies make an extra flooding or 

 drainage necessary, and this increases the demand for 

 water. Further than this, in order that the crop may 

 be the best, the water must not remain long stagnant, 

 and this requires either alternate flooding and drain- 

 ing, or else a considerable steady surplus flow of water 

 over the fields. 



In order to secure more economical methods of 

 seeding and harvesting the rice fields, this crop is 

 extensively grown on naturally dry lands, which may 

 be readily checked off into flooding basins, to which 

 the water may be admitted and withdrawn at pleasure. 

 In these cases, there is added to the demands for 

 water already mentioned the loss from seepage. This 

 loss from seepage may be so large that rice irrigation 

 cannot be economically practiced on uplands unless 

 they are quite fine and close in texture, so that the 

 rate of seepage will be small, or unless the normal 

 level of the ground -water is within a few feet of the 

 surface. Even here the subsoil must be pretty close, 

 or the loss of water by under -drainage will be too 

 large. 



The various available sources of data regarding the 

 duty of water in rice irrigation place the amounts of 

 water used as varying all the way from one second -foot 

 for 25, 28, 30, 35, 40, 55 and 66 acres of rice, thus 

 making an average of 38.6 acres per cubic foot of 

 water per second, and this is equivalent to covering 

 the surface with water about 6.2 inches deep every 10 

 days. 



