412 



I. INAGAKI. 



porates from the plant itself, and 42 cb. m. from the surface of 

 the water in which the rice is cultivated. 



To obtain the actual quantity of water necessary for rice- 

 culture, we must, however, take the filtration water into considera- 

 tion. It is said that 50 cb. m. of water per day, is commonly lost 

 per ha in this way, so that we have, by adding 50, 



41 +42 + 50=133. cb. m. per day, per ha. 

 or 1 .539 litre per ha, per second. 



But as a matter of fact all this quantity of water is not needed for 

 irrigation, because there is a supply of rain, which according to the 

 observations of the Central Signal Office in Tokyo, amounts on the 

 average of 18 years, to 0.582 litre per Ha, per second during the 

 main vegetation period of rice. Deducting this quantity from the 

 above figure we have as remainder the necessary quantity for 

 irrigation : 



1.539 — 0.582 = 0.957 litre per ha, per second. 

 I have studied the subject also practically in connection with 

 irrigation water in Iwashiro district which I hav^e personally in- 

 spected. There the water is supplied from Lake Inawashiro through 

 a large canal which divides itself into 8 during its course. The 

 quantity of water that passes through each of these branches and 



the area of the rice fields irrigated by it is s 

 table : 



hown in the following 



Branch of Canal. 



iVi'ca of irrig.ited field, 

 ha. 



Water passhig through per 

 second, in litres. 



1st Branch. 



713.658 



704-536 



2nd „ 



992.927 



• 932.68S 





341.712 



276.601 



4th 



251.141 



293.077 



5th „ 



1579.701 



1884.364 



6th 



133-492 



99.165 



7th 



170.208 



1S1.581 



8th 



252.855 



366.002 



Sum. 



4435 6S9 



473S.014 



