IRRIGATION 



67 



be allowed to remain in pools for long periods, say not 

 over several days. Water which has stood ten days 

 or over is dangerous. The use of metal, cement, or tile 

 irrigation ditches, which prevent lateral flow except 

 where wanted, will greatly help in lessening the vast 

 number of mosquitoes now produced in or by poorly 



FlG. 31. Showing overflow or drain ditch ordered dug to prevent large standing pools. 

 The pool in the foreground was due to drainage water from irrigation. 



kept ditches. The running water in the ditches need 

 not be feared, — mosquitoes do not breed in running 

 water. The current in ditches with little slope may 

 be sluggish and further retarded by weeds, and under 

 such conditions mosquitoes may breed. The ditch 

 must be kept free from weeds and the edges should be 

 smooth. It is suggested that borrow pits in the con- 

 struction of ditches should be at some distance from 

 the ditch and from higher ground if possible ; otherwise 

 they may fill with seepage water and breed mosquitoes. 



