METHODS OF DRAINAGE 



249 



rain, until such time, after the subsidence of a general 

 overflow as it may be removed. They may serve to 

 remove the water accumu ated as the result of an over- 

 flow. In every such case their efficiency depends upon 

 taking advantage of the natural inequalities of the sur- 



Fig. 80. Surface ditches for drainage in a grain field. Such drains are usually 



of low efficiency. 



face of the land. One phase of this practice is to plow the 

 land in narrow beds, so that the frequent "dead fur- 

 rows" serve as surface drains and as temporary storage 

 for the surface water. 



As sub-surface drains, their efficiency depends upon 

 their depth being sufficient to permit percolation lrom 



