THE WATER OF SOILS. 



253 



That this will not happen when the head ditches are nearer 

 together, is shown in the lower diagram. 



HCtD DITCH or 



!vg; ; ^ 



v -/' 1 



FIGS 46, 47. Diagram showing loss by percolation when head ditchers are too far apart. 



The means of avoiding the mechanical losses have already 

 been discussed, and may be summarized thus : tightening of 

 leaky ditches; use of w r ater in deep furrows; and ascertaining 

 the rapidity of percolation (see p. 242) so as to obtain a 

 proper gauge for the time during which water should run. and 

 for the distances at which head ditches or furrows should be 

 placed. 



The importance of thus diminishing the losses of water is 

 obvious when it is considered that if the duty of water can be 

 reduced to twenty instead of forty or fifty acre-inches, twice 

 the area can be irrigated with the same amount of water, or 

 the cost of , water correspondingly reduced. It should be noted 

 that when the land is leachy it may be pure waste to continue 

 the flow beyond a few hours ; but the irrigation must then be 

 more frequently repeated. 



EVAPORATION. 



Alongside of and supplementary to the best possible utiliza- 

 tion of the rainfall and irrigation water, the prevention of un- 

 necessary evaporation has to be considered. Evaporation 

 from the soil's surface .implies not only unnecessary loss of 

 water that should have remained for the use of the crop, but 



