CHAPTER XV 



REGULATING THE WATER SUPPLY 



A 50-BUSHEL corn crop requires 7 to 10 inches of water 

 for the use of the plant, besides that to be allowed for 

 run-off, seepage, and evaporation. In Nebraska, with a 

 29-inch rainfall, the division of this water between the 

 four sources of losses is estimated as follows, when a 50- 

 bushel crop is grown : — 



Water required by the plants 8 inches 



Water lost by run-off 3 inches 



Water lost by seepage 2 inches 



Balance lost by evaporation 16 inches 



Total 29 inches 



The proportion of total rainfall lost by the different 

 means will vary with the region, but it is probable that 

 in most cases evaporation is twice the amount required by 

 the crop. 



106. Not all evaporation is undesirable. Whenever the 

 soil reaches its water-holding capacity, as is often the case 

 in early spring, then it must be dried by evaporation before 

 cultivation can be practiced. Run-off is desirable after the 

 soil reaches saturation, if the run-off takes place in such a 

 way as not to cause erosion, since the taking up of this 

 water by the soil would increase the loss by drainage, and 

 excessive drainage means a slow leaching of the soil. 

 The amount of run-off necessary in order to care for ex- 

 cessive rainfall, or of evaporation necessary in order to 

 dry out the soil, will varj^ with the rainfall. In fact, all 

 the water above that actually used by the crop is exces- 



161 



