14 AGRICULTURE IN THE SEMIARID GREAT PLAINS. 



produce a crop. The water actually contained in the crop at any 

 time is so small as not to be worth considering. It is the water that 

 passes through the plants into the air and the amount lost from the 

 soil which determine the amount necessary for the welfare of the 

 crop. The amount of water within reach of the roots of plants is of 

 no greater importance than the rate at which it escapes through the 

 leaves and stems. The water used by the plants is that which passes 

 through them and the small amount retained in their bodies. The 

 balance of the precipitation in this region is nearly all lost by evapo- 

 ration directly from the surface of the soil, very little escaping 

 through seepage. 



The amount of water used by plants is far from uniform for all 

 parts of the region, being greatest in the warmest and windiest parts 

 and growing less as temperature and wind velocity decrease. For 

 this reason an inch of water in the Panhandle of Texas is not com- 

 parable with an inch of water in North Dakota.^ The amount of 

 water lost through plants in the semiarid region, or, in other words, 

 the amount of water necessary to produce a crop if all loss from the 

 soil could be prevented, is not very well known. It is, however, 

 known to be far in excess of that required in more humid sections. 

 Experimenters in several States have determined the amount of water 

 lost by various plants in their particular localities and in publishing 

 the results have usually stated that they applied to their particular 

 conditions only; but, in spite of this, results obtained in Wisconsin 

 have frequently been quoted to show what a small quantity of water 

 was needed on the dry Plains. Records indicate that in the drier 

 portion of the Plains the air is about twice as dry as at Madison, 

 Wis. Obviousl}^, results in Wisconsin have no relation to Plains con- 

 ditions. At the Utah Agricultural Experiment Station it has been 

 found that about 50 tons of water passed through wheat plants for 

 every bushel of grain produced, or the equivalent of 12 inches of 

 water actually passed through the plants to produce 27 bushels of 

 wheat to the acre. To this must be added the water lost from the 

 soil by evaporation and by seepage in order to determine what was 

 required to produce the crop. 



The loss of water is controlled mainly by the same factors as the 

 evaporation from an open water surface, namely, the dryness of the 

 air, the temperature of the evaporating surface, the wind velocity, 

 and the lightness of the air. From these facts it is plain that the 

 amount of water necessary for a crop is very variable and is not 

 likely to be the same in the s'ame field in any two consecutive seasons. 

 It must vary from season to season approximately as the dryness of 

 the air, the wind velocity, the temperature of the air, the soil, and 

 the plants vary. 



215 



1 Bulletin 188, Bureau of Plant Industry. 



