114 



slight differences in consumptive use in relation to depth to 

 water table are unusual. The difference between the first two 

 tanks indicates a nearly uniform use of water for all depths to 

 15 inches. Between 24 and 38 inches, consumptive use is nearly- 

 constant. In plotting these values a straight-line relation ap- 

 pears to exist for all depths except 24 inches. 



NORTHEASTERN COLORADO 



Weeds . --As has appeared from previous discussions in this 

 report, investigations of use of water by crop or noncrop plants, 

 conducted by engineers and others interested in irrigation or 

 water supply, usually measure the quantities of water consumed 

 in Inches of depth for the growing season. This measure is com- 

 parable to rainfall and may be converted into units of flowing 

 water. Other investigators more interested in transpiration 

 losses as a function of the plant determine the ratio of the 

 weight of water absorbed to the weight of dry matter harvested. 

 In each case there is a similarity in technique but the results 

 are in units that are not comparable. 



Probably the most extensive investigation of the latter 

 kind to include native vegetation was conducted at Akron, Colo. , 

 by the Bureau of Plant Industry, United States Department of 

 Agriculture, from 1911 to 1917 (27). A study less extensive as 

 regards native vegetation was also carried on by the same Bureau 

 at Mandan, N. Dak., from 1919 to 1922 (11). 



The term "water requirement," as defined by the investiga- 

 tors, "indicates the ratio of weight of water absorbed by the 

 plant during its growth period to the weight of dry matter har- 

 vested." Other investigators have defined the term as "the total 

 quantity of water required by crops for normal growth under field 

 conditions." (13) (See also p. 1.) The water required is dis- 

 posed of by transpiration from the plant, evaporation from the 

 soil, deep percolation, and other iinavoidable losses. The Bureau 

 of Plant Industry sealed the soil and roots in closed containers 



