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CHAPTER 7 

 RELATION OF CONSUMPTIVE USE TO EVAPORATION 



Throughout this report the relation between consumptive 

 use and evaporation, first mentioned in the Introduction, has 

 been stressed as a basis of estimating water used by plants when 

 only evaporation is known. The relation varies month by month, 

 reaching a maximum in summer and a minim\im during the cooler 

 months of the growing season. Thus, the relation for any period 

 is an average which may have a considerable departure from the 

 value for any single month. For the more water-loving species 

 summer consumptive use exceeds evaporation, but for many dry-land 

 plants it is less. Since consumptive use becomes less with in- 

 creased depth to ground water, its relation to evaporation is 

 partly governed by the position of the water table. 



Few attempts have been made to determine, by experiment, 

 the consumptive use-evaporation relation. The Victorville, Calif, 

 investigation with tules, previously described, is probably the 

 most significemt and perhaps the only experiment undertaken di- 

 rectly for this purpose (4). The results indicated, for the 

 particular region in which the investigation was carried on, that 

 annual use of water by tules was equal to 95 per cent of the an- 

 nual evaporation from a standard Weather Bureau pan with monthly 

 values ranging from 57 to 122 per cent. For other areas in the 

 same climatic territory where evaporation records are available, 

 use of water by tules may be computed as a percentage of the 

 evaporation. At Los Griegos, the annual use of water by tules 

 was as low as 83 per cent of the evaporation. 



A graphical comparison of monthly use of water by tules 

 growing under natural conditions and evaporation from a Weather 

 Bureau pern, in four southwestern localities, is shown in Figure 16. 

 Table 42 gives the percentage relation for tules by months at 

 various locations. 



