Cannon : Transpiration of Fouquieria 



399 



The significance of the small rainfall of the year is a factor 

 which must be taken into account, and, although it seems difficult 

 to express this concretely, it may be better appreciated if we con- 

 sider the relation of the precipitation of the year to the possible 

 total evaporation. The normal annual evaporation at Tucson is 

 estimated to be 90 inches.* The ratio between the normal annual 

 rainfall and the evaporation Is therefore i : y.y. If we suppose 

 for the moment that the decrease in 1904 of approximately 25 



Ja F Mr Ap My Je Jl Au S O N D 



Figure i. The rainfall and the mean maximum temperature at Tuc 



4Jt 



normal 



rainfall is represented by a solid line ; the rainfall in 1904 by a broken one ( ) ; 



5 mm. = one-half inch precipitation. The mean maximum temperature for the year is 

 given by the second broken line ( ) ; 5 ram. — 10 degrees Fahrenheit. 



per cent, in the precipitation was accompanied by a proportional 

 mcrease in the evaporation for the year, the ratio would be greatly 

 altered. An increase of 25 per cent, in the total evaporation 

 ^vould make 1 12.5 inches ; and the ratio between precipitation and 



evaporation becomes 



I 



12.7. 



Coville & MacDougal, /. c. 27. 



