i9i 2] WATSON— PLANT GEOGRAPHY OF NEW MEXICO 197 



thoroughly saturates the soil; much more 



summer 



which quickly run off. The writer 

 has been surprised to observe how brief an influence these summer 

 rains have on the mountain streams and springs. A day or two 

 after a heavy shower they are nearly as low as before, although they 

 may have poured out a deluge for an hour or two. A heavy winter 

 snow, on the contrary, maintains a steady flow throughout most 



summer 



TEMPERATURE 



Because of its altitude and southern latitude, the climate is char- 



acterized by a comparatively low mean annual range of temperature 



and a high daily range. Although the thermometer is known to go 



to zero or below at night, the mean for January is 34 F. (Magnusson, 



loc. cit). This is due to the high temperature in the middle of the 



day (average maximum 46 ). For July the mean is 76. 4 F., the 



average maximum 89 , 4 and the average minimum 63. 5 . The 



absolute maximum for the ten years was 104 F., and it has exceeded 



100 F. on three different occasions. It is the occasional low 



temperatures which render it impossible for the larger, thicker 



cacti and century plants, so characteristic of southern Arizona and 



Mexico, to grow here. They have been planted repeatedly on the 



campus of the University of New Mexico, only to perish during 

 the winter. 



EVAPORATION FROM A FREE WATER SURFACE 



The following data (Magnusson, loc. cit.), giving evaporation in 



inches, show that the ratio of evaporation to rainfall is more than 

 10 to 1 : 



Jan. Feb. Mch. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

 2.04 2.63 6.17 6.82 10.08 12.63 n.78 10.21 8.00 4.38 1.73 1.4 



Total for the year 77.87 inches. 



SOIL MOISTURE 



Measurements of soil moisture gave the following results : sandy 

 soil in the valley in December (dry season) 0.8 in. below the 



surface, 30 per cent; 



(ed 



4 A striking characteristic of the arid southwest is the great difference in tem- 

 perature in the sun and in the shade. 



