6 



BULLETIN 367, U. S. DEPARTMENT OF AGRICL^LTURE, 



tervals should show something of the changes taking place, and the 

 more accurately they can be drawn the more valuable will be the in- 

 formation obtainable from such a series. 



CLIMATIC CONDITIONS. 



The importance of those factors known as climatic conditions do 

 not need to be argued, especially in relation to the arid grazing 

 lands, where the whole crop of forage is so patently dependent upon 

 them. The peculiarities of the seasons upon the Santa Rita Eange 

 Reserve have already been discussed by Dr. Griffiths,^ who calls par- 

 ticular attention to the two growing seasons and shows that they 

 depend upon the amount and distribution of rainfall. 



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Fig. 4. — Curves showing the variations in tlie total monthly precipitation at two stations 

 on the Santa Rita Range Reserve, Ariz., through a period of sis years. 



The spring growing season is dependent upon the rain of the 

 previous fall and winter, taken with what may fall in the spring 

 proper. In April and May, and in at least a part, if not all, of 

 June, there usually occurs a period of dry weather, during which 

 most growth ceases and the spring annuals dry up. July, August, 

 September, and sometimes part of October constitute the sunnner 

 grooving season, since it is during this period that the greater part 

 of the rain falls and, the temperature being high, rapid growth 

 occurs. 



Records of the rainfall by months at McCleary's house haA'e been 

 kept since July 1, 1901. In June of 1909 a rain gauge was placed 

 at MacBeath's house and the records from both these stations are 

 given in Table I. A comparison of the two records by months is 

 shown in the diagram (fig. 4). 



^ See Bureau of Plant Industry Bulletin 67, pp. 38-44. 



