FARM ORGANIZATION IN SOUTHERN ARIZONA. 5 
mitigated by a very dry atmosphere, and sunstroke is unknown. The 
nights are always cool, a range of 50° between day and nighl tem- 
peratures being not uncommon during the summer months. The 
following table shows the highest, lowest, and mean temperatures 
for each of the months during 21 years at Phoenix, which is approxi- 
mately the center of the irrigated section: 
Table 1. — Temperature data at Phoenix, Ariz, for 21 years, 1896 to 1916, inclusive. a 
Jan. 
Feb. 
Mar.- 
Apr. 
May 
June 
July 
Aug. 
Sep. 
Oct. 
Nov. 
Dec. 
o F _ 
81 
16 
. F 
91 
24 
°F. 
94 
31 
°F. 
102 
37 
o F 
114 
39 
°F. 
116 
49 
°F. 
117 
63 
o F 
113 
62 
°F. 
112 
49 
°F. 
100 
II 
°F. 
92 
28 
° F. 
80 
22 
50.0 
54.4 fiO.5 
66.6 
74.8 
84.8 
90.4 
89.0 
81.4 
70.2 
58 7 Kl Q 
o U. S. D. A. Wo.ither Bureau, Summary of Climate-logical Data for the United States by sections. Sec- 
tion III, and hi I or annual reports. 
The rainfall averages about 7.27 inches per year. Most of the 
precipitation occurs during two short rainy seasons, one in July and 
August and the other during the winter months. The summer rains 
; are entirely local in origin and character, while the winter rains are 
, of a more general nature. The rains are seldom copious enough to 
take the place of irrigation, and crop production depends entirely 
upon irrigation for its water supply. 
There is abundant evidence that this valley was once irrigated by 
ancient peoples, probably the ancestors of the semicivilized tribes of 
Indians now living in or near the valley. Old canals aggregating 
150 miles in length have been located and it is estimated that at 
I least 140,000 acres were under cultivation at that time. Much of 
I the adobe and other heavy surface soils now found at different places 
l in the valley probably were deposited from muddy water used for 
irrigation. 
The modern development of the valley began in 1867 with the 
, construction of the Swilling Canal. Between this date and 1892 a 
: dozen or more canals were constructed which were about equally 
\ distributed between the north and the south sides of the river. 
With but few exceptions the dams and headgates for these canals 
, were of a temporary nature and were washed out with every flood. 
The dam for the Arizona Canal (see fig. 2), the largest one in the 
, valley, was the only one which had any semblance of permanency, 
and it was partly washed out in 1891 and completely destroyed by 
the floods of 1905. These conditions resulted in chronic water 
famine. When the river was full of water there were no dams to 
divert it to the land, and when there were dams in the river there 
was not enough water to irrigate all the lands under the canals. 
Water rights for 151,360 acres of land were adjudicated in 1889, 
