90 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
at two depths—for the hill, wash, and river-plain at 15 and 30 cm., and 
for the Larrea slope at 10 and 20 cm. ‘The localities of the four stations 
are shown on the map, plate 26, where the hill station is marked I, that 
for the Larrea slope II, that for the wash III, and that for the river-plain 
IV. All samples for the same station were taken within the same area 
of a few square meters, the small excavations made in obtaining them 
being immediately refilled, and no later sample being taken within 50 cm. 
of the soil thus disturbed. On December 13 it was found necessary, 
on account of irrigation water, to remove the station for the river-plain 
from the position marked IV, on the map to that marked IV,. Otherwise 
the stations remained fixed throughout the period. The deeper sample 
in the wash was considerably more sandy than the shallower one. After 
December 3, 1907, the samplings and moisture determinations were 
performed by Mr. J. C. Blumer, to whose careful work the series of obser- 
vations is fundamentally due. 
Records for Tucson, extending over a period of 15 years (Publication 
No. 6 of the Carnegie Institution of Washington, pp. 26-27, or Publication 
No. 50, p. 23) show a mean annual precipitation of 30.10 cm. (11.74 in.), 
occurring mainly in two rainy seasons. The mean monthly precipita- 
tions, together with the actual precipitations for the months from July, 
1907, to March, 1908, are given in table 4. 
TABLE 4.—Mean Monthly Precipitations at Tucson, from Records of 15 years, and observed 
Precipitations for the Months, July, 1907, to March, 1908. 



Month. Mean. 1907. Month. Mean. 1908. 
cm. in. cm. mM. cm. nN. cm. mM. 
July. 2. se ch Oe15.| 2.40411, 9.54267 My January: oe) 625030100 790 ia ban ee oo 
AUZUSts. . 6 6.67 | 2.60 | 9.32 | 4.67 || February...) 2.31 | 0.90] 4,78 \41..90 
Septembers 2067 f Gic t64G 294. | toe Marcht@ no ©. 977) 02777) 1-359 os 
October 2... wit «64 OLG4 ee. AL 0505 eh Dill eae @:60 WOSe7 aS 2 Crises 
November. -] 2.08 |0.81 171-88") @.74 || May O 26500 214 Ale ieee nee 
Decemberay) h2556" (05300 5\0'.001) O00 i Juneraa tee, O867> 110 326 ieee. t iene 












The summer rains began on July 3, 1907, with a shower amounting 
to 0.08 cm. (0.03 in.). The first effective shower, however, occurred 
on July 7, at which time 1.52 cm. (0.60 in.) of rain fell. After this the 
rains were well distributed throughout the remainder of July and August. 
It is apparent from the table that, until February, the period of the soil 
observations was exceedingly dry, although of course not as dry as is the 
ustial spring and early summer. Before the beginning of the soil records, 
the last effective shower occurred on September 3, amounting to 0.53 cm. 
(0.21 in.), and on September 17 occurred a shower of 0.12 cc. (0.07 in.). 
The rain-gage is located only a few meters from the station for soil 
observations on the hill, and there is no question as to the applicability 
of its records to the soil data for that station. Owing to the local nature 
of many showers in this region, it is possible that the precipitation recorded 
