496
BERNARD L. FONTANA
has seen in a half a decade [see Shreve and Wiggins (1964, p. 21) for
a rainfall map and Hastings and Turner (1965, p. 11) for a detailed and
up-to-date discussion of the problem].
In spite of the variability, however, the threefold division of Pimeria
Alta based on the areal distribution of water, western, central, and riverine,
holds up extremely well (see Fig. 2, showing rainfall lines and villages).
1. The Arid West
a. Climate. The southwestern corner of Arizona and the northwestern
corner of Sonora are among the driest, hottest places in America. San Luis,
situated at the northwestern point of Sonora, received only 0.01 cm of rain
in 1956, and its mean annual rainfall for the years 1963-1967 was a
parched 2.3 cm, or 0.9 inches (Hastings, 1964b, Hastings and Humphrey,
1969b, p. 68). In Arizona, and moving from west to east across western
Pimeria Alta, Yuma gets about 8.84 cm annual rainfall; the Lechuguilla
Desert, 10.9 cm; Tinajas Altas, 12.9 cm; Tule Tank, 10.5 cm; Agua Dulce,
17.5 cm; and Growler Pass, 17.5 cm (Turnage and Mallery, 1941, pp.
4-5). In the Pinacate Plateau in Sonora the annual rainfall is 14.2 cm.
At Quitovac and Sonoita, both also in northwestern Sonora, the mean annual rainfall for the years between 1963-1967 was 28.6 cm and 19.6 cm,
respectively (Hastings and Humphrey, 1969b, pp. 29 and 33).
The rain which falls in the Sonoran Desert tends to come during two
seasons: summer and winter. The periods of greatest precipitation are
July-August and December-January. The summer "monsoons" are convective storms, often no more than 1 mile in diameter, and the rain from
these is widely scattered. Even so, the summer rains, which move in from
the Gulf of Mexico heading west, provide dependable sources of water
year in and year out. The winter rains, which are widespread and cyclonic
in nature, come to the Sonoran Desert from the Pacific (Hastings and
Turner, 1965, pp. 11-12).
This biseasonal nature of precipitation is crucial with regard to plant
growth. In western Pimeria Alta, about half the rain falls in winter and
half in summer, with winter slightly favored. The percentage of summer
rainfall increases moving from west to east and from south to north over
the region (Hastings and Turner, 1965, pp. 13-15.).
Just as aridity is greater in the west, so is the temperature. At Yuma,
temperatures ranging from —5.5° to 49°C over a 65-year period have
provided a yearly average temperature of 23.9°C. At Ajo, Arizona, at the
eastern edge of this western zone of Pimeria Alta, it is cooler, 21.9°C
average annual temperature, with 44-year extremes of —8.3° to 46.1°C.
The highs and lows are almost consistently reached in July and January,
BERNARD L. FONTANA
has seen in a half a decade [see Shreve and Wiggins (1964, p. 21) for
a rainfall map and Hastings and Turner (1965, p. 11) for a detailed and
up-to-date discussion of the problem].
In spite of the variability, however, the threefold division of Pimeria
Alta based on the areal distribution of water, western, central, and riverine,
holds up extremely well (see Fig. 2, showing rainfall lines and villages).
1. The Arid West
a. Climate. The southwestern corner of Arizona and the northwestern
corner of Sonora are among the driest, hottest places in America. San Luis,
situated at the northwestern point of Sonora, received only 0.01 cm of rain
in 1956, and its mean annual rainfall for the years 1963-1967 was a
parched 2.3 cm, or 0.9 inches (Hastings, 1964b, Hastings and Humphrey,
1969b, p. 68). In Arizona, and moving from west to east across western
Pimeria Alta, Yuma gets about 8.84 cm annual rainfall; the Lechuguilla
Desert, 10.9 cm; Tinajas Altas, 12.9 cm; Tule Tank, 10.5 cm; Agua Dulce,
17.5 cm; and Growler Pass, 17.5 cm (Turnage and Mallery, 1941, pp.
4-5). In the Pinacate Plateau in Sonora the annual rainfall is 14.2 cm.
At Quitovac and Sonoita, both also in northwestern Sonora, the mean annual rainfall for the years between 1963-1967 was 28.6 cm and 19.6 cm,
respectively (Hastings and Humphrey, 1969b, pp. 29 and 33).
The rain which falls in the Sonoran Desert tends to come during two
seasons: summer and winter. The periods of greatest precipitation are
July-August and December-January. The summer "monsoons" are convective storms, often no more than 1 mile in diameter, and the rain from
these is widely scattered. Even so, the summer rains, which move in from
the Gulf of Mexico heading west, provide dependable sources of water
year in and year out. The winter rains, which are widespread and cyclonic
in nature, come to the Sonoran Desert from the Pacific (Hastings and
Turner, 1965, pp. 11-12).
This biseasonal nature of precipitation is crucial with regard to plant
growth. In western Pimeria Alta, about half the rain falls in winter and
half in summer, with winter slightly favored. The percentage of summer
rainfall increases moving from west to east and from south to north over
the region (Hastings and Turner, 1965, pp. 13-15.).
Just as aridity is greater in the west, so is the temperature. At Yuma,
temperatures ranging from —5.5° to 49°C over a 65-year period have
provided a yearly average temperature of 23.9°C. At Ajo, Arizona, at the
eastern edge of this western zone of Pimeria Alta, it is cooler, 21.9°C
average annual temperature, with 44-year extremes of —8.3° to 46.1°C.
The highs and lows are almost consistently reached in July and January,