Climate and Vegetation Types of the Central Arizona Highlands
Climate is essentially the condition of the weather resulting from the
interactions between air temperature, precipitation, and land form. Climate
has a major influence over plant distribution (Figure 1) and the characteristic
of streamflow generation in the area. The climate of the southwestern
United States is a cycle of winter precipitation, spring drought, summer
precipitation, and fall drought. Winter precipitationmoisture received
between October and Aprilusually comes from a northwesterly directions
from the Pacific Ocean. This precipitation is associated with frontal
or large regional storms falling as snow at elevations above 6,000 feet
and mainly as rain below the 3,000 foot level. Winter precipitation
is responsible for 80 to 95% of the annual streamflow produced in the
Riparian EcosystemsClimatic characteristics
of riparian ecosystems exhibit a wide range of conditions due to large
elevational differences and distributions of associated mountain ranges
and highlands. The key characteristic of the riparian system is the availability
of water throughout the year or at least during the growing season.
Riparian ecosystems occur in elevations from near sea level up to 10,000
feet and occupy about 0.1 million acres in Arizona. Water yield averages
from 6 to 24 inches depending on precipitation, elevation, soils, and
Riparian areas provide habitat for a numerous variety of wildlife species,
grazing for livestock and wildlife, and areas for fishing, hiking, bird
watching, picnicking, and camping.
High elevation riparian area
Low elevation riparian area
Mixed Conifer ForestsMixed conifer
forests grow between elevations of 7,000 to 10,000 ft and occupy about
0.4 million acres in Arizona. Annual precipitation in the high elevation
mixed conifer forests (above 9,500 feet) ranges from 30 to 45 inches and
is normally in excess of potential evapotranspiration—the amount of water
required by plants to grow normally. As a result of this excess precipitation,
streams originating in this area are often perennial—flow year round.
Streams originating in the lower elevation mixed conifer forests (8,000
to 9,500 feet) are mostly intermittent. Snowmelt runoff is the primary
source of annual runoff.
Mixed Conifer Forests: Water yield from natural forest stands range from
3 to 5 inches depending on precipitation, elevation, soils, and overstory
density. Potential increases in water yield of 3 to 4 inches have been
obtained by manipulating the forest overstory with 1.5 inches expected
if one maintains one-third of the watershed in openings for site with
4 to 5 inches of normal water yield. See Mixed
Conifer Treatments and Results for more information.
Mountain grasslands ranges from 30 to 45 inches of annual precipitation,
with 50% occurring during the summer season. The general weather patterns
of these areas coincide with those of the adjacent forests.
Mountain Grassland site
Plains grasslands average about 17 inches of average annual
precipitation, with extremes of approximately 10 and 20 inches. About
70% of the total annual precipitation falls between April and September.
Desert grasslandsThe climate of the desert grasslands
is generally characterized by warm to hot summers, and by mild and open
winters. This grassland type is the most arid of all North American
Plains Grassland site
Desert Grassland site
Precipitation varies from 1 to 12 inches annually, averaging about 6
inches. Most of the precipitation occurs as liquid rainfall during two
seasons, summer and winter. Summer rains are largely in the form of thunderstorms,
with high-intensity events, with rainfall occurring intermittently for
several days. Normally, relative humidities are low throughout the year,
except during the storm periods.
Temperatures and wind velocities are often high in the desert grasslands,
and evaporation rates are high. The high temperature and wind velocities,
coupled with the low humidity, account for the comparatively high evaporation
Ponderosa Pine ForestsThe ponderosa pine
forest type is found between elevations of 6,000 and 9,000 feet on the
Mogollon Plateau and on scattered "sky island" areas in the
Southwest and occupy nearly 6 million acres in Arizona. High rates of
water use by plants and low soil moisture can often curtail the growth
of plants in the ponderosa pine forests, which receive 20 to 30 inches
of annual precipitation. Normally, little summer rain is converted into
streamflow. Winter precipitation is the major source of runoff.
Ponderosa Pine Forests: Water yield from natural forest stands range from
2 to 6 inches depending on precipitation, elevation, soils, and overstory
density. As you go higher in elevation, you generally get more precipitation,
and conditions are more favorable for less evapotranspiration loss. Potential
increases in water yield of 0.1 to 1.0 inch can be expected by manipulating
the forest overstory (1 to 3 inches in the short term, 3 to 10 years,
have been measured (Baker, 1986)). See Ponderosa
Pine Treatments and Results for more information.
Pinyon-Juniper WoodlandsThe coniferous
woodlands or pinyon-juniper types generally lie below the ponderosa pine
forests, at elevations of 4,500 to 7,500 feet and occupies 19.9 million
acres in Arizona. Again, there are wide fluctuations in weather patterns
throughout the pinyon-juniper woodlands. Annual precipitation varies from
12 to 24 inches. Winter precipitation is usually rain with occasional
snow. Evapotranspiration (evaporation from soil, leaves, and needles)
rates are relatively high in the growing season. Only during the coldest
months of December through February is precipitation greater than the
potential evapotranspiration rates. Streamflow is typically ephemeral—flowing
only in the winter season during periods of precipitation excess.
Pinyon-Juniper Woodlands: Water yield from natural stands ranges from 1 to
3 inches depending on precipitation, elevation, soils, and overstory density.
Potential increases of 0.5 inch or less can be expected from the pinyon-juniper
woodlands. See Pinyon-Juniper
Woodlands Treatments and Results for more information.
Thermal heating during winter storms is less pronounced than typically
occurs during summer storm events. Upslope air movement is relatively
slow, cloudiness is common, and precipitation tends to fall over a large
area and at relatively low rainfall rates. These conditions have
major implications on energy available to produce soil erosion and movement
Chaparral ShrublandsChaparral shrublands
occupy 3.5 million ac in Arizona. Mean annual precipitation ranges from
16 inches at the lower limits of the chaparral (3,000 ft) to over 25 inches
on the wetter sites (6,000 ft). About a third of the winter precipitation
falls as snow. Summer storms are often intense, but produce less streamflow
than the larger, less intense winter storms, which yield about 90% of
the annual streamflow. Flow is intermittent under natural conditions.
Water yield ranges from 1 to 4 inches depending
on precipitation, elevation, soils, and overstory density. Potential increase
from less than 1 up to 5 inches have been measure as the result of vegetation
conversion or overstory density reduction. See Chaparral
Shrublands Treatments and Results for more information.
Lower Density Chaparral site
Higher Density Chaparral site
Desert ShrublandsThe diversity of vegetative
cover found within the desert shrub cover type would seem to make it difficult
to characterize the climatic features of the area. Temperature is not
necessarily a determining factor in the production of vegetation on a
desert. High temperatures may accelerate the loss of water, as, conversely,
continued temperatures below the physiological activity of the plants
may produce drought conditions for the plants. Moisture, is the determining
factor of production, with its more significant attribute being the distribution
of the annual supply.
Average precipitation in the northern desert shrub type is about 10 inches
annually, with a general variation of 5 to 14 inches. Annual precipitation
in the southern desert shrub type varies from 3 to 12 inches, but averages
approximately 6 inches.
The climatic conditions associated with the desert shrub type are extreme:
high temperatures are combined with relatively low annual precipitation
amounts. Additional physiological stress is placed upon the vegetation
by the fact that the moisture supply is not evenly distributed throughout
the growing season.
Desert Shrubland site
Desert Shrubland site
Although winter precipitation accounts for only 50 to 60% of the annual
precipitation in the southwestern United States, it is responsible
for 80 to 95% of the annual streamflow produced in the region.
Winter precipitation—moisture received between October and April—usually
comes from a northwesterly directions from the Pacific Ocean. This precipitation
is associated with frontal or large regional storms falling as snow at
elevations above 6,000 feet and mainly as rain below the 3,000 foot level.
Because winter precipitation normally occurs as low intensity rain or
snow fall and streamflow is the product of either low intensity rainfall
or snowmelt, erosion potential is energy limited and the dominant parent
materials often limits the supply of sediment (e.g., heavy clay soil characteristic
of volcanic-derived soil) or the transport of the dominant sediment size
particles unless runoff is substantially concentrated (e.g., coarse textured
soils derived from sedimentary and granitic parent materials).
The major source of moisture for summer storms is the Gulf of Mexico.
This moisture moves into Arizona from the southeast, passes over highly
heated and mountainous terrain, and thermal heating causes it to rise
rapidly, cool, and condense. Summer storms, therefore, are primarily
convectional, often intense, and usually local or small in size (a square
mile or less) rather than being widespread, regional storms. This characteristic
influences the amount and distribution of energy available for producing
erosion. Summer rains typically begin in early July, breaking the
prolonged spring drought and providing relief from the hot weather of
June and July.
AnnualVariation in annual precipitation is probably the
most important characteristic governing the climate of the Southwest.
Winter precipitation is more variable than summer precipitation in both
amount and time of occurrence from year-to-year. However, yearly variations
in precipitation are generally less at higher elevations than at the lower
SeasonalSpring droughtMay and Juneis often more severe
than the fall droughtOctober-on most plants and animals in the region.
This is due largely to the higher air temperatures and the higher activity
levels of the plants and animals in the spring.