Climate of the Beaver Creek Watershed

The Beaver Creek Experimental Watershed (BCEW), similar to other areas in the state, is characterized by a cyclic climatic regime of winter precipitation, spring drought, summer precipitation, and fall drought. Precipitation usually comes from the northwest in the winter and from the southeast in the summer. Winter precipitation, often snow at higher elevations, is associated with frontal storms moving into the region from the Pacific Northwest. Surface thermal heating in the winter is less pronounced than in the summer; upslope air movement is relatively slow; cloudiness is common; and precipitation is usually widespread and relatively low in intensity.

Winter scene

The major source of moisture for summer rains is the Gulf of Mexico. This moisture moves into the Highlands from the southeast, passes over highly heated and mountainous terrain, rises rapidly, cools, and condenses. Summer storms, primarily convectional, are often intense and local rather than widespread. Summer rains typically begin in early July, breaking the prolonged spring drought and provides relief from the hot weather of June and July.

Winter precipitation is more variable than summer rainfall in amount and time of occurrence from year-to-year. However, yearly variations in precipitation generally decreases with increases in elevation. Spring drought is often more detrimental to most plants and animals in the region than the fall drought, due to the higher temperatures and wind conditions during the beginning of the growing season. There are definite relationships between precipitation, elevation, and the type of vegetation in the Southwest.

Relationship between precipitation, elevation, and vegetation

High transpiration rates and soil moisture deficiencies can curtail the growth of plants in ponderosa pine forests, which receive 500 to 750 mm (20 to 30 inches) of annual precipitation. These high elevation forests tend to have greater frequencies and amounts of precipitation than the lower elevation woodlands. Usually only a small amount of summer rain is converted into streamflow. Winter precipitation accounts for 80 to 95% of the annual runoff.

Histogram of monthly precipitation and runoff in the pine

There are wide fluctuations in weather patterns throughout the pinyon-juniper woodlands. Annual precipitation varies from 300 to 600 mm (12 to 24 inches). Winter precipitation is usually rain with occasional snow. Evapotranspiration rates are relatively high in the growing season. Only during the coldest months of December through February is precipitation greater than the evapotranspiration rates.

Histogram of monthly precipitation and runoff in the pinyon-juniper

The Beaver Creek watershed lies in the north-central area of Arizona in what is called the plateau climatic region, which includes the Kaibab, Coconino, and Mogollon Plateaus and the San Francisco and White Mountains. January temperature in the region averages -3^oC (26^o F) and the July temperatures 19 ^o C (66^o F). Average annual temperature in the pine type has been 7^oC (45^o F) and 13 and 10^oC (56^o F and 50^o F), respectively, in the Utah and alligator juniper woodlands.

Monthly mean temperatures by vegetation type

The estimated annual evapotranspiration (ET) is 430 mm (17 inches) in the Utah juniper, 400 mm (16 inches) in the alligator juniper, and 500 mm (20 inches) in the ponderosa pine type, determined by difference between annual precipitation and streamflow. This calculation is a reasonable approach for BCEW because deep seepage is minimal and soil-water deficits are similar each year at the end of September (end of the water year).

The average growing season at Montezuma Castle (near Camp Verde) is a period of 210 days. The average date of the latest frost in spring is April 4; the latest recorded date of a killing frost in spring is May 13. The average date of the earliest frost in the fall is October 31; the earliest recorded date of a killing frost in the fall is October 13.

The average growing season at Flagstaff is a period of 121 days. The average date of the latest frost in spring is May 30; the latest recorded date of a killing frost in spring is June 18. The average date of the earliest frost in the fall is September 28; the earliest recorded date of a killing frost in the fall is September 12.

Selected References on Climate

Baker, M.B., Jr. 1982a. Hydrologic regimes of forested areas in the Beaver Creek watershed. USDA Forest Service, General Technical Report RM-90.

Beschta, R. L. 1974. Climatology of the ponderosa pine type in central Arizona. Arizona Agricultural Experiment Station, Technical Bulletin 228.

Campbell, R.E.; Ryan, M.G. 1982. Precipitation and temperature characteristics of forested watersheds in central Arizona. USDA Forest Service, General Technical Report RM-93.

Campbell, R.E.; Stevenson, O.G. 1977. Solar radiation measurement in northern Arizona. USDA Forest Service, Research Note RM-339.

McAda, D.P. 1978. Indexing solar radiation by clouds for snowmelt modeling. MS Thesis, University of Arizona, Tucson, Arizona.

McAda, D.P.; Ffolliott, P.F. 1978. Solar radiation as indexed by clouds for snowmelt modeling. Hydrology and Water Resources in Arizona and the Southwest 8:175-181.

McAda, D.P.; Ffolliott, P.F. 1987. Predicting solar radiation from cloud cover for snowmelt modeling. Hydrology and Water Resources in Arizona and the Southwest 17:29-34.

Solomon, R.M.; Ffolliott, P.F.; Thompson, J.R. 1976. Correlation between transmissivity and basal area in Arizona ponderosa pine forests. USDA Forest Service, Research Paper RM-318.

Williams, J.A.; Anderson, T.C., Jr. 1967. Soil survey of Beaver Creek area, Arizona. USDA Forest Service, Soil Conservation Service, and Arizona Agriculture Experiment Station.