Chaparral Shrublands

Climate

The Central Arizona Highlands, similar to other areas in the state, are characterized by a cyclic climatic regime. Winters in the chaparral type are cool and wet through March, followed by warm, dry weather for 3 or 4 months until summer rains start in July. Dry weather usually returns by October and persists until winter rains begins in November or December.

Precipitation

Annual—The climate is semi-arid with average annual precipitation of about 26 inches (30-yr average); 63 % occurs from October through May, and37 % from June through September (Davis 1993). About a third of the winter precipitation falls as snow. The 2 driest months are May and June, which are followed by the summer monsoonal months of July through September, when thunderstorms develop as moist air from the south moves into the area. Summer storms are often intense, but produce less streamflow than the larger, less intense winter storms of December through April, which yield about 90 % of the annual streamflow (nearly 50 % occurs during February and March). Flow is intermittent under natural conditions.

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) (Fig. 5). The proximity of mountains and other physiographic factors that control flow and cooling of air combine with elevation to produce the local climate. Some snow falls each year, but normally snow is not an important factor in the hydrology of the chaparral type (Hibbert et al 1974). Annual potential evapotranspiration rates can approach 35 inches (Hibbert 1979).

Although mean annual precipitation is commonly used to describe the moisture regime of an area, the mean indicates nothing about the frequency and seasonal distribution of rain, and little about the magnitude of dry and wet years, critical factors in the survival and maintenance of plant cover. Available data suggest that the driest years receive about one-half the mean precipitation while the wettest years get about double the mean (Hibbert et al 1974). Thus, as little as 8 inches of rain may fall in dry years while in very wet years the wetter sites might received as much as 50 inches.

Winter—About 55 % of the annual precipitation falls as rain or snow between November and April. Winter moisture usually comes from the Pacific as frontal-type storms which become heavier as the moist air is forced over the central highlands (Hibbert et al 1974). These storms can produce several inches of water in 24 hours, but their intensity is relatively low.

Summer— Summer moisture from the Gulf of Mexico in the form of local convective storms accounts for about 35 % of the annual rainfall during July, August, and September. These storm are often intense, dropping as much as 3 inches of water in an hour. The remaining 10 % of the annual precipitation comes in May, June, and October. The water year used in the chaparral type is calculated from the first of July to coincide with the end of a 2- to 3-month dry period when the moisture regime varies the least from year to year.

Variability—Winter precipitation is more variable than summer precipitation in both amount and time of occurrence from year-to-year. However, yearly variations in precipitation generally decrease with increases in elevations. 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.