Precipitation in October and November (about 14 percent of annual rainfall) is quite variable from year to year and is often a result of severe storms or Pacific hurricanes that “graze” the region. These storms can produce flooding, often over large areas. These rainfall patterns are distinctive of the Sonoran Desert and explain the extraordinary vegetation of the area. The Mohave Desert to the west of Tucson does not receive as much summer rain, and the Chihuahuan Desert to the east gets less winter rain than the Sonoran Desert.

The year-to-year variation of precipitation in the Tucson Basin is quite substantial. (See Figure 1- 4.) Global phenomena such as El Niño and La Niña affect the distribution and magnitude of precipitation. Winter precipitation in 1992/93 and 1997/98 was as much as 55 percent higher than the winter average in the Tucson Basin.

Figure 1-4 Climatological factors 1987-1998.Source: U.S. National Weather Service

Evapotranspiration

Clear skies and a relatively low latitude make Tucson one of the warmest areas in the United States. Average summer highs are in the upper 90s with peaks above 110° F. These high temperatures, along with low relative humidity, contribute to very high water loss through evapotranspiration. (Evapotranspiration is the combined effect of surface evaporation and transpiration by plants.) The potential evapotranspiration rate averages about 77 inches per year which is about 6.5 times greater than the approximate total annual precipitation in the area. Most of the precipitation that falls in summer storms evaporates without being used by plants or people or being recharged into the aquifer.

Figure 1-5 Location of cites along 30 degree "arid one"