Ecoregion Definition: Seasonal Distribution of Rainfall

Sonoran Desert (Arizona Uplands division): biseasonal rainfall (approaching equal quantities of summer & winter rains); more rain

Chihuahuan Desert: rainfall 70-80% in summer (mid-june to mid-september); less rain

In what follows, we make the data err in the direction of a "Chihuahuan" bias, measuring as "summer rain" everything falling from the first of May through the end of September. (Shreve, Brown, and Turner each give somewhat differing specifications month-by-month of this seasonal contrast.)

The two Cascabel stations we consider here lie along the San Pedro River. Their locations straddle the confluence of Hot Springs Canyon Wash with the San Pedro River.


Cascabel Rainfall: the Teran Wash Confluence (Milepost 20) (years 1971-2000)

Summer (May through September):


Winter: October through May:


(51.4% summer rain)

Total Rain per year: 14.39"

Cascabel Rainfall: near El Potrero (Milepost 26) (years 1983-2003)

Summer (May through September):


Winter: October through May:


(50.3% summer rain)

Total Rain per year: 15.57"

Both of these stations show averages very close to a 50/50 "biseasonal rainfall" pattern, so it seems fair to conclude that the Cascabel area (centering around the confluence of Hot Springs Canyon Wash with the San Pedro River) displays a "Sonoran Desert" rainfall pattern (at least over the past 30 years). Note also that the total amounts are high, even for the Arizona Uplands. (The Cascabel pattern may be somewhat skewed toward winter by the fact that 1983, the first year of record, had a major flooding rainfall event in October.)

When we move southward (and upward in elevation) to Benson, AZ, in contrast, we see a rainfall pattern -- both in seasonality and amount -- shifted more in the direction of the "Chihuahuan" ecoregion (note: this set strikes an average based on more than 80 years):

Benson Rainfall (years 1894 to 1975)

Summer (May through September)


Winter (October through May)


(64% summer rain)

Total Rain per year: 11.33"