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Climate of the Southeastern Arizona Planning Area

Annual average precipitation in the planning area is 14.7 inches, with over 52% coming in July, August, and September (Figure 3.0-6).  This planning area receives the most summer precipitation in the state because of its proximity to the core monsoon region in Mexico.  The monsoon is strongest in northwestern Mexico, and Arizona usually only receives the northernmost fringes of precipitation.  Pool and Coes (1999) noted that trends in seasonal precipitation at four stations in the southern half of the Upper San Pedro Basin showed a general trend of increasing winter precipitation and decreasing wet-season (summer) precipitation during the period 1956-1997.

Figure 3.0-6 Average Monthly Precipitation and Temperature in the Southeastern Arizona Planning Area, 1930-2002

Monthly Precipitation and Temperature

Data are from selected Western Regional Climate Center cooperative weather observation stations.

(http://www.wrcc.dri.edu/summary/climsmaz.html). Figure author: CLIMAS.

Summer precipitation from thunderstorms is less hydrologically efficient than winter precipitation, because monsoon storm cells are spatially discontinuous and high summer temperatures result in high evaporation rates.  About 35% of planning area precipitation occurs during winter months (November – April), mostly from frontal storm systems.  At higher elevations, this precipitation falls as snow.  Slow water release from high elevation spring snowmelt and low evaporation rates make winter precipitation more hydrologically efficient because there is less runoff and greater gain to streams.

As in other areas of Arizona, precipitation is extremely variable, both spatially and from year to year.  For example, during the 2005-2006 winter, the planning area received 6.3 inches less precipitation than during the 2004-2005 winter.  This variability can also be observed on longer time scales.  The 1950s were a relatively dry decade with an average annual precipitation deficit of -1.46 inches, while the 1980s were a relatively wet decade with an average annual precipitation surplus of 1.86 inches (Figure 3.0-7).  Annual average temperature in the planning area is 61.6° F, compared to the statewide average of 59.9° F.  As in other parts of Arizona, temperatures have been increasing the past several decades. Temperature observations are consistent with global temperature trends; however, some warming may be attributed to changes in land-cover resulting from population growth.

Figure 3.0-7 Average Temperature and Total Precipitation in the Southeastern Arizona Planning Area from 1930-2002

Average temperature and total precipitation

Horizontal lines are average temperature (61.6 F) and precipitation (14.7 inches), respectively.  Light lines are yearly

values and highlighted lines are 5-year moving average values.  Data are from selected Western Regional Climate

Center Cooperative weather observation stations. Figure author: CLIMAS

Winter precipitation records dating to 1000 A.D. reconstructed from tree rings show extended periods of above and below average precipitation in every century (Figure 3.0-8) in the area encompassed in Climate Division 7, which includes the planning area and parts of others.  A climate division is a region within a state that is generally climatically homogeneous. Arizona has been divided into seven climate divisions. These decadal and shorter time period shifts are related to circulation changes in the Pacific Ocean.  On time scales of 10-30 years, precipitation variability is likely related to shifts in Pacific Ocean circulation patterns, such as the El Niño-Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO).  On time scales of 2-7 years, the ENSO, with its phases of El Niño and La Niña, is associated with precipitation variations in the region, most notably during winter months (November-April).  During El Niño episodes, there are greater chances for above-average winter precipitation, while La Niña conditions are usually associated with below-average winter precipitation.  However, El Niño winters can also produce below-average precipitation.  Generally, La Niña conditions are associated with drought in the region. The ENSO phases also impact precipitation and monsoon strength in the region.

Figure 3.0-8 Arizona NOAA climate division 7 (southeastern Arizona; Graham, Greenlee, Cochise, Santa Cruz and Pima Counties)

winter (November-April) precipitation departures from average, 1000-1988

Precipitation departures from average

Data are presented as a 20-year moving average to show variability on decadal time scales. Values

shown for each year are centered on a 20 year period. The average winter precipitation for 1000-1988 is 4.9 inches.

Data: Fenbiao Ni, University of Arizona Laboratory of Tree-Ring Research and CLIMAS. Figure author: CLIMAS.

 

For additional information on Climate in individual basins in the Southeastern Plateau Planning area see Climate on the menu to the right.

 

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