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Lower Colorado River Planning Area Climate

The Lower Colorado River Planning Area is characterized by the highest average annual temperature in the state, 71.5°F, which is much warmer than the statewide average of 59.5°F.  Average annual precipitation in the planning area is 4.6 inches, though totals are considerably higher in mountainous areas where precipitation is not recorded.  Annual precipitation totals vary widely across the planning area, from 6-9 inches at Organ Pipe Cactus National Monument, Aguila, and Kofa Mine stations to less than 3 inches at Yuma Airport. On average, the Lower Colorado River exhibits the bi-modal precipitation seasonality characteristic of Arizona (Figure 7.0-6); however, the northwestern part of the planning area, near Parker, exhibits a stronger late winter peak, more typical of the Mohave Desert.

Figure 7.0-6  Average Monthly Precipitation and Temperature, 1930-2006

Figure 7.0-6

Data are from the Western Regional Climate Center. Figure author: CLIMAS.

Frontal storm systems moving west-to-east, guided by the jet stream, deliver the area’s winter and spring precipitation.  Summer monsoon thunderstorms deliver abundant moisture to the eastern part of the Lower Colorado River Planning Area.  The planning area shows a very strong response to El Niño conditions, with winters registering wet conditions 59% of the time and dry conditions only 24% of the time.  Strong El Niño years, such as 1941, 1982, 1983, 1992 and 1993, show high precipitation (Figure 7.0-7).  The precipitation response to La Niña conditions is not as pronounced with dry winters occurring only 50% of the time.  Neutral El Niño-Southern Oscillation conditions yield dry planning area winters 57% of the time – a strong indication of the extreme aridity in this region.

Average annual temperatures in the Lower Colorado River Planning Area have been increasing since the 1930s, and especially rapidly since the mid-1970s (Figure 7.0-7). The long-term trend is superimposed on decadal variability generated primarily by Pacific Ocean and atmosphere variations. Decadal variations are particularly obvious in the instrumental record of precipitation. Drought conditions were present for the decades of the 1940s-1960s and since the mid-1990s; the 1980s and early 1990s were relatively wet. This part of the state exhibits Arizona’s highest year to-year precipitation variability, with especially high variability during the dry 1940s-1960s.

Figure 7.0-7 Average Annual Temperature and Total Annual Precipitation for the Lower Colorado River Planning Area from 1930-2002

Figure 7.0-7

Horizontal lines are average temperature (71.5 °F) and precipitation (4.6 inches), respectively. Light
lines are yearly values and highlighted lines are 5-year moving average values. Data are from the
Western Regional Climate Center. Figure author: CLIMAS.

Winter precipitation records dating to 1000 A.D. estimated from tree-ring reconstructions for Arizona climate divisions show extended periods of above and below average precipitation in every century (Figure 7.0-8).  A climate division is a region within a state that is generally climatically homogeneous.  Arizona has been divided into seven climate divisions and most of the Lower Colorado River Planning Area is within Climate Division 5, which includes La Paz and Yuma counties.  Markedly dry periods in Climate Division 5 include the late 1000s, mid-1100s, the late 1200s, late 1500s, and several shorter, but very intense, periods during the last 300 years. Winters were relatively wet during the late 1400s, early 1600s, much of the 1800s, and the early 1900s.

Figure 7.0-8 Winter (November - April) Precipitation Departures from Average 1000-1988 (Climate Division 5)

Figure 7.0-X

Data are presented as a 20-year moving average to show variability on decadal time scales. Data:
Fenbiao Ni, University of Arizona Laboratory of Tree-Ring Research and CLIMAS. Figure author:


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