U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 119-02

2002

The Colorado Plateau, one of the most visually captivating parts of the United States, 

covers 210,000 km2 (130,000 mi2) of Utah, 

Colorado, New Mexico, and Arizona (fig. 1). 

Geologically, the plateau contains extensive 

areas of colorful, nearly horizontal, and essentially bare sedimentary rocks that are sculpted 

into plateaus, mesas, and deeply incised canyons, of which Grand Canyon is the classic 

example. The average elevation of the plateau 

is 1,500 m (5,000 ft), with peaks that rise to 

more than 3,700 m (12,000 ft). The region is 

arid to semiarid with sparse vegetation in the 

lowlands, forests in the mountains, and lush 

vegetation along the watercourses. The spectacular scenery, diverse ecosystems, and numerous important archeological sites are protected in 30 national parks and monuments.

Ranchers, environmental organizations, and 

agencies responsible for resource management 

in the parks, monuments, and other Federal 

and state lands of the region have a need to 

understand its long-term climate and how its 

climate may change in the near future. Regional precipitation varied substantially in the 

20th century and is expected to decrease over 

the next 20 to 30 years. Scientists from diverse 

disciplines recently participated in a workshop 

to consider how landscapes and ecosystems in 

the Southwest might respond to a drier climate 

(Schmidt and Webb, 2001). A change to drier 

climate—particularly reduced winter rainfall—

would have a variety of interrelated effects 

on plateau ecosystems. These may include 

U N D E R S TA N D I N G E A R T H S U R FA C E P R O C E S S E S A N D G E O L O G Y — A K E Y T O E N S U R I N G F U T U R E P R O S P E R I T Y

Precipitation History of the Colorado Plateau Region, 1900–2000

The Colorado Plateau covers 210,000 km 2 

(130,000 mi 2) of Utah, Colorado, New 

Mexico, and Arizona. Management of this 

region’s resources requires an understanding of how its climate has varied in the 

past and may change in the near future. 

Recent studies by U.S. Geological Survey 

(USGS) and other scientists suggest that the 

region may become drier for the next 2 to 3 

decades, in a pattern similar to the drought 

of 1942–1977. The region’s population has 

increased fourfold since the mid-1950s, creating the possibility of severe consequences 

if such a drought were repeated.

reduced groundwater recharge, lower baseflow 

in perennial streams, increased frequency of 

dust storms and strong winds, weakening of 

biological soil crusts, reduction of plant cover 

and possible changes in species composition, 

remobilization of sand from previously stable 

dunes, and increased frequency of forest and 

range fires. This fact sheet discusses the 20th 

century precipitation patterns of the plateau, 

shows their relation to global climatic indices, 

and points out how precipitation may change in 

upcoming decades.

Long-Term Variability of Annual 

Precipitation

Average annual precipitation on the Colorado Plateau, based on analysis of daily 

records from 97 long-term weather stations 

(fig. 1), ranges from 136 to 668 mm/yr (5.4 

to 26.3 in/yr), with a median precipitation 

of 300 mm/yr (11.8 in/yr). Annually, precipitation varied substantially during the 

20th century (fig. 2). Three multidecadal 

precipitation regimes are apparent in the precipitation history: 1905–1941, 1942–1977, 

and 1978–1998. Although the choice of 

limiting dates for these regimes is somewhat 

subjective, the middle period was clearly 

dry and was sandwiched between two wetter 

episodes. The early part of the dry regime 

(1942–1956) was recognized as a drought 

in the 1960s (Gatewood and others, 1964), 

perhaps the severest drought in the past 400 

years in New Mexico (Swetnam and Betancourt, 1998). This drought affected much 

of the Southwest and lasted until 1977, 

although precipitation increased somewhat 

after 1956. The first 5 years of the 20th century (1900–1904) was the culmination of an 

Figure 1. Regional setting of the Colorado Plateau (white outline), showing the 97 long-term 

weather stations (yellow dots) used to document the region’s precipitation history. 

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11-year drought in the Southwest that began 

in 1893 (Gatewood, 1962).

The annual variability of precipitation is 

consistent among the stations. Wet years, defined as having precipitation above the 20th 

century regional median, typically have more 

than 50% of the stations reporting precipitation above the median. From 1942 to 1977, 

moisture was below normal at more than 50% 

of the stations for 32 of the 36 years. Excluding the 1900–1904 drought and 1999–2000, 

the relatively wet precipitation regimes (fig. 

2) were characterized by wet conditions in 18 

of 37 and 14 of 21 years, respectively. Aside 

from the severe drought of 1989, climate from 

1978 to 1998 was consistently wet compared 

with the preceding 36 years.

Seasonal Precipitation

Regional precipitation is biseasonal, occurring mostly during the cool and warm 

seasons, defined here as October 16 to April 

15 and June 15 to October 15, respectively. 

Cool-season precipitation is caused primarily 

by frontal systems originating in the eastern 

North Pacific Ocean. Warm-season rainfall is 

related to the Mexican monsoon, a seasonal 

reversal of atmospheric circulation that transports moisture over the plateau from the Gulf 

of Mexico and (or) the Gulf of California. 

Seasonal precipitation is expressed in figure 

3 as a standardized anomaly index (SAI); indices near zero indicate normal precipitation, 

while those substantially above or below zero 

indicate relatively wet or dry conditions. Seasonal precipitation has three multidecadal patterns similar to those of annual precipitation. 

The two wet regimes were characterized by 

several unusually moist seasons, whereas during the 1942–1977 dry regime neither season 

was unusually wet except the winter or cool 

season of 1973.

Colorado Plateau Precipitation and 

Global Climate

Precipitation variability on the Colorado 

Plateau is linked spatially and temporally 

with events in the tropical Pacific and northern Pacific Ocean. Specifically, episodes of 

unusually wet or dry climate result from interrelated global-scale fluctuations of sea-surface 

temperature (SST), atmospheric pressure, 

and atmospheric circulation patterns. These 

fluctuations operate on two timescales, providing an important means of understanding 

and predicting precipitation patterns. Shortterm climate variation, with a period of 4 to 7 

years, is associated with El Niño and La Niña 

activity as expressed by several indicators, including the Southern Oscillation Index (SOI) 

and equatorial SST. Multidecadal climate 

variation follows a pattern best expressed by 

the Pacific Decadal Oscillation (PDO), a phenomenon of the northern Pacific Ocean.

Short-Term Variation—El Niño and 

La Niña

The SOI is the standardized difference in 

sea-level atmospheric pressure between Darwin, Australia, and Tahiti. A sustained negative value of the SOI portends the large-scale, 

anomalous warming of SST in the tropical 

eastern Pacific Ocean. This phenomenon is 

generally referred to as El Niño, a term originally applied to the weak, seasonal (usually 

late December), warm, and south-flowing current off the coast of Peru (Trenberth, 1997). 

Warm SST in the eastern equatorial Pacific 

Ocean and sustained negative SOI indicate El 

Niño conditions, whereas cool SST and sustained positive SOI indicate La Niña conditions. The fully developed interaction between 

atmosphere and ocean is termed ENSO (El 

Niño-Southern Oscillation). El Niño conditions tend to bring wet winters to the Southwest and increased streamflow through southerly displacement of storm tracks, although 

drought may also occur, whereas La Niña 

conditions reliably bring dry winters (Cayan 

and others, 1999).

Seasonal precipitation in the Colorado 

Plateau region correlates negatively with the 

SOI (fig. 4). The strength of the correlation is 

modest, but the probability (P) that precipitation is independent of SOI is negligible (that 

is, probability of no relation is less than 0.05 

or P<0.05). Time series of ENSO activity 

and seasonal precipitation are given in figure 

5. Identification of ENSO events and their 

influence on global and regional climate is the 

subject of considerable climate research. Indeed, there is no universal definition of what 

constitutes an ENSO event (Trenberth, 1997). 

The ENSO chronology plotted in figure 5 is 

based on the SOI, the SST in the equatorial 

region of the Pacific Ocean at 5°N to 5°S, 90° 

to 150°W (called the Niño3 region), and the 

chronologies of Trenberth (1997), Ropelewski 

(1999), and the Climate Prediction Center 

(2001). In the following analysis, Colorado 

Figure 2. Average annual precipitation recorded at 97 weather stations (see fig. 1) 

on the Colorado Plateau from 1900 to 2000. Unusually wet and dry years are labeled. Horizontal lines show the average precipitation for three multidecadal precipitation regimes.

Figure 3. Standardized anomaly index (SAI) of seasonal precipitation for the Colorado Plateau. 

Blue lines are cool seasons and red lines are warm seasons. Solid circles are the 10 wettest and 

driest seasons for the period 1900–2000.

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Plateau seasonal precipitation was classified 

as ENSO (El Niño or La Niña ) or non-ENSO 

(that is, background climate) on the basis of 

the above sources and the conditions of SST 

and SOI from the midpoint to 6 months preceding a particular season.

The relation between ENSO and Colorado 

Plateau precipitation is complicated and 

contradictory when examined in detail (fig. 

5). Nevertheless, several general relations 

are statistically significant (P<0.05). During El Niño, the frequency of above-normal 

cool-season precipitation increased above 

the background climate; on the other hand, 

precipitation during La Niña is typically less 

than precipitation during El Niño. ENSO activity varies in strength, which in turn affects 

precipitation (fig. 6). During both the cool and 

warm seasons, weak El Niño conditions were 

usually associated with below-normal precipitation. The variability of warm-season precipitation during strong El Niño events exceeded 

non-ENSO conditions. Briefly, in both seasons, strong El Niño episodes increased the 

variability (warm season) of precipitation or 

the frequency (cool season) of above-normal 

precipitation relative to non-ENSO climate, 

whereas strong La Niña episodes typically 

produced normal, relatively low variability 

warm-season precipitation and typically 

below-normal cool-season precipitation. 

Long-Term Variation—The PDO

Recent and possible future climate variation related to the PDO and other ENSO-like 

indicators of multidecadal climate variability 

is a recently developed tool for climatological research. The PDO is partly related to 

SST and atmospheric pressure of the northern Pacific Ocean (Mantua and Hale, 2002). 

Changes in these parameters evidently 

trigger sharp transitions from one climate 

regime to another, altering the climate of 

North America for periods of 2 to 3 decades 

(Zhang and others, 1997). Phase shifts of the 

PDO are thought to affect the spatial connection between ENSO and precipitation in 

the Western United States (McCabe and Dettinger, 1999). The PDO is not a completely 

independent predictor of climate variability, 

because it correlates with and is dependent 

Figure 4. Seasonal precipitation (standardized 

anomaly index, SAI) for the Colorado Plateau, 

classified by El Niño-Southern Oscillation 

(ENSO) or non-ENSO influence as a function 

of the Southern Oscillation Index (SOI). Purple, 

light green, and orange symbols are El Niño, 

La Niña, and non-ENSO, respectively. SOI was 

averaged over a 7-month window centered on 

the midpoint of the season. Figure 5. Seasonal precipitation (standardized anomaly index, SAI) for the Colorado Plateau compared 

with El Niño-Southern Oscillation (ENSO) chronology of sea-surface temperature (SST; Niño3 Indices) 

and Southern Oscillation Index (SOI). Color pattern same as figure 4. SST and SOI were smoothed with 

5-month running average with ENSO threshold conditions for SST of 0.4 and –0.3°C and SOI threshold 

of ±0.5 standard deviation. 

Figure 6. Seasonal precipitation anomalies for 

the Colorado Plateau classified by strength of 

El Niño-Southern Oscillation (ENSO) activity. 

Color pattern same as figure 4.

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on ENSO variability, although it produces 

essentially the same patterns of temporal 

variability as more robust indicators.

Precipitation in the Colorado Plateau region 

is modestly correlated (P<0.05) with the PDO 

(fig. 7). The three precipitation regimes shown 

by the annual (fig. 2) and seasonal (fig. 3) 

precipitation time series are largely in phase 

with the PDO (fig. 8). This in-phase relation is 

shown well by the dry episode of 1942–1977, 

which corresponds to a period of low indices 

and a prolonged cool phase of the PDO. The 

early phase of the PDO is associated, although 

in a complicated manner, with the relatively 

wet conditions from 1905 to 1941. The strong 

warm phase of the PDO beginning around 

1977 is readily associated with the wet climate 

beginning in 1978. Of particular interest is the 

downward shift in the PDO beginning in 1999, 

with concomitant decreased precipitation that 

continued through the winter of 2002. The 

weather, SST, and surface-pressure patterns 

of 1999–2002 suggest to climatologists that 

the transition to another regime is presently 

underway (Ze’ev and Smith, 2001), signaling 

the end of the warm PDO phase and the wet 

climate of 1978–1998.

Implications

Precipitation in the Colorado Plateau region 

has varied substantially during the past century. 

This multidecadal variability has implications 

for ecosystem processes and land management. Precipitation, along with other climate 

variables, affects the spatial scale, frequency, 

and magnitude of natural disturbances to the 

ecosystem (Swetnam and Betancourt, 1998) as 

well as the recovery rates from natural and human disturbances. For example, the results of 

studies of floral and faunal population dynamics and the affects of grazing are dependent on 

the prevailing climate. Inferences and projections based on these studies may not be valid or 

may need adjustment or reappraisal if applied 

during a different climate regime.

Recent trends in Colorado Plateau precipitation and the PDO suggest that the climate of 

the region may become drier for the next 2 

to 3 decades in a pattern that could resemble 

the drought of 1942–1977. Although there are 

many uncertainties and assumptions, including 

using a single index (PDO) to predict multidecadal climate variability (Schmidt and Webb, 

2001), it is important to consider the potential 

affects of climate variation on the human and 

natural resources of the region. Water resources 

were heavily affected during the early part of 

the 1942–1977 drought (Gatewood and others, 1964). The population of the region has 

increased fourfold since the mid-1950s, substantially increasing the demand for water in a 

region without abundant supplies and creating 

the possibility of severe or catastrophic consequences if such a drought were repeated.

The work of USGS and other scientists is 

leading to a better understanding of the past and 

probable future climate of the Colorado Plateau. 

This work is only part of USGS efforts to provide information crucial to sound planning policies for land use and energy and other resource 

management. These efforts are also helping to 

protect lives and property from drought, landslides, and other hazards.

Figure 7. Combined cool and warm season (October 16–October 15) precipitation for the Colorado 

Plateau as a function of the Pacific Decadal 

Oscillation (PDO). Ten wettest and driest years 

shown in red. PDO is averaged over a 14-month 

window with 7 months preceding and 6 months 

following the midpoint of the combined season.

Figure 8. Smoothed (15-month moving window using the Savitzsky-Golay procedure) monthly indices 

of the Pacific Decadal Oscillation (PDO) (upper) and combined seasonal precipitation (standardized 

anomaly index, SAI) (lower) for the Colorado Plateau.

For more information contact:

Scientist-in-Charge

U.S. Geological Survey

2255 North Gemini Drive

Flagstaff, AZ 86001

(928) 556-7024

or

http://climchange.cr.usgs.gov/info/sw/

This fact sheet and any updates to it are available online

at: http://geopubs.wr.usgs.gov/fact-sheet/fs119-02/

Richard Hereford, Robert H. Webb, and

Scott Graham

Edited by James W. Hendley II and Peter H. Stauffer

Graphic design by Sara Boore and Susan Mayfield 

References

Cayan, D.R., Redmond, K.T., and Riddle, L.G., 1999, ENSO and 

hydrologic extremes in the western United States: Journal of 

Climate, v. 12, p. 2881-2893.

Gatewood, J.S., 1962, The meteorologic phenomenon of drought 

in the Southwest: U.S. Geological Survey Professional Paper 

372-A, p. A1-A43.

Gatewood, J.S., Wilson, A., Thomas, H.E., and Kister, L.R., 1964, 

General effects of drought on water resources of the Southwest, 1942-1956: U.S. Geological Survey Professional Paper 

372-B, p. B1-B55.

Mantua, N.J., and Hare, S.R., 2002, The Pacific decadal oscillation: 

Journal of Oceanography, v. 58, p. 35-42.

McCabe, J.G., and Dettinger, M.D., 1999, Decadal variations in 

the strength of ENSO teleconnections with precipitation in the 

western United States: International Journal of Climatology, v. 

19, p. 1399-1410.

Ropelewski, C., 1999, The great El Niño of 1997-1998—Impacts on 

precipitation and temperature: Consequences, v. 5, p. 17-25.

Schmidt, K.M., and Webb, R.H., 2001, Researchers consider U.S. 

Southwest’s response to warmer, drier conditions: Eos, Transactions, American Geophysical Union, v. 82, p. 475, 478.

Swetnam, T.W., and Betancourt, J.L., 1998, Mesoscale disturbance and ecological response to decadal climate variability in 

the American Southwest: Journal of Climate, v. 11, p. 3128-3147.

Trenberth, K.E., 1997, The definition of El Niño: Bulletin of the 

American Meteorological Society, v. 78, p. 2271-2777.

Ze’ev, G., and Smith, D.J., 2001, Interdecadal climate variability 

and regime-scale shifts in Pacific North America: Geophysical 

Research Letters, v. 28, p. 1515-1518.

Zhang, Y., Wallace, J.M., and Battisti, D.S., 1997, ENSO-like 

interdecadal variability—1900-93: Journal of Climate: v. 10, p. 

1004-1020.

Websites

Climate Prediction Center, Previous ENSO events (1877–present): http://www.cpc.ncep.noaa.gov/.../ensoyears_1877-

present.htm; accessed August 31, 2001.

Niño3 Indices, http://ingrid.ldeo.columbia.edu/SOURCES/

.KAPLAN/.Indices/.NINO3/.avGOSTA/; accessed June 20, 2001.

Pacific Decadal Oscillation data: N. Mantua http://

tao.atmos.washington.edu/pdo; accessed June 19, 2000.

SOI Indices, http://www.cpc.ncep.noaa.gov/data/indices/

index.html; accessed July 9, 2001.

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