Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 75

events (commonly known as El Niño conditions), cool ENSO events (commonly known
as La Niña conditions), and other conditions (also see the discussion in Chapter 3).
Precipitation during El Niño conditions generally is high, although notable dry periods
have occurred as well. Winter precipitation is the most affected; the effects of El Niño on
summer precipitation are weak at best. The largest floods in the region tend to be associated
with El Niño conditions. 9 La Niña conditions are predominantly dry, and most significant
regional droughts are associated with this climate state. Anything except extreme wet or
dry periods can occur during other conditions.
Despite geographic variation in hydroclimatology, decadal-scale climatic fluctuations
appear to affect the region in a relatively uniform fashion (see Chapter 3). While all of
the region may simultaneously experience dry or wet conditions, the magnitude and
persistence of unusual climatic conditions varies. While wet conditions generally are
uniform from the Mojave River through southern Arizona, droughts seldom are uniform
in severity or length. Orographic effects and seasonality of precipitation add to the
complexity, making general statements about climatic variability difficult. Finally, average
temperatures are related to precipitation; temperatures tend to be annually or seasonally
high during droughts and may be relatively low or high during wet periods.
Analyses of climatic trends provide a general framework of decadal climatic fluctuations
affecting hydrology of the Southwest. 2,12,17,18 The period of 1880 through 1891 was generally
wet, with numerous regional-scale storms that caused channel downcutting and generally
led to the observation that “rainfall follows the plow.” The most severe drought, and the
one that affected the largest amount of the region, occurred between 1891 and 1904. The
combination of overstocking of the range and the drought led to the death of half of
the cattle in the region between 1891 and 1896. El Niño conditions in 1904 and 1905 ended
the drought, and the wettest period in the region's history began in 1909 and extended
through about 1920. This period continues to cause water problems in the southwestern
United States because above-average flows in the Colorado River were divided among
seven western states according to the Colorado River Compact, resulting in the current
problem of overallocation of water supplies in this critical river. 2
Climate was regionally variable between 1920 and the early 1940s, ending with the strong
El Niño conditions of 1941 through 1942. In southern Arizona, conditions were relatively
dry with few significant winter storms. From the Mojave River through southern Utah,
conditions were generally wet, punctuated with a mild drought during the Dust Bowl
years of the early 1930s. Between the mid-1940s and the early 1960s, drought conditions
prevailed with considerable regional variation in intensity. The mid-century drought,
centered on the La Niña conditions of 1954 through 1956, was most severe in the Mojave
Desert, in southern Utah, and to the east in New Mexico. Normal and above-average
summer precipitation mitigated this drought in central and southern Arizona.
Beginning in the early 1960s, and fueled by several significant El Niño periods, the
climate of the region became significantly wetter and warmer. Numerous strong storms in
fall and winter occurred between 1970 and 1995, leading to significant floods in central and
southern Arizona and above-average precipitation in the Mojave Desert and the Colorado
Plateau. Notable periods of El Niño conditions occurred from 1978 through 1980, 1982 and
1983, and 1993 through 1995. Brief droughts interrupted this wet period in 1986 and 1989
through 1991, with the latter event having severe effects in the Mojave Desert.
Despite El Niño conditions in 1997 through 1998 and 2002 through 2003, winter
drought generally prevailed at the end of the twentieth century through 2004. The
drought centered on 2002 created several record extremes, including the lowest flow in