Environmental Engineering Reference
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110°W
120°W
Wyoming
Salt Lake City
Utah
40°N
Nevada
Denver
Colorado
Virgin
River
California
Las Ve gas
San Juan
River
Rio
Grande
Little
Colorado
River
Albuquerque
Verde
River
35°N
Mojave
River
New
Mexico
Los Angeles
Salt River
River
Phoenix
Gila
Arizona
San
Pedro
River
Santa
Cruz
River
N
El Paso
Te xas
200 km
FIGURE 4.1
Map of the southwestern United States showing the locations of principal rivers.
is dominated by drought.* What once was thought to be an endless dependable water supply
beneath the ground has turned into declining water tables and land subsidence. The desert
cycle of floods and droughts leaves water managers and residents alike scratching their
heads and wondering just what in the hydrologic world they can depend on.
Understanding of the desert hydrologic cycle begins with a recognition of climate and
the influence of climate variability on surface water and groundwater. It extends to rainfall,
runoff, and recharge, the three “r's” of hydrology. Rainfall is spatially and temporally variable
in arid and semiarid regions, and its deficit defines the deserts. Runoff can turn into floods,
which can both damage and nurture. Recharge is the deposit in our hydrologic savings
account, hopefully not to be withdrawn at a higher rate than the additions. Interactions
between surface water and groundwater, often neglected under legal protections, both help
and hurt riparian ecosystems. Taken together, these processes can tell us what to expect in
the hydrologic future in the desert Southwest, and how we might help thwart the persistent
attempts of the harsh desert to keep our water-loving civilization at bay.
4.2 Hydrologic Settings in the Desert Southwest
Especially given its complex systems of water development, the desert region in Arizona
and parts of adjacent states (Figure 4.1) have the most complex hydrologic systems in North
*
Considerable uncertainty exists in future climate predictions; see Chapter 3. The early twenty-first century
drought may or may not be over, depending upon one's definition of drought as a specific period of concur-
rent time with below-average rainfall, or whether a more general drought period, punctuated by wet years but
overall with below-average precipitation is used. In this sense, we are applying the latter concept.
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