Water Resources in the Desert Southwest - Design with the Desert: Conservation and Sustainable Development - page 74

ET

ET

Inflow from
surface water

Bedrock

Aquifer

Outflow to
surface water

FIGURE 4.2
Generalized hydrologic cycle in the southwest depicting surface water-groundwater interactions that are
typical of many alluvial aquifer systems in the Basin and Range. ET, evapotranspiration.

requires recognition of global- and hemispheric-scale climatic processes that affect deliv-
ery of moisture to the region. Flood frequency, in particular, is affected by hydroclimatic
processes (see Chapter 5). 9
In terms of land area, most of the region receives less than 10 in. of precipitation, most
of which occurs as rainfall. 10 However, headwaters of many rivers in the region are
above 8000 ft and receive more than 20 in. of precipitation, most of which arrives as snowfall
in late fall, winter, and spring. Summer storms are common, with the exception of the
western Mojave Desert and the headwaters of the Mojave River. Reliable runoff, generated
during the winter and spring months, has high sediment concentrations; summer-storm
runoff is more episodic, generally is in lower volume, and contains very high sediment
concentrations. As a general rule, the climatic and physiographic setting dictates that most
of the runoff is generated at higher elevations while most of the sediment comes from
lower elevations.
Summer rainfall generally results from intense thunderstorms of local extent. This sea-
sonal precipitation tends to be reliable and strongly affects regional vegetation patterns,
essentially defining the spatial extent of the Sonoran Desert (see Chapter 7). 11,12 In addition
to the normal summer thunderstorms, tropical cyclones, a generic term that includes tropi-
cal depressions, tropical storms, and hurricanes, contribute a significant, although unreli-
able, amount of precipitation at any time from June through October. 13 These storms form
in either the eastern North Pacific Ocean, the Gulf of Mexico, or the Atlantic Ocean. Few
tropical storms or hurricanes have made landfall in the southwestern United States; most
dissipate over the ocean, and their leftover moisture is advected into the region, either in
weak monsoonal flow or strong systems from the North Pacific.
The El Niño-Southern Oscillation (ENSO) phenomenon of the Pacific Ocean strongly
affects interannual variation in precipitation in the Southwest (see Chapter 3 of this
volume). 9,12,14-17 ENSO effects can be separated into three general categories: warm ENSO