6
ERDC TR-04-1
1.5
LANDSCAPE FEATURES AND THEIR
USEFULNESS AS OHWM INDICATORS
Hydrology
Physical features that develop within arid stream channels are a result of
hydrometeorological events that produce flows in the stream channel. Therefore,
understanding the nature of these events is critical to the development of OHWM
field indicators. Climatically the region is influenced by three large-scale patterns
in precipitation that influence channel morphology and OHWM field indicators.
These are winter North Pacific frontal storms, summer convective thunderstorms,
and late-summer eastern North Pacific tropical storms (Ely 1997). Either sepa-
rately or in combination, these precipitation patterns influence channel discharges
seasonally in the region. In general, winter North Pacific frontal storms make up
an increasing proportion of the total rainfall as one proceeds toward the north in
this region (Andrade and Sellers 1988). Convective thunderstorms, or the so-
called "summer monsoons," typically are associated with Arizona and New
Mexico. The North Pacific tropical storms occur across Arizona, New Mexico,
southern California, and southern Utah. Changes in water temperature in the
Pacific Ocean and subsequent El Nio patterns can cause increased precipitation
in winters in California, shifts in monsoonal precipitation, and increased fall to
spring precipitation throughout parts of the region. These patterns also cause
shifts in decadal-scale precipitation and flood frequency. The region is well
known for highly variable spatial and temporal rainfall (Reid and Frostick 1977,
Graf 1988, Pilgrim et al. 1988), which can result in intense rains that can fall in
one watershed or portions of a watershed while an adjacent watershed or portion
remains dry (Chapter 2). Interannual variability in precipitation is also high in
arid regions, with a high ratio of record peak to average annual discharge (Graf
1988).
In addition to a high ratio of peak to average annual discharges, there are
other factors that make the occurrence of extreme discharges more common in
the arid Southwest. The cumulative effects of interception of precipitation by
vegetation, evaporation and transpiration, transmission losses, and topography
play an important role in discharge rates. Rapid runoff rates are due in part to
lower infiltration and interception rates combined with greater rainfall intensities
in arid versus more humid climates (Pilgrim et al. 1988). Topography exerts a
strong influence on the type, location, and amount of precipitation. Because of
orographic effects, higher elevations experience more precipitation than low-
lying areas. At higher elevations there also is an increase in vegetation cover and
the possibility of snow. These factors result in more consistent runoff throughout
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