Chapter 2. Hydrology
29
Transmission losses (the loss of discharge due to infiltration of flow into the
channel bed) are significant along ephemeral streams (Reid and Frostick 1997,
Tooth 2000). As a result, runoff can actually decrease in a downstream direction
despite the increase in drainage area. For headwater streams, especially where
bedrock is near the surface, transmission loss is not an important factor. Rainfall
runoff modeling must account for transmission losses, but it is difficult to model
because the rate of loss varies from point to point along a channel and with the
degree of saturation of the channel bed from prior events (Pilgrim et al. 1988).
Arid regions have a characteristically rapid onset of runoff after the start of a
rainfall event; consequently, only a small amount of rain is needed to initiate
runoff (Pilgrim et al. 1988). This is in part due to lower infiltration and intercep-
tion rates compared to humid climates and in part due to generally greater rainfall
intensities in arid climates. These differences underscore the need to avoid
applying to arid regions assumptions about runoff characteristics developed for
humid climates (Pilgrim et al. 1988).
Topography exerts a strong influence on the type, location, and amount of
precipitation throughout the Southwest. Higher elevations generally experience
greater precipitation than low-lying areas because of orographic effects (e.g.,
Sierra Nevadas; Fig. 2 and 3). Snowfall in higher regions during the winter can
prevent what would have been severe flooding if the precipitation had fallen as
rain. Conversely, large floods often occur when warm heavy rain falls on a deep
snowpack created during earlier colder frontal storms. The presence of a snow-
pack results in more consistent runoff through the spring, and even summer,
along higher-elevation mountain streams while adjacent lower-elevation streams
run dry. Greater vegetation at higher elevations also tends to stretch runoff over
longer periods because of the increased infiltration rates. Northsouth-trending
mountain ridges throughout the Southwest, especially in the Basin and Range
Province, block moisture from eastward-moving frontal systems, creating rain
shadows, or dry regions, on the east side of the mountain ridges (Fig. 2, 3, and 9).
This phenomenon is manifested most dramatically by the Sierra Nevada Moun-
tains, with some of the driest regions of the country found in eastern California
(e.g., Death Valley) and western Nevada (Fig. 9).
The cumulative effect of all the factors controlling runoff described above is
for arid-region river systems to have greater flow magnitudes, a more rapid
response to rainfall, and shorter-duration flows than equivalent humid-region
counterparts (Fig. 10). As a consequence, arid-region rivers are typically dry (i.e.,
they have no base flow) prior to a storm event. In many deserts, particularly
where precipitation intensities are high, the runoff hydrograph characteristically
rises very steeply as the result of limited infiltration capacity and then falls
sharply in response to transmission losses (Fig. 10) (Cooke et al. 1993). The
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