Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 113

FIGURE 3.22 Influence of snowpack and glacier cover on runoff is illustrated by data from side-by-
side basins of the same size but different elevations. Water year (Oct.-Sept.) hydrographs showing
mean daily discharge (1938-1999) for high-elevation and low-elevation subbasins of the Nooksack
River, Washington. The high-elevation North Fork has a mean elevation of 1,311 m, 6 percent gla-
cier cover, and a mean annual discharge of 22.0 m 3 /s. The lower-elevation South Fork has a mean
elevation of 914 m, no glacier cover, and a mean annual discharge of 20.8 m 3 /s. (Daily discharge
data from USGS, from Bach 2002.)
High-elevation snowsheds are important to regional water supplies, as they provide
water for domestic, industrial, and agricultural users, as well as for recreation, hydro-
electric power, and habitat. They also produce flood hazards. Rapid population growth,
increasing environmental concerns, and resulting changes in the character of demands
for water have led to increased competition for water even under normal flow con-
ditions. Water management practices, storage infrastructure, and patterns of use are
tuned to the expected range of variation in surface runoff and groundwater availability.
The abundant surface water supply from mountainous regions has promoted a historic
reliance on this resource in adjacent lowlands. Effective water development planning
and policy making must recognize how changes in upper watershed conditions will im-
pact lowland water resources (Wohl 2004). New reservoirs and water transfer systems
require considerable lead time to plan and construct. As discussed in Chapter 12, such
structures will be necessary to deal with changing water supplies in the future.
Winds
Mountains are among the windiest places on Earth. They protrude into the high atmo-
sphere, where there is less friction to retard air movement. While there is not a con-
stant increase in wind speed with altitude, measurements from weather balloons and
aircraft show a persistent increase at least up to the tropopause, where, in middle latit-
udes, the wind culminates in the jet streams. Similar increases occur in mountains, al-
though the conditions at any particular site are highly variable. Wind speeds are greater
in middle latitudes than in tropical or polar areas, in marine than in continental loca-
tions, in winter than in summer, and during the day than at night—and, of course, the
velocity of the wind is dependent on the local topographic setting and the overall syn-
optic conditions (Smith 1979; Gallus and Klemp 2000). The wind is usually greatest in
mountains oriented perpendicular to the prevailing wind, on the windward rather than
the leeward side, and on isolated, unobstructed peaks rather than on those surrounded
by other peaks. The reverse situation may exist in valleys, since those oriented perpen-
dicular to the prevailing winds are protected while those oriented parallel to the wind
may experience even greater velocities than the peaks, owing to funneling and intens-
ification (Mass 2008). Table 3.5 lists the mean monthly wind speeds during the winter
for several representative mountain stations in the northern hemisphere.
Mountains greatly modify the normal wind patterns of the atmosphere (Smith 1979;
Insel et al. 2010). Their effect may be felt for many times their height in both horizontal
and vertical distance. The question of whether the wind speed is greater close to moun-
tains or in the free air has long been problematic. The two basic factors that affect wind
speeds over mountains operate in opposition to on another. The vertical compression