Mountain Climate - Mountain Geography: Physical and Human Dimensions - page 112

within and between mountains, making generalities about mountain hydrology difficult.
Two unique aspects of mountain hydrology are the generation of flood events and the
influences of snowpack meltwater.
Snow meltwater has four principal impacts on watershed hydrology: (1) lowering
stream temperature; (2) sudden contributions to discharge resulting from rapid melting
(rain on snow events); (3) an increase in melt-season discharge and decrease in snow-
accumulation season discharge; and (4) a decrease in annual and especially seasonal
variations in runoff (Wohl 2004). The changes in average seasonal discharge because of
snowmelt are illustrated in Figure 3.22. Differences in discharge in these side-by-side
mountainous watersheds of the same size are related to the elevational effects on snow-
fall (Bach 2002). During October, the beginning of the wet season, both basins have
similar discharges. Between November and about April, two differences become appar-
ent in the flow characteristics: The higher-elevation basin discharge becomes smaller in
volume and less variable than the lower basin (Fig. 3.22). The decrease in volume is due
to more precipitation falling as snow and accumulating in the upper basin, while rain
falls throughout the winter in the lower basin and runs off. The greater temporal vari-
ability in the lower basin is related to the chaotic timing of storms. In March, temperat-
ures begin to warm and the snowmelt season begins—a few weeks earlier in the lower
basin (Fig. 3.22). The variability of discharge decreases in both basins (lines become
smoother), indicating a change from storm event-dominated runoff to temperature-driv-
en snowmelt runoff (Peterson et al. 2000). The peak in the snowmelt flood of the lower
basin occurs about one month earlier and is only 70 percent the size of the upper basin,
reflecting the difference in snowpack volume. These runoff characteristics are further
confounded by the presence of glaciers in a watershed (Clow 2010).
Besides the daily to weekly variations caused by storm events, and the seasonal vari-
ations throughout a year, streamflow regimes in mountains are prone to interannual and
secular variations related to large-scale patterns of climate variations, such as the El
Niño/Southern Oscillation (Wohl 2004). The type of climatic variations vary around the
globe, but generally result in extreme weather and climate conditions such as flooding,
droughts, different storm frequency and precipitation, or changes in snowmelt season
(Viles and Goudie 2003). Global warming is likely to increase the frequency and mag-
nitude of these climatic variations and their impacts on the hydrological system (Xie et
al. 2010).