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Multiscale Hydrologic Remote Sensing: Perspectives and Applications
driven by changes to precipitation for each scenario. The 2030 and 2080 “Wet”
scenarios produced significant increases in the annual streamflow of 13.4% and
19.5%, respectively, which were slightly larger than the respective annual percentage changes in precipitation for the study area; the two “Dry” scenarios produced
slight decreases in the annual flow volumes, –5.4% for 2030 and –1.9% for 2080,
which were less than the reductions in annual precipitation; the two “Mid” scenarios produced increases in annual flow volumes of 0.5% and 5.0%, which were
quite close to the annual percentage changes in precipitation corresponding to those
two scenarios. The exacerbation of annual flow volume for the two “Wet” scenarios
and the reduction for the two “Dry” scenarios of annual flow volume relative to
the percentage changes of precipitation are primarily the result of the time of year
the increase or decrease of precipitation occurs. The climate in this region generally has wet winters and dry summers. The six climate scenarios generally show
an increase in winter precipitation and a decrease in summer precipitation. The
increase in winter precipitation is more pronounced for the “Wet” scenarios, and
the decrease in summer precipitation is more pronounced for the “Dry” scenarios,
resulting in the relative exacerbation or reduction in annual stream flow volumes
noted above.
The temporal distribution throughout the year of perturbations to temperature
and precipitation has a significant impact on how a climate scenario impacts snowmelt runoff. For the 2030 period, annual average temperature increases ranged from
+0.86°C to +1.50°C, and annual percentage changes to precipitation ranged from
–7.64% to +12.93%. For the 2080 period, annual average temperature increases
ranged from +2.56°C to +4.38°C, and annual percentage changes to precipitation
ranged from –6.75% to +17.41%. In our usage, the temperature and precipitation perturbations from the GCMs were specified on a monthly basis as shown in Table 10.4,
and for several of the scenarios, there was substantial monthly variability around the
annual averages listed above. Most notably, the mean annual temperature increase of
4.38°C corresponding to the 2080 Dry scenario has most of the temperature perturbations, which fall above the mean occurring during July through September, when
most of the snowmelt has already occurred. Also for the Dry scenario at both 2030
and 2080, large double-digit percentage reductions to precipitation occur during the
summer and early fall, which are historically the dry months in the study region.
Both of these temperature and precipitation extremes on the hot/dry side occur at
times of year, which somewhat dampen out their effect with regard to snowmelt
runoff compared to what their effect would be if they occurred in the spring. In contrast, the large percentage increases in precipitation from the “Wet” GCM are largely
driven by increases in precipitation during winter months, which tends to exaggerate
their effect on snowmelt runoff through large increases in precipitation during some
of the wettest months.
It is useful to compare the magnitude of simulated changes from a range of climate scenarios with the range of historical observations. In our case study, the simulated changes to average annual stream flow volume ranged between about –5% and
+20%. The historical range of precipitation relative to the median annual precipitation during the base period was –36% to +52%; during this same period, the range of
the annual stream flow volume relative to the median was –37% to +83%.
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
driven by changes to precipitation for each scenario. The 2030 and 2080 “Wet”
scenarios produced significant increases in the annual streamflow of 13.4% and
19.5%, respectively, which were slightly larger than the respective annual percentage changes in precipitation for the study area; the two “Dry” scenarios produced
slight decreases in the annual flow volumes, –5.4% for 2030 and –1.9% for 2080,
which were less than the reductions in annual precipitation; the two “Mid” scenarios produced increases in annual flow volumes of 0.5% and 5.0%, which were
quite close to the annual percentage changes in precipitation corresponding to those
two scenarios. The exacerbation of annual flow volume for the two “Wet” scenarios
and the reduction for the two “Dry” scenarios of annual flow volume relative to
the percentage changes of precipitation are primarily the result of the time of year
the increase or decrease of precipitation occurs. The climate in this region generally has wet winters and dry summers. The six climate scenarios generally show
an increase in winter precipitation and a decrease in summer precipitation. The
increase in winter precipitation is more pronounced for the “Wet” scenarios, and
the decrease in summer precipitation is more pronounced for the “Dry” scenarios,
resulting in the relative exacerbation or reduction in annual stream flow volumes
noted above.
The temporal distribution throughout the year of perturbations to temperature
and precipitation has a significant impact on how a climate scenario impacts snowmelt runoff. For the 2030 period, annual average temperature increases ranged from
+0.86°C to +1.50°C, and annual percentage changes to precipitation ranged from
–7.64% to +12.93%. For the 2080 period, annual average temperature increases
ranged from +2.56°C to +4.38°C, and annual percentage changes to precipitation
ranged from –6.75% to +17.41%. In our usage, the temperature and precipitation perturbations from the GCMs were specified on a monthly basis as shown in Table 10.4,
and for several of the scenarios, there was substantial monthly variability around the
annual averages listed above. Most notably, the mean annual temperature increase of
4.38°C corresponding to the 2080 Dry scenario has most of the temperature perturbations, which fall above the mean occurring during July through September, when
most of the snowmelt has already occurred. Also for the Dry scenario at both 2030
and 2080, large double-digit percentage reductions to precipitation occur during the
summer and early fall, which are historically the dry months in the study region.
Both of these temperature and precipitation extremes on the hot/dry side occur at
times of year, which somewhat dampen out their effect with regard to snowmelt
runoff compared to what their effect would be if they occurred in the spring. In contrast, the large percentage increases in precipitation from the “Wet” GCM are largely
driven by increases in precipitation during winter months, which tends to exaggerate
their effect on snowmelt runoff through large increases in precipitation during some
of the wettest months.
It is useful to compare the magnitude of simulated changes from a range of climate scenarios with the range of historical observations. In our case study, the simulated changes to average annual stream flow volume ranged between about –5% and
+20%. The historical range of precipitation relative to the median annual precipitation during the base period was –36% to +52%; during this same period, the range of
the annual stream flow volume relative to the median was –37% to +83%.