Hydrologic Impacts of Climate Change

Author: B.C. Hydro

Hydrological climate impact studies, including the three studies that produced planning data sets described in this document (PCIC, WC2N, and UW-CIG), usually follow a similar approach when assessing the impact of climate change on hydrology. In most climate change assessments, numerical modelling, a commonly used method for solving complex water resources problems, is the central tool. Hydrological assessments of future climate change impact relies on a numerical modeling chain that usually entails (i) macro-economic and systems-engineering models to generate future greenhouse gas emission scenarios, (ii) general circulation models (GCMs) to resolve the large-scale global circulation and simulate present and future climate, (iii) downscaling techniques to add regional detail to the coarse GCM simulations, and (iv) hydrological models to convert climatic scenarios into discharge scenarios at the watershed scale.

There is substantial uncertainty in the future projections arising from variations among GCMs, model parameter uncertainty, uncertainty in the downscaling of GCM output, model structural uncertainty, uncertainty in recent glacier volume change, etc. Up until the 2050s, GCMs are found to contribute more too overall uncertainty than the different emission scenarios, i.e. the unknowns in future greenhouse gas emissions. Hydrologic modelling uncertainty remains a relatively large source of uncertainty at that forecast horizon. Beyond the 2050s, emission scenarios become more important and emerge from all other uncertainties. Despite all these uncertainties, some general conclusions can be drawn from the climate change impact studies:
– Historical trends in annual reservoir inflows are small and not significant. There is some evidence for a modest historical increase in annual inflows into BC Hydro’s reservoirs.
– There is evidence for historical changes in the seasonality of inflows. Fall and winter inflows have shown an increase in almost all regions; there is weaker evidence for a possible modest decline in late-summer flows for those basins driven primarily by melt of glacial ice and/or seasonal snowpack.
– For the period of inflow records (35 to 47 years, depending on the reservoir), the severity of
year-to-year fluctuations in annual reservoir inflow volumes has not changed. ·
– Projected warming in the 21st century shows a continuation of patterns similar to those of
recent decades.
– All emission scenarios project higher temperatures in all seasons in all areas of British Columbia during the 21st century that will very likely be larger than those observed during the 20th century.
– Precipitation projections suggest likely increases in winter, spring, and fall for all study areas under all scenarios.
– A modest increase in annual water availability is likely for BC Hydro’s hydroelectric system.
– Annual discharge in most Upper Columbia watersheds is projected to likely increase.
– In the Columbia and Kootenay regions, late fall and winter flows will increase slightly; the onset of the snowmelt freshet will be earlier; spring and early-summer flows will be substantially higher; earlier peak flows and higher monthly peak flows can be expected; and late-summer and early-fall flows will be substantially lower.

There is substantial uncertainty in the future projections arising from variations among GCMs, model parameter uncertainty, uncertainty in the downscaling of GCM output, model structural uncertainty, uncertainty in recent glacier volume change, etc. Up until the 2050s, GCMs are found to contribute more too overall uncertainty than the different emission scenarios, i.e. the unknowns in future greenhouse gas emissions. Hydrologic modelling uncertainty remains a relatively large source of uncertainty at that forecast horizon. Beyond the 2050s, emission scenarios become more important and emerge from all other uncertainties. Despite all these uncertainties, some general conclusions can be drawn from the climate change impact studies:
– Historical trends in annual reservoir inflows are small and not significant. There is some evidence for a modest historical increase in annual inflows into BC Hydro’s reservoirs.
– There is evidence for historical changes in the seasonality of inflows. Fall and winter inflows have shown an increase in almost all regions; there is weaker evidence for a possible modest decline in late-summer flows for those basins driven primarily by melt of glacial ice and/or seasonal snowpack.
– For the period of inflow records (35 to 47 years, depending on the reservoir), the severity of
year-to-year fluctuations in annual reservoir inflow volumes has not changed. ·
– Projected warming in the 21st century shows a continuation of patterns similar to those of
recent decades.
– All emission scenarios project higher temperatures in all seasons in all areas of British Columbia during the 21st century that will very likely be larger than those observed during the 20th century.
– Precipitation projections suggest likely increases in winter, spring, and fall for all study areas under all scenarios.
– A modest increase in annual water availability is likely for BC Hydro’s hydroelectric system.
– Annual discharge in most Upper Columbia watersheds is projected to likely increase.
– In the Columbia and Kootenay regions, late fall and winter flows will increase slightly; the onset of the snowmelt freshet will be earlier; spring and early-summer flows will be substantially higher; earlier peak flows and higher monthly peak flows can be expected; and late-summer and early-fall flows will be substantially lower.