Download Introduction - UW Hydro | Computational Hydrology

Climate Related Hydrologic and Water Resources Impacts in British Columbia and
the Pacific Northwest
Alan Hamlet
JISAO Center for Science in the Earth System
Department of Civil and Environmental Engineering
University of Washington
Introduction
Canada is likely to face a wide ranging set of uncertain, but potentially significant,
climate change impacts and associated challenges to risk assessment, long-range
planning, monitoring, and adaptation over the next century. In addition to relatively well
understood physical changes in the annual quantity or variability of water supplies
(primarily associated with changes in precipitation), or seasonal changes in the timing of
streamflows in snow melt dominant watersheds (associated with temperature changes),
Canada will also have to face escalating (and already evident) physical and ecological
impacts to boreal forests and tundra ecosystems, threats due to extreme weather events
(e.g. floods and droughts), uncertain threats to agricultural systems, human health, and a
long list of direct and indirect potential socioeconomic impacts.
In transboundary areas such as Columbia River basin, Canada is also likely to encounter
indirect stresses on Canadian water resources associated with climate impacts in the US.
Such changes may influence important transboundary agreements between Canada and
the US such as the Columbia River Treaty (1964).
Hydrologic Impacts Associated with Climate Change Scenarios
Climate change studies for the Columbia River basin conducted at the UW based on
climate model scenarios have quantified some of the potential hydrologic responses to
climate change in the Pacific Northwest (PNW). The study results for the Columbia
basin, although representing only a small portion of BC explicitly, are instructive, and
probably can be generalized to help understand a number of snowmelt dominant
watersheds in BC. The studies have shown that the Northern parts of the Columbia basin
are likely to experience relatively modest shifts in streamflow timing and are most
sensitive to relatively uncertain changes in precipitation in the scenarios. Some high
elevation northern watersheds may experience localized increases in snowpack if
precipitation increases significantly in winter. In the US portions of the Columbia basin,
by comparison, hydrologic impacts are more equally influenced by temperature induced
timing shifts (which are present in all the scenarios) and precipitation changes (which
vary significantly between scenarios and are more uncertain). In the US portions of the
basin, increases in precipitation are generally shown not to offset losses of snowpack due
to temperature increases, and changes in streamflow timing and water availability result.
The changes in runoff and streamflow timing are also shown to influence the annual
cycles of soil moisture and evapotranspiration. Areas that are sensitive to these kinds of
physical changes (e.g. rangelands, dryland agriculture) may experience impacts.
Impacts to Managed Water Resources Systems
In the PNW, many water systems have developed historically to take advantage of natural
water storage as mountain snowpack, and many systems have very small amounts of
reservoir storage in comparison to natural streamflow. Most of water systems on the
western slopes of the Cascades in the US, for example, have reservoir storage to annual
streamflow volume ratios of less than 10%. The Columbia basin upstream of The Dalles
has a storage/streamflow ratio of about 30%, and the Snake River upstream of Milner has
a storage/streamflow ratio of about 60%. British Columbia, by comparison, has a number
of very large reservoirs including the reservoir behind Mica Dam. These characteristics
are important in that they determine the nature of the risks. The water supply systems on
the west side of the Cascades in the US are believed to be relatively sensitive to changes
in climate because they show strong changes in streamflow timing associated with small
increases in temperature and have limited ability to transfer water from winter to summer
without natural storage. By comparison, the water systems in BC with large amounts of
storage are likely to be most strongly affected by long-term changes in precipitation, and
particularly by long-term drought. Given that changes in streamflow timing in BC are
expected to be relatively modest for some time coupled with BC’s abundant reservoir
storage in comparison with the rest of the PNW suggests that BC’s water systems may be
capable of regulating the effects of altered interannual variability provided longer term
changes (i.e. decadal or longer) in precipitation are not encountered.
In the Columbia basin, however, BC’s potential robustness to climate change impacts
may present a double edged sword. To give one example, if streamflow timing shifts
occur as expected in the US portions of the basin, US water management agencies may
experience difficulty in maintaining acceptable levels of streamflow in the lower basin in
late summer due to limited reservoir storage. Increased use of US and Canadian storage
could be the only viable response strategy, but such an adaptation would require a
coordination mechanism between Canada and the US beneficial to both parties. Existing
transboundary agreements may provide a basic framework for such an agreement, but
would also require substantial modifications to both the transboundary agreements and
the Columbia basin reservoir operating policies.
Some Primary Areas of Uncertainty
1.
2.
3.
4.
5.
Precipitation variability
Changes in extreme weather statistics
Groundwater resources and groundwater/surface water interactions
Ecosystem impacts
Uncertain socioeconomic impacts (e.g. impacts to human health, food production,
immigration)