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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)