One of the most confident predictions concerning global climate change is that average temperatures over land masses will increase. Warmer temperatures during the winter will affect the form of precipitation, with more of the precipitation falling as rain rather than snow. This may lead to increased streamflow during winter, earlier peak runoff in spring and reduced late summer streamflow. An average temperature change of only a few degrees during the melt season would very likely alter the timing of spring runoff.
In the Pacific Northwest, both the Portland Water Bureau and Seattle Public Utilities have begun to examine how such changes in seasonal runoff could affect their ability to meet customer demands, particularly during the summer months, as well as instream flows to support fish habitat. Their efforts can provide lessons for many other cities in the western United States as well as many in Europe, Northern Asia and Canada who face similar potential impacts on system reliability.
Portland and Seattle are alike in that both utilities rely heavily on surface water coming from protected watersheds where runoff timing depends partly on snowmelt. Both utilities are located in a region characterized by large seasonal differences in precipitation. Most of the precipitation occurs during the winter, and summers are relatively dry. The utilities differ, however, in the options available for supply enhancement. The direction and emphasis of their planning and management efforts reflect those differences. The Portland Water Bureau has explored various climate change scenarios to examine the robustness of its supply system. Based on this examination, utility managers are developing adaptive strategies to cope with the possible changes in seasonal flows, and other likely effects of climate change.
The Portland Water Bureau supplies water to approximately 800,000 people in the Portland metropolitan area, with deliveries totaling about 40 billion gallons per year. Its primary water source is the Bull Run Watershed, an unfiltered surface supply that has two reservoirs with a combined usable storage capacity of 10 billion gallons. The utility also has a backup groundwater supply located along the south shore of the Columbia River. Precipitation over the watershed typically ranges between 59 to over 80 inches per year, with most falling during the winter months. Therefore, supplying water during the summer months is the greatest challenge for this utility. Summer water demand can peak at over 220 million gallons a day, which is double the average daily use. Climate change could increase these difficulties in coming decades, but the Portland Water Bureau has already begun to prepare for such an eventuality by considering changes in long-term water demand and supply in their current infrastructure planning.
To evaluate the implications of future warming, the Portland Water Bureau conducted an analysis based on future climate scenarios derived from four different climate models (Palmer and Hahn 2002). They ran the climate scenarios through watershed hydrology, regional population growth, and system management models to simulate the impacts on system reliability and reservoir conditions. While the specific results vary, general trends such as increased winter precipitation, earlier snowmelt, and drier summers were consistent across the models. Such scenarios suggest that in the absence of additional supply infrastructure, future climate change scenarios may lead to decreased reliability of supply with a concurrent increase in summer water demand, leading to an increase in overall system vulnerability. Already, the utility is preparing for an increase in seasonal demand of between 8 and 10 percent, primarily resulting from population growth. This is equivalent to an additional billion gallons of water demand during the summer, which is about 10 percent of the current storage capacity of existing surface reservoirs. Under current climate conditions, it would be possible to meet this additional demand by fully exploiting the summer groundwater supply, but using groundwater as a primary source rather than a backup would remove a safety net for drought years and thereby decrease system reliability.
Current scientific evidence suggests that the primary threat that climate change poses for Portland’s water supply is not a reduction in annual average precipitation but rather a change in runoff timing that aggravates a deficiency in storage capacity. The Portland Water Bureau’s analysis suggests that changes in runoff timing could result in winter flows increasing by as much as 15 percent, accompanied by a 30 percent decrease in late spring flows. The impact of such changes, together with an increase in summer demand, would result in a 2.8–5.4 billon gallon decrease in reservoir storage volume by the end of the drawdown period or about 15 to 30 percent of the current storage. The utility is considering these factors as it evaluates the feasibility of further expanding the existing groundwater supply and/or expansion of existing source-water reservoirs. The proposed projects will increase system reliability under both current conditions and various climate change scenarios, and therefore constitute a precautionary strategy. Greater surface storage will also decrease the frequency of groundwater use, and therefore increase the sustainability of that emergency supply.
In addition to supply augmentation, the Portland Water Bureau is evaluating other strategic measures, such as conjunctive use strategies that coordinate the optimal use of existing surface and groundwater supplies, including use of aquifer storage and recovery (ASR). Since the exact nature of climate change is unknown, Portland is also emphasizing flexibility in infrastructure development. For example, as an unfiltered surface water supply, Portland may be especially vulnerable to extreme storm events in winter months that would result in elevated turbidity, making the surface water supply temporarily unusable. While Portland anticipates remaining unfiltered for the foreseeable future, water treatment options have been considered that might readily accommodate the addition of filtration equipment in the future.
Seattle Public Utilities is actively working with the University of Washington’s Joint Institute for the Study of the Atmosphere and Oceans (JISAO) Climate Impacts Group. This is a group of scientists and policy analysts at the university that is examining the potential impacts of climate change within the Pacific Northwest region (which is defined as the states of Washington, Oregon and Idaho, and the Columbia River Basin). The UW JISAO Climate Impacts Group believes that by planning now, and by incorporating information about climate variability and change into decisions about how the region manages its natural resources, resource managers and decision-makers can reduce the negative impacts of (and take better advantage of the opportunities brought by) both human-caused climate change and natural variability.
Seattle Public Utilities supplies water to 1.3 million people and businesses in the Seattle metropolitan region. Its sources provide approximately 50 billion gallons per year to its retail and wholesale customers. Nearly all this water is from the 90,000-acre Cedar River Watershed and the 13,300-acre South Fork Tolt River Watershed located on the western slopes of the Cascade Mountains in eastern King County, WA. Water use is approximately the same as it was in the mid-1960s, despite population growth, due to conservation savings and system efficiencies. Recent climate statistics inform us that the Seattle area receives over 75 percent of its annual precipitation from the beginning of October through the end of March. Snowpack accumulations in the mountain watersheds normally peak around the beginning of April and runoff from snowmelt usually ends by July. Peak snowpack accumulations for the Cedar and Tolt watersheds above elevation 2500 feet average around 30 inches of snow water equivalence. The summer months are typically mild with minimal amounts of precipitation. The average annual precipitation for the Seattle area is about 37 inches; Cedar Lake at elevation 1560 feet receives about 100 inches, and the Tolt weather station at elevation 2000 feet receives about 90 inches. Average annual air temperature in Seattle is about 52°F; Cedar Lake is about 47°F, and Tolt weather station is about 44°F.
Systematic monitoring and collection of meteorological and hydrological data such as precipitation, air temperature and streamflow began as early as the late 1800s in the central Puget Sound region, and agencies such as the US Geological Survey and the National Weather Service maintain historical records. Traditionally, water planners assume that these historical records, when they are long enough, represent sufficient variability to support analysis of present and future water system performance and behavior. Indeed, water projects have been built and instream flow requirements have been established in the region based primarily on available historical weather and streamflow data.
Under the current hydrological regime, the storage reservoirs on both Seattle sources are refilled in the spring by snowmelt and rainfall. Water stored in the form of snowpack is an important element in managing reservoir refill because, once it has accumulated, it becomes a known quantity that can be relied on to refill the reservoirs. On the other hand, spring rainfall events are uncertain and difficult to predict. This uncertainty is significant in balancing the need for full reservoirs at the start of summer water use with the need for storage capacity to regulate downstream flows during steelhead spawning and incubation periods. Seattle Public Utilities is well aware of the sensitivity of its system to changing snowpack, and routinely monitors the condition of the snowpack when making decisions regarding reservoir operations. The utility has been managing reservoir levels using a “dynamic rule curve” that adjusts reservoir refill targets based on real-time snowpack and soil moisture conditions. This approach uses actual conditions to adjust reservoir management and increase the likelihood of full reservoir refill prior to the summer reservoir drawdown period.
Seattle Public Utilities has also started to examine a set of climate change scenarios. Like Portland, they are now sponsoring climate change research work by the UW JISAO Climate Impacts Group to explore and develop analysis techniques that will enable regional water planners and decision-makers to incorporate global climate change information into local long-range water supply planning processes.
Water planners at Seattle Public Utilities believe it is important to recognize that translating climate change scenarios down to regional and local scales with confidence is a significant problem for researchers and practitioners. One of the management challenges that a climate change study presents is that observed temperature changes over the past 100 years are not uniform throughout the world. Some places show trends of greater warming, while other locations are actually getting cooler, despite the fact that on average the Earth as a whole has been getting warmer. Similarly, there are probably regional and local differences. For example, we may find that impacts to the Central Puget Sound region may be different than those projected for the Columbia River Basin and eastern Washington, and there may even be differences between local watersheds within the Central Puget Sound region. Historic surface air temperature data for mid-elevation in the Cedar River watershed, for example, show a cooling trend expressed over the last 70 years.
The research work that Seattle Public Utilities is sponsoring by the UW JISAO Climate Impacts Group will first examine the current state-of-the-art climate change prediction models used by the Intergovernmental Panel on Climate Change (IPCC). This examination will focus on the levels of uncertainty associated with climate change scenarios generated by these climate prediction models, and the steps necessary to prepare climate change model results for use in existing regional or watershed-level models (a.k.a. downscaling techniques). The research effort will then be poised to develop useful analysis techniques using computer simulation modeling methods to evaluate potential future climate change impacts to local water resources. The research team will give special attention to identifying and documenting the important uncertainties and complexities associated with the methods studied to give water planners an understanding of the limitations of the method as well as to help identify focus areas where the method can be improved in future efforts. The water planner can then keep these uncertainties in mind when expensive or long-term water projects are considered.