Implications of 21st Century Climate Change for the Hydrology of Washington StateMarketa M Elsner1, Lan Cuo2, Nathalie Voisin2, Jeffrey S Deems2, Alan F Hamlet1,2, Julie A Vano2, Kristian EB Mickelson2, Se-Yeun Lee2, and Dennis P Lettenmaier1,2AbstractThe hydrology of the Pacific Northwest (PNW) is particularly sensitive to changes in climate because seasonal runoff is dominated by snowmelt from cool season mountain snowpack, and temperature changes impact the balance of precipitation falling as rain and snow. Based on results from 39 global simulations performed for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), PNW temperatures are projected to increase an average of approximately 0.3°C per decade over the 21st century, while changes in annual mean precipitation are projected to be modest, with a projected increase of 1% by the 2020s and 2% by the 2040s. Based on IPCC AR4 projections, we updated previous studies of implications of climate change on the hydrology of the PNW. In particular, we used results from 20 global climate models (GCMs) and two emissions scenarios from the Special Report on Emissions Scenarios (SRES): A1B and B1. PNW 21st century hydrology was simulated using the full suite of GCMs and 2 SRES emissions scenarios over Washington, as well as focus regions of the Columbia River basin, the Yakima River basin, and those Puget Sound river basins that supply much of the basin’s municipal water supply. Using two hydrological models, we evaluated projected changes in snow water equivalent, seasonal soil moisture and runoff for the entire state and case study watersheds for A1B and B1 SRES emissions scenarios for the 2020s, 2040s, and 2080s. We then evaluated future projected changes in seasonal streamflow in Washington. April 1 snow water equivalent (SWE) is projected to decrease by an average of approximately 27-29% across the State by the 2020s, 37-44% by the 2040s and 53-65% by the 2080s, based on the composite scenarios of B1 and A1B, respectively, which represent average effects of all climate models. In three relatively warm transient watersheds west of the Cascade crest, April 1 SWE is projected to almost completely disappear by the 2080s. By the 2080s, seasonal streamflow timing will shift significantly in both snowmelt dominant and transient, rain-snow mixed watersheds. Annual runoff across the State is projected to increase by 0-2% by the 2020s, 2-3% by the 2040s, and 4-6% by the 2080s; these changes are mainly driven by projected increases in winter precipitation.1JISAO Climate Impacts Group, University of Washington, Seattle, Washington2Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington3: Hydrology and Water Resources69CHAPTER 3: Hydrology and Water Resources: Washington State1. IntroductionThe Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) states that warming of Earth’s climate is unequivocal and that anthropogenic use of fossil fuels has contributed to increasing carbon dioxide concentrations and thereby warming of the atmosphere (IPCC, 2007). The hydrology of the Pacific Northwest (PNW - which typically includes the Columbia River basin and watersheds draining to the Oregon and Washington coasts) is particularly sensitive to changes in climate because of the role of mountain snowpack on the region’s rivers. In this paper, we utilize archived climate projections from the IPCC AR4 to evaluate impacts on regional hydrology, with focus on Washington, which includes the lower Columbia River basin in the eastern and southern part of the State, as well as coastal drainages, including the Puget Sound basin (Figure 1).Washington is partitioned into two distinct climatic regimes by the Cascade Mountains. The west side of the Cascades on average receives approximately 1,250 mm of precipitation annually, while the east side receives slightly more than one-quarter of this amount. Washington, like much of the western US, relies on cool season precipitation (defined as October through March) and resulting snowpack to sustain warm season streamflows (defined as April through September). Approximately 75% of the annual precipitation in the Cascades falls during the cool season (Snover and Miles, in review). A changing climate affects the balance of precipitation falling as rain and snow and therefore the timing of streamflow over the course of the year. Figure 2 illustrates simulated historical mean annual runoff over the period 1916-2006 using the Variable Infiltration Capacity hydrologic model (further described below) and shows the importance of the State’s mountainous regions with respect to water supply for various natural resources.Small changes in temperature can strongly affect the balance of precipitation falling as rain and snow, depending on a watershed’s location, elevation, and aspect. Washington, and the Pacific Northwest as a whole, is often characterized as having three runoff regimes: snow-melt dominant, rain dominant, and transient (Hamlet and Lettenmaier 2007). In snowmelt dominant watersheds, much of the winter precipitation is stored in the snowpack, which melts in the spring and early summer resulting in low streamflow in the cool season and peak streamflow in late spring or early summer (May-July). Rain dominant watersheds are typically lower in elevation and mostly on the west side of the Cascades. They receive little snowfall. Streamflow in these watersheds peaks in the cool season, roughly in phase with peak precipitation (usually November through January). Transient watersheds are characterized as mixed rain-snow due to their mid-range elevation. These watersheds receive some snowfall, some of which melts in the cool season and some of which is stored over winter and melts as seasonal temperatures increase. Rivers draining these watersheds typically experience two streamflow peaks: one in winter coinciding with seasonal maximum precipitation, and another in late spring or early summer when water stored in snowpack melts. Hydrographs of simulated average historic streamflow, which are representative of the three watershed types, are shown in Figure 3. 70CHAPTER 3: Hydrology and Water Resources:
View Full Document
Unlocking...