Implications of 21st century climate change for the hydrology of Washington State


Elsner, M.M., Cuo, L., Voisin, N., Deems, J., Hamlet, A.F., Vano, J.A., Mickelson, K.E.B., Lee, S-Y., Lettenmaier, D.P. 2009. Implications of 21st century climate change for the hydrology of Washington State. Chapter 3.1 in The Washington Climate Change Impacts Assessment: Evaluating Washington’s Future in a Changing Climate, Climate Impacts Group, University of Washington, Seattle, Washington.


The 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.