Puget Sound Past, Present, and Future: 150 years of hydrologic change
The Puget Sound basin has changed remarkably since Anglo settlement in the mid-1800s. The primary agents of change have been urbanization, primarily in the lowlands, forest harvest (lowlands and uplands), and a warming climate. To the extent that changes in the fluxes of freshwater to Puget Sound have occurred, they have potentially important implications not only for the circulation within the Puget Sound estuary, and hence the ecology of the Sound, but also for the fluxes of pollutants and other constituents into the Sound.
APPROACH AND RESULTS
Land cover maps of the Puget Sound basin from the late 1800s show that most of the basin was then mature conifer, which has transitioned to extensive urbanization in the lowlands, and reduced forest maturity due to logging across most of the non-urbanized portion of the basin at present (Figure 1). At the same time, there has been a general warming, mostly manifested by increases in daily minimum temperatures (trends are less apparent in daily temperature maxima). Precipitation trends over the last century, in contrast, mostly have been not statistically significant (Cuo et al., 2009). We used a spatially distributed hydrology model to deconvolve the concurrent effects of changing climate and land cover on the discharge of major rivers in the basin. Our results show that land cover and temperature change effects on streamflow have occurred differently at high and low elevations. In the lowlands, land cover has occurred primarily as conversion of forest to urban or partially urban land use, and here the land cover signal on streamflow dominates temperature change. In the uplands, both land cover and temperature change have played important roles. Temperature change is especially important at intermediate elevations (so-called transient snow zone), where the winter snow line is most sensitive to temperature change—notwithstanding the effects of forest harvest over the same part of the basin. Our model simulations show that current land cover in the lowlands has resulted in mostly higher fall, winter and early spring streamflow but lower summer flow in the lowlands; with higher annual maximum flows as compared with pre-development conditions. For the upland portion of the basin, shifts in the seasonal distribution of streamflow (higher spring flow and lower summer flow) are clearly related to rising temperatures, but annual streamflow has not changed much.
We also evaluated the implications of future (mid-21st Century) climate and land cover on the basin as compared with current climate and land cover (Figure 2). We used global climate change simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), statistically downscaled and used as input to a spatially distributed hydrologic model. We represented future land cover by a 2027 projection, extrapolated to 2050 (Cuo et al., 2011). In general, the results are similar to the historic reconstructions. Future climate change projections are mostly for warmer temperatures, with precipitation, averaged across global models, showing only modest changes. Changes in predicted future streamflow are evidenced primarily through changes in the timing of winter and spring runoff, from the uplands, with much more modest changes in lowland streams. Upland land cover-related changes are primarily associated with modest projected reductions in forest maturity, resulting in slight increases in annual runoff. The most climatically sensitive parts of the uplands are in areas where the current winter precipitation is in the rain–snow transition zone. Changes in land cover are generally more important than climate change in the lowlands, where a substantial change to more urbanized land use and increased runoff is predicted. Both the annual total and seasonal distribution of freshwater flux to Puget Sound are more sensitive to climate change than to land cover change, primarily because most of the runoff originates in the uplands. Both climate and land cover change slightly increase the annual freshwater flux to Puget Sound (Figure 2). Changes in the seasonal distribution of freshwater flux are mostly related to climate change, and consist of double-digit increases in winter flows and decreases in summer and fall flows.
Cuo, L., D.P. Lettenmaier D.P., M. Alberti, and J.E. Richey, 2009: Effects of a century of land cover and climate change on the hydrology of Puget Sound basin, Hydrological Processes, 23, 907-933.
Cuo, L., T.K. Beyene, N. Voisin, F. Su, D.P. Lettenmaier, M. Alberti, and J.E. Richey, 2010: Effects of mid-twenty-first century climate and land cover change on the hydrology of the Puget Sound basin, Washington. Hydrological Processes, 25, 1729–1753, doi: 10.1002/hyp.7932.
Dennis Lettenmaier (Department of Civil and Environmental Engineering, University of Washington) firstname.lastname@example.org
Marina Alberti (Urban Ecology Research Lab, University of Washington) email@example.com
Jeff Richey (School of Oceanography, University of Washington) firstname.lastname@example.org