Land Cover Change and Climate Change Effects on Streamflow in Puget Sound Basin, Washington

1. Land Cover Change and Climate Change Effects on Streamflow in Puget Sound Basin, Washington Lan Cuo1, Dennis Lettenmaier1, Marina Alberti2, Jeffrey Richey3 1: Department of Civil and Environmental Engineering, University of Washington 2: Department of Urban Design and Planning, University of Washington 3: Department of Chemical Oceanography, University of Washington February 21, 2007 University of Washington

2. Background Early settlement started in the mid 1800s in the Puget Sound Basin. Population has increased by 17 times since 1900. 70% of Washington state population lives in the Puget Sound Basin. Land cover change is mainly caused by logging and urbanization. Temperature is changing in the Puget Sound. • Objectives How does land cover change affect streamflow in the Puget Sound Basin? How does temperature change affect streamflow in the Puget Sound Basin?

3. Methodology • Study Area - Puget Sound Basin • Area: 30,807 sqr.km • Bounded by the Cascade and Olympic Mountains • Maritime climate, annual precipitation 600 mm - 3000 mm, October – April • Land cover: 82% vegetation 7% urban 11% other

4. Methodology • Generate forcing data and land cover maps for the study area. • Calibrate hydrology model. • Study land cover change effects by removing the long term trend in temperature. • Study climate change effects using temperature regime detrended to 1915, temperature regime detrended to 2002, and historical temperature regime.

5. Methodology • Interception • Evapotranspiration • Snow accumulation and melt • Energy and radiation balance • Saturation excess and infiltration excess runoff • Unsaturated soil water movement • Ground water recharge and discharge • Model: Distributed Hydrology Soil Vegetation Model

6. Forcing Data –Basin Averaged Historical Annual Precipitation Eastern Puget Sound Basins

7. Forcing Data –Basin Averaged Historical Annual Precipitation Western Puget Sound Basins

8. Forcing Data –Basin Averaged Historical Annual Tmin Eastern Puget Sound Basins

9. Forcing Data –Basin Averaged Historical Annual Tmin Western Puget Sound Basins

10. Forcing Data – Basin Averaged Historical Annual Tmax Eastern Puget Sound Basins

11. Forcing Data – Basin Average Historical Annual Tmax Western Puget Sound Basins

12. Data: 2002 Land Cover Map (Alberti et al., 2004)

13. Data: Reconstructed 1883 land cover • Source: • Department of Interior, Density of Forests-Washington Territory, 1883 • 2. Historical records of Puget Sound county population development

14. Results: Calibration

15. Results:Calibration

16. Results: Monthly Statistics of Calibrated and Measured Streamflow

17. Results: Land Cover Change Effects: Seasonal Flow Eastern Puget Sound Basins

18. Results: Land Cover Change Effects: Seasonal Flow Western Puget Sound Basins

19. Results: Land Cover Change Effects: Seasonal Flow 71% urbanization Urbanization Affected Gages 64% urbanization 31% urbanization

20. Results: Mean Annual Streamflow

21. Results: Daily Peak Flow Eastern Puget Sound Basins

22. Results: Daily Peak Flow Western Puget Sound Basins

23. Results: Daily Peak Flow 71% urbanization Urbanization Affected Gages 64% urbanization 31% urbanization

24. Mann-Kendall Trend Analysis on Measurement and Model Residuals for Upland Gages Annual Maximum Daily Peak Flow (AMDPF) • No significant trend was found in monthly and annual streamflow at the above gages. • Although model simulation shows increase trend in AMDPF and annual streamflow for upland basins, the trend might not be statistically significant.

25. Climate Change Effects: Seasonal Flow Eastern Puget Sound Basins

26. Climate Change Effects: Seasonal Flow Western Puget Sound Basins

27. Climate Change Effects: Seasonal Flow 71% urbanization Urbanization Affected Gages 64% urbanization 31% urbanization

28. DJF: winter months, JJA: summer months

29. Climate Change Effects: Daily Peak Flow Eastern Puget Sound Basins

30. Climate Change Effects: Daily Peak Flow Western Puget Sound Basins

31. Climate Change Effects: Daily Peak Flow 71% urbanization Urbanization Affected Gages 64% urbanization 31% urbanization

32. Climate Change Effects: Mean Annual Flow Change

33. Mann-Kendall Trends of Raw Measurement: Combination of Climate Change Effects and Land Cover Change Effects For upland basins, land cover is not a dominant effect in changing streamflow.

34. Pacific Decadal Oscillation (PDO) • Positive phase (+): warmer and dryer climate • Negative phase (-): colder and wetter climate • In upland basins, PDO perhaps play a more important role than land cover change effects.

35. Conclusions • In upland basins, fall, winter and spring streamflows are higher under current land cover condition because of lower ET. Summer streamflow is lower in 2002 scenario because of less water storage in the basin. • On average, mean annual streamflows are slightly higher under current land cover condition which might not be statistically significant. • Peak flows are affected by the combination of ET and infiltration excess runoff. Peak flows tend to be higher under current land cover condition for most basins. • Chances of getting peak flows are higher under current land cover condition.

36. Conclusions • Climate change mainly affects upland basins where snow occurs. Temperature change mainly affects seasonal distribution of streamflow. Warmer temperature regime tends to generate higher winter flow but lower summer flow due to less snow occurrence, early snow melt and less basin snow storage. • Simulation shows that land cover change might be more important than climate change in affecting the streamflow in lowland urbanizing basins. • Trend study in upland gauged stations shows that land cover change is not the dominant factor that influences peak flows, monthly and annual flows in the upland basins. • Regional climate system such as PDO perhaps plays a more important role in affecting streamflow in the upland basins.

37. Thank You !