Land Cover Change and Climate Change Effects on Streamflow in Puget Sound Basin, Washington Lan Cuo 1, Dennis Lettenmaier 1, Marina Alberti 2, Jeffrey Richey 3 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 March 1, 2007 Climate Impact Group University of Washington

Background Early settlement started in the mid 1800s in the Puget Sound Basin. Population has increased by 17 times since % 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?

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

Methodology Generate 1/16 th degree gridded 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 temperature change effects using temperature regime detrended 1915, temperature regime detrended 2002, and historical temperature regime.

Methodology Model: Distributed Hydrology Soil Vegetation Model  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

Forcing Data –Basin Averaged Historical Annual Precipitation Eastern Puget Sound Basins Mean annual Prcp: 1200 mm – 2500 mm

Forcing Data –Basin Averaged Historical Annual Precipitation Western Puget Sound Basins Mean annual prcp: 1600 mm – 3000 mm

Forcing Data –Basin Averaged Historical Annual Tmin Eastern Puget Sound Basins Mean annual Tmin: -0.5 – 4.5 C

Forcing Data –Basin Averaged Historical Annual Tmin Western Puget Sound Basins Mean annual Tmin: -1.8 – 2.5 C

Forcing Data – Basin Averaged Historical Annual Tmax Eastern Puget Sound Basins Mean annual Tmax: 10 – 15 C

Forcing Data – Basin Average Historical Annual Tmax Western Puget Sound Basins Mean annual Tmax: 9 – 13 C

Data : 2002 Land Cover Map (Alberti et al., 2004) Land Cover TypesProportion (%) Dense urban (>75% impervious area) 2.41 Light-mediu urban (<75% impervious area) 3.97 Bare ground0.42 Dry ground1.30 Native grass0.05 Grass/crop/shrub5.36 Mixed/deciduous forest32.19 Coniferous forest36.41 Regrowth vegetation0.61 Clear cuts0.50 Snow/rock/ice7.85 Wetlands0.34 Shoreline0.13 Water8.46

Data : Reconstructed 1883 land cover Land Cover TypesProportion (%) Light-mediu urban (<75% impervious area) 0.40 Grass/crop/shrub7.43 Mixed/deciduous forest29.61 Coniferous forest48.23 Snow/rock/ice6.38 Water7.96 Source: 1.Department of Interior, Density of Forests-Washington Territory, Historical records of Puget Sound county population development

Results : Model Calibration

Results: Monthly Statistics of Calibrated and Measured Streamflow Basin (gage)Observation Mean (cms) Simulation Mean (cms) Correlation Coefficient RMSE (cms) Model Efficiency Cedar ( ) Deschutes ( ) Green ( ) Nisqually ( ) Puyallup ( ) Snohomish ( ) Stillaguamish ( ) Duckabush ( ) Quilcene ( ) Hamma Hamma ( ) Skokomish ( )

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

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

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

Results: Mean Annual Streamflow Basin (gage)1883 Land Cover (cms) 2002 Land Cover (cms) 2002 vs Change (%) Cedar ( ) Deschutes ( ) Green ( ) Nisqually ( ) Puyallup ( ) Snohomish ( ) Stillaguamish ( ) Quilcene ( ) Duckabush ( ) Hamma Hamma ( ) Skokomish ( ) Springbrook Creek ( ) Upper Mill Creek ( )

Results: Daily Peak Flow Eastern Puget Sound Basins Controlled Basin

Results: Daily Peak Flow Western Puget Sound Basins

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

Mann-Kendall Trend Analysis on Measurement and Model Residuals for Upland Gages Gage LocationGageStart PeriodEnd PeriodConfidence level Slope Cedar river near Cedar falls Duckabush river near Brinnon NF Skokomish at Hoodsport SF Skykomish at Index SF Stillaguamish at Granite Falls 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. Annual Maximum Daily Peak Flow (AMDPF)

Temperature Change Effects: Seasonal Flow Eastern Puget Sound Basins

Temperature Change Effects: Seasonal Flow Western Puget Sound Basins Warmer T regime Detrended 1915 Colder T regime Detrended 2002

Temperature Change Effects: Seasonal Flow Urbanization Affected Gages 71% urbanization 64% urbanization 31% urbanization

Basin (gage)Detrended 1915 vs. Historical Detrended 2002 vs. Historical DJFJJADJFJJA Cedar ( )-25%18%33%-21% Green ( )-10%7%10%-7% Nisqually ( )-9%14%7%-9% Puyallup ( )-8%7%9%-6% Snohomish ( )-6%-3%6%3% Stillaguamish ( )-10%9%10%-8% Quilcene ( )-3%11%2%-7% Duckabush ( )1%-5%-1%5% Hamma Hamma ( )2%-9%-2%9% Skokomish ( )2%-15%-2%14% Deschutes ( )0-4%04% Springbrook Creek ( )-0.3%-0.5%0.3%1% Upper Mill Creek ( )-0.2%-0.8%0.4%1% DJF: winter months, JJA: summer months Potential problems in summer

Temperature Change Effects: Mean Annual Flow Change Basin (gage)Detrended 1915 vs. Historical Detrended 2002 vs. Historical Cedar ( )-3%3% Deschutes ( )-0.2%0.2% Green ( )-0.4%0.5% Nisqually ( )-0.9%0.9% Puyallup ( )-0.7%0.7% Snohomish ( )-0.7%0.8% Stillaguamish ( )-0.3%0.3% Quilcene ( )-0.2%0.2% Duckabush ( )-0.5%0.5% Hamma Hamma ( )-0.5%0.4% Skokomish ( )-0.7%0.7% Springbrook Creek ( )-0.4%0.6% Upper Mill Creek ( )-0.3%0.5%

Temperature Change Effects: Daily Peak Flow Eastern Puget Sound Basins

Temperature Change Effects: Daily Peak Flow Western Puget Sound Basins

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

Mann-Kendall Trends of Raw Measurement: Combination of Climate Change Effects and Land Cover Change Effects GagesMaximum Daily Peaks Monthly QAnnual Q Confidence level SlopeConfidence level SlopeConfidence level Slope For upland basins, land cover is not a dominant effect in changing streamflow.

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.

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 in upland basins. 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.

Conclusions Temperature 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 streamflows in the upland basins. Regional climate system such as PDO perhaps plays a more important role in affecting streamflow in the upland basins.

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