Land Cover 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 December 11, 2006 San Francisco, CA

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

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, November – April Land cover: 82% vegetation 7% urban 11% other

Methodology Generate forcing data and land cover maps for the study area. Calibrate and validate hydrology model in pilot basins. Apply the calibrated model to other basins, calibrate again if necessary. Remove the long term trend in temperature to isolate land cover change effects.

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

Methodology Forcing Data: 1 January 1915 – 30 September 2003, 1/16th degree grid, Hamlet and Lettenmaier (2005) Detrended Tmin Tmax Historical

Methodology: 2002 Land Cover Map (Alberti, et al., 2004) Land Cover Types percent Dense urban (>75% impervious area) 2.41 Light-mediu urban (<75% impervious area) 3.97 Bare ground 0.42 Dry ground 1.30 Native grass 0.05 Grass/crop/shrub 5.36 Mixed/deciduous forest 32.19 Coniferous forest 36.41 Regrowth vegetation 0.61 Clear cuts 0.50 Snow/rock/ice 7.85 Wetlands 0.34 Shoreline 0.13 Water 8.46

Methodology: Reconstructed 1883 land cover Land Cover Types 1883 Light-mediu urban (<75% impervious area) 0.40 Grass/crop/shrub 7.43 Mixed/deciduous forest 29.61 Coniferous forest 48.23 Snow/rock/ice 6.38 Water 7.96 Source: Department of Interior, Density of Forests-Washington Territory, 1883 2. Historical records of Puget Sound county population development

Methodology: Basins Cedar Basin Stillaguamish Hamma hamma Skokomish Green Puyallup Deschutes Mill Creek and Springbrook Creek Basins

Results: Calibration in Cedar Basin and Urban basin Daily Statistics 12115000 Obs. Mean 7.92 Sim. Mean 8.55 RMSE 4.16 Model Efficiency 0.75 Model Efficiency: Nash & Sutcliff (1970)

Results: Validation in Cedar Basin Daily Statistics 12115000 12117000 Obs. Mean 6.91 2.65 Sim. Mean 6.76 2.89 RMSE 4.64 1.53 Model Efficiency 0.55 0.64

Results: Calibration in Green, Deschutes, Puyallup, Stillaguamish, Skokomish, and Hamma Hamma

Results: Land Cover Change Effects: Seasonal Flow Cedar (12115000) Hamma hamma (12054500) Mill Creek (12113349) 71% Urbanization Springbrook Creek (12113346) 64% Green (12104500) Skokomish (12056500) Deschutes (12078720) Stillaguamish (12161000) 31% Puyallup (12094000) Upland Gauges Lowland Gauges

Results Annual Q (units: cms) Gages Land cover 1883 Land cover 2002 Change in Percentage 12104500 10.81 11.95 11% 12115000 6.75 7.20 7% 12056500 17.11 19.62 15% 12054500 9.86 10.99 12% 12161000 31.17 31.70 2% 12078720 0.98 1.06 8% 12113349 0.43 0.53 23% 12113346 0.20 0.24 20%

Results: Land Cover Change Effects: Daily Peak flow Urbanization = 31% Urbanization = 71% Urbanization = 64% Controlled Basin

Land Cover Change Effects: Daily Peak Flow Peak flow timing difference in urbanized basins Gages Number of Peak flow Cases Studied Cases with 1 Leading Day in 2002 Scenario 12078720 267 32 12113349 115 12113346 114 Days having peak flows above annual mean maximum peak flow in 2002 scenario Gages Land cover 1883 Land cover 2002 12104500 30 35 12115000 34 38 12056500 33 36 12054500 12161000 31 12078720 43 47 12113349 2 12113346 27

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. In lowland basins, the proportion of urbanization determines basin response in seasonal streamflow. On average, mean annual streamflows are slightly higher under current land cover condition. Peak flow timing is mainly affected by infiltration excess runoff and therefore peak flows tend to occur earlier for highly urbanizing 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.

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