1. 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

2. 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?

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

4. 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.

5. 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

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

7. 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

8. 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

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

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

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

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

13. Results: Land Cover Change Effects: Seasonal Flow
Cedar ( )
Hamma hamma ( )
Mill Creek ( )
71% Urbanization
Springbrook Creek ( )
64%
Green ( )
Skokomish ( )
Deschutes ( )
Stillaguamish
( )
31%
Puyallup ( )
Upland Gauges
Lowland Gauges

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

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

16. 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
267 32
115
114
Days having peak flows above annual mean maximum peak flow in 2002 scenario
Gages
Land cover 1883
Land cover 2002 30 35 34 38 33 36 31 43 47 2 27

17. 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.

18. Thank You !