1. Trends in Runoff and Soil Moisture in the Western U.S. 1916-2003
Center for Science in the Earth System
Climate Impacts Group
and Department of Civil and Environmental Engineering
University of Washington
December, 2004
Alan F. Hamlet,
Phil Mote, Martyn Clark,
Dennis P. Lettenmaier

2. Changes in Simulated April 1 Snowpack for the Cascade Range in Washington and Oregon for Middle-of-the-Road Climate Change Scenarios
Current Climate
“2020s” (+1.7 C)
“2040s” (+ 2.5 C)
-44%
-58%
April 1 SWE (mm)

3. Seasonal Water Balance Naches River Current Climate 2040s Scenario
More runoff in winter and early spring, less in summer
2040s Scenario
(+ 2.5 C)

4. In temperature sensitive areas of the West, we should be able to see the effects of global warming in the historic snow and streamflow records.
Using models we should be able to more fully analyze these changes, as well as other hydrologic effects which are not typically measured.
What can we say about less sensitive areas?

5. Schematic of VIC Hydrologic Model and Energy Balance Snow Model
PNW CA
CRB GB
Snow Model

6. Overview of Simulation and Analysis
Met Data
Linear Trend
Analysis
VIC
SWE
(warm to cool PDO)
(cool to warm PDO)
with (warm to warm PDO)
Linear Trends:
Base—combined effects of temp and precip trends
Static Precip—effects of temperature trends only
Static Temp—effects of precipitation trends only
Experiments:

7. Trends in April 1 SWE
Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS (in press)

8. Trends in SWE 1916- 1997 a) 10 % Accumulation b) Max Accumulation
c) 90 % Melt
Trends in
SWE
1916-
1997
Change in Date
Change in Date
Change in Date
DJF Temp (C)
DJF Temp (C)
DJF Temp (C)
Change in Date
Change in Date
Change in Date
DJF Temp (C)
DJF Temp (C) FP
DJF Temp (C)
Change in Date
Change in Date
Change in Date
DJF Temp (C)
DJF Temp (C)
DJF Temp (C) FT
Change in Date
Change in Date
Change in Date

9. winter flows rise and summer flows drop
As the West warms,
winter flows rise and summer flows drop
Stewart IT, Cayan DR, Dettinger MD, 2004, Changes toward earlier streamflow timing across western North America, J. Climate (in review)
Spring snowmelt timing has advanced by days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

10. June March Trends in fraction of annual runoff 1947-2003 (all cells)
Relative Trend (% per year)

11. Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April 1)
March
June
Relative Trend (% per year)

12. Runoff June June Effects of temp and precip Effects of temp only
1916-
2003
Relative Trend (% per year)

13. Trends in Total Column Soil Moisture 1947-2003 (% per year)
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep

14. Soil Moisture Apr 1 Apr 1 Effects of temp and precip
Effects of temp only
Apr 1
Apr 1
1925-
2003
Relative Trend (% per year)

15. Soil Moisture Sep 1 Sep 1 Effects of temp and precip
Effects of temp only
Sep 1
Sep 1
1925-
2003
Relative Trend (% per year)

16. Conclusions
Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West as a result of increasing temperatures.
Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in late summer and fall soil moisture.
Because these effects are shown to be predominantly due to temperature changes, we expect that they will both continue and increase in intensity as global warming progresses in the 21st century.