Climate Warming and Water Budget Partitioning in
Willamette Valley Grasslands
Our Hypotheses
1) Climate warming will accelerate plant
phenological development and enhance
evapotranspiration during the spring
growing season.
2) Due to enhanced ET, groundwater recharge
will be reduced during the spring, and
therefore annually.
50
0
20
Spring Rainfall
Plant Growing Season
30
0
25
50
20
100
15
150
10
200
5
250
0
15
10
5
0
-5
April 1, 2010
April 2, 2010
April 3, 2010
April 4, 2010
Jan
Feb
Mar
Apr
May
June
July
Aug
Apr
June
Symmetrical
Warming,
March, 2009
Aug
Oct
Many Willamette Valley soils (and ours shown above)
contain large fractions of clay particles with structures
that are prone to shrinking and swelling during drying
and wetting cycles. The photo above was taken only
three days after a rain event in March, 2011.
We speculate that warming treatments may enhance
soil cracking and macropore development due to more
persistently dry soil conditions (figure 1b). This may
explain (1) generally greater R under warming
treatments in the fall, following the summer drought,
and (2) similar recession behavior in the spring, despite
lower soil water content under warming scenarios.
Dec
200
20
Volumetric Soil Moisture
On May 1, 2010:
ETsym was 26% (30 mm) > ETamb
ETasy was 32% (37 mm) > ETamb
0
0.5
30
Precipitation [mm d-1]
10
Figure 1a.
Cumulative daily evapotranspiration
under ambient climate (green), symmetric warming
(red), and asymmetric warming (blue). Error bars
represent +/- 1 standard deviation.
Warming
treatments enhanced ET, though there is
considerable variance among chambers within any
treatment.
40
Figure 1b. Average daily soil moisture content for
ambient climate (green), symmetric warming (red),
and asymmetric warming (blue) treatments.
A
volume weighted average soil moisture for each
chamber was calculated using TDR measurements at
5, 15, 35, 55, and 75 cm soil depth. A treatment
average was calculated from the four chambers per
treatment. Error bars represent +/- 1 standard
deviation.
0.4
0.3
0.2
0.1
Recharge [mm
R
0
300
d-1]
ΔS
Cumulative ET [mm]
400
On May 1, 2009:
ETsym was 43% (39 mm) > ETamb
ETasy was 40% (37 mm) > ETamb
0
20
16
May - June 2009:
Ramb was 189% (10.8 mm) > Rsym
Ramb was 237% (11.6mm) > Rasy
12
8
May 2009:
Ramb was 159% (10.2 mm) > Rsym and Rasy
4
0
2008
2009
2010
200
2007-2008
Figure 1c.
Average daily groundwater
recharge for ambient climate (green),
symmetric warming (red), and asymmetric
warming (blue) treatments.
Error bars
represent
+/1
standard
deviation.
Reductions in groundwater recharge caused by
warming-enhanced evapotranspiration become
evident only for late-spring rain events in May
of each year.
2008-2009
2009-2010
Figure 2b. Average annual groundwater recharge among temperature
treatments for three different water years. Error bars represent +/- 1
standard deviation from the average (n=4).
15
Results 1: Warming Generally Enhances ET, Though the Response is Highly Variable; A
Reduction in R is Observed During Late-Spring Rain Events
100
400
Results 3: Soil Structure and Hydraulic Function Moderate the Response of R to Imposed
Warming Treatments?
Sep
Ambient
Temperature,
March, 2009
Feb
600
Water Year
300
Hypothesized Acceleration and
Enhancement of ET During the Spring
asymmetric
Minimum T = 5 C > ambient
Maximum T = 2 C > ambient
Ambient
Symmetric
Asymmetric
0
10 11 12 1 2 3 4 5 6
10 11 12 1 2 3 4 5 6
Figure 2a. Average monthly groundwater recharge among temperature treatments for
three different water years. Error bars represent +/- 1 standard deviation from the
average (n=4).
Summer Drought
Plant Senescence
Evapotranspiration
symmetric
3.5 C > ambient
ET is measured by condensing chamber water
vapor and routing to tipping bucket; soil
moisture is measured using TDR probes at 5 soil
depths; drainage (groundwater recharge) is
measured by tipping bucket.
ET
Recharge (mm/year)
100
Imposed Temperature Treatments
ambient
800
150
25
Temperature [C]
 Ambient - (continuously replicates
conditions at local weather station
 Symmetric warming – 3.5°C > ambient
 Asymmetric warming – 3.5°C > ambient, with
Tmin 5°C and Tmax 2°C > ambient, respectively
200
Groundwater Recharge [mm/d]
Therefore, we pose two hypotheses:
The “Terracosm” research facility (TERA,
www.teraglobalchange.org) includes 12 sun-lit,
climate controlled growth chambers (2 m2
ground
area)
housing
native
grassland
communities that were planted in 2007, on a
native soil series contained by lysimeter (1 m
depth).
The chambers are irrigated with natural rainfall
in real time, there is continuous air circulation
and relative humidity control, and three
temperature treatments are imposed (n=4
chambers/treatment):
Winter Rainfall
Plant Dormancy
Average Monthly Temperature [C]
Projected increases in average annual temperature in the Pacific Northwest region of
the United States over the next century range from 1.6°C to 5.4°C (Mote and Salathe
Jr., 2010). Increasing surface air temperatures are expected to alter the landsurface hydrologic cycle (Huntington, 2006). We utilize a world-class climate change
research facility to examine how a 3.5°C warming signal alters the annual partitioning
of precipitation to groundwater recharge (R) and evapotranspiration (ET) in grassland
ecosystems native to the Willamette Valley.
2009-2010
Numeric Month
The climate in Oregon’s Willamette Valley is
temperate Mediterranean, with seasonal
rainfall occurring primarily during November
through April, and a seasonal drought period
during the summer. Native grasslands achieve
peak productivity during the spring, resulting
in a transient period during which plant
evapotranspiration draws from the same pool
of soil water that contributes to groundwater
recharge.
Our Approach
2008-2009
Ambient
Symmetric Warming
Asymmetric Warming
10 11 12 1 2 3 4 5 6
Average Monthly Rainfall [mm]
How will projected climate warming in the next century alter the annual
partitioning of precipitation to groundwater and evapotranspiration?
250
2007-2008
10
5
Figure 3a:
Storage versus R among
ambient
and
warming
treatments.
Volumetric soil moisture is the average
daily soil moisture by treatment, and R is
average daily groundwater recharge by
treatment. The R data were selected
exclusively from recession periods to
evaluate the influence of enhanced ET on
recession flow. A power function was fit
to each data set. R2 values were 0.55,
0.59, and 0.41 for ambient, symmetric,
and asymmetric warming treatments,
respectively.
Ambient
Symmetric Warming
Asymmetric Warming
data4
fit 2
fit 3
R = f(S) varies among temperature
treatments; similar R rates during
recessions occur despite less soil
water storage under warming
scenarios.
0
0.25
0.3
0.35
0.4
0.45
0.5
Volumetric Soil Moisture
15
Groundwater Recharge [mm/d]
Our Question
www.teraglobalchange.org
Recharge (mm/month)
Luke Pangle1, Jillian Gregg2, Jeffrey McDonnell3
1Water Resources Graduate Program, Oregon State University
2Terrestrial Ecosystems Research Associates, Corvallis, OR
3Forest Engineering, Resources and Management, Oregon State University
Results 2: 3.5°C Warming Causes No Reduction in Annual Groundwater Recharge;
Reductions in Spring Recharge Constitute Less Than 4% of Annual Total Among Years.
Figure 3a: Relative Storage versus R
among ambient and warming treatments.
The average daily soil moisture for each
treatment was divided by the maximum
daily soil moisture to obtain a relative
value. R2 values are similar as in Figure
3a.
10
5
0
0.5
Variability in R = f(S) among
ambient and warming treatments
diminishes when the relative soil
moisture is considered
0.6
0.7
0.8
0.9
1
1.1
Relative Soil Moisture
Conclusions
 We show that an average temperature increase of 3.5°C does not
significantly alter the annual partitioning of precipitation to
groundwater recharge in a grassland ecosystem in the Willamette Valley (Table 1).
A 3.5° temperature increase generally enhances spring ET, though
corresponding reductions in groundwater recharge during the spring
comprise less than 4% of annual recharge among the three years we examined.
We suggest that the 3.5°C temperature increase may promote soil cracking in
shrink/swell clay soils , which likely contributed to the generally greater
recharge rates in fall following the summer drought, and to the lack of
sensitivity of recharge recession flow to the enhanced ET under warming treatments.
Annual R/P
Water Year
Ambient
Symmetric
Warming
Asymmetric
Warming
2007-2008
0.44 (0.03)
0.42 (0.06)
0.47 (0.08)
2008-2009
0.31 (0.09)
0.35 (0.09)
0.34 (0.10)
2009-2010
0.37 (0.10)
0.39 (0.06)
0.37 (0.10)
Table 1. Ratios of annual groundwater recharge to annual precipitation. Values
in parentheses are standard deviations from the treatment averages (n=4).
REFERENCES
- Mote, P.W. and E.P. Salathe Jr. 2010. Future Climate in the Pacific Northwest. Climatic Change 102: 29-50.
-Huntington, T.G. 2006. Evidence for Intensification of the Global Water Cycle: Review and Synthesis. Journal of Hydrology 319: 83-95.
ACKNOWLEDGEMENTS
Thanks goes to Terrestrial Ecosystems Research Associates, the Water Resources Graduate program at OSU, and the OSU College of Forestry,
who have all helped support this research.