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How Changing Species and Warming Temperatures Impact Streamflow in a Western U.S. Coniferous Forest
20
of Geography, University of California at Santa Barbara: garcia@geog.ucsb.edu; 2Bren School of Environmental Science and Management, University of California at Santa Barbara
RHESSys
Method of Investigation
100
80
Flow(mm/day)
40
60
20
0
Respiration: maintenance, and
growth f(T, N and biomass)
varies with type and size of
plant components
0
Current Climate
+2 oC
+4 oC
Base
+2 oC
+4 oC
30
!20
!10
0
10
20
+2 oC
+4 oC
!30
Percent Change in Streamflow
Total Monthly Streamflow Change Relative
to Current Climate: Douglas fir
Oct
Feb
Apr
Jun Jul Aug
+2 oC
+4 oC
!10
0
10
20
30
Dec
Total Monthly Streamflow Change Relative
to Current Climate: 90m Alder + Douglas fir
Oct
Dec
Feb
Apr
Jun Jul Aug
30
1
+4 oC
Percent Change in Streamflow
3000
2500
2000
1500
Streamflow (mm)
1000
500
0
3000
+2 oC
Total Average [10!yr] Annual Streamflow,
with Warming 90m alder + Doug fir
Total Monthly [10!yr average] Streamflow Change
Relative to Doug fir as baseline
Apr
Funded by a Grant from the USDA Forest Service
Feb
Jan
Aug
Oct
May
Mar
Sep
Jun
Jul
+2 oC
+4 oC
150
50
100
Old!growth Douglas Fir
90m Alder at Stream
0
Percent Change in Transpiration
Current Climate
+2 oC
+4 oC
Base
Percent Change in Transpiration
Relative to Current Climate Conditions
!50
500
200
300
400
20
Transpiration (mm)
Largest changes (as percent) in streamflow occur during the summer (20% decrease).
These reductions in summer streamflow with a shift to alder in riparian areas are
consistent with those reported by Hicks et al (1991) based on a 30 yr paired catchment
study in the HJ Andrews. Basin wide change to Western Hemlock (Tsuga heterophylla), a
conifer common to the HJA, highlights the impact on streamflow of a less productive (for
this current climate and site) conifer species
100
Aug
Annual Average [10!yr] Transpiration,
with Warming for Doug fir
0
Jun
10
Apr
0
Feb
!10
Dec
Percent Change in Streamflow
Oct
!20
90m Alder
+ Doug fir
Western Hemlock
60m Alder in Riparian Zone
90m Alder in Riparian Zone
!30
0
Doug fir
60m Alder + Doug fir
90m Alder + Doug fir
The maximum ~25% decrease in
streamflow with warming is of a similar
magnitude as the effect of a species
change scenario. However, the timing of
loss differs. In warming scenarios largest
reductions occurs in the early spring with
species change in the summer.
Scenarios that include alder in the
riparian zone have more productivity and
transpiration in the late summer. That
water use can be tied to a decrease in
total monthly streamflow.
Current Climate
+2 oC
+4 oC
Base
2500
Aug
A 10-yr average of percent difference in
monthly streamflow shows that the
greatest decrease occurs in early spring
and is due to a change in the timing of
snowmelt and earlier spring ET.
2000
Jun
A uniform temperature increase of 2 and
4 oC results in a basinwide decrease in
total annual streamflow, reflecting overall
gain in total evapotransporation (ET)
(largely due to increases in spring ET,
not shown)
Total Average [10!yr] Annual Streamflow,
with Warming Doug fir Forest Structure
1500
5
Transpiration (mm/day)
2
3
4
340
!
Apr
Old growth Douglas fir
60m Alder + Doug fir
90m Alder+ Doug fir
400
380
360
!
320
Transpiration (mm)
!
Feb
Daily [10 yr] Average Transpiration
Additional
Water Use
Occurs Late
Summer
!
Dec
Warmer Temperatures
Streamflow (mm)
Oct
Comparison of net primary productivity
(NPP) and transpiration between oldgrowth Douglas fir (Pseudotsuga
menziesii) forest and scenarios of red
alder (Alnus rubra) within 60- and 90meter riparian zones in combination with
the Douglas fir suggests that the addition
of alder results in greater annual PSN
and transpiration. Extension of alder
into areas farther from stream (90 vs 60)
however begins to show decline in both
water use and PSN (e.g so for higher
elevations Douglas fir is more
productive)
1000
PSN (gC/m2/day)
!5
0
5
!10
!15
90m Alder
+ Doug fir
10!yr Average of Total Annual Transpiration
300
Mack Creek is a 5.5 km2
subwatershed of Lookout
Creek Watershed, located in
the HJ Andrews Experimental
Forest in Western Oregon.
The temperate forest is
composed largely of
Douglas-fir (Pseudotsuga
meziesii) ,fir (Abies amabilis),
and western hemlock (Tsuga
heterophylla).
500
15
Old growth Douglas fir
60m Alder + Doug fir
90m Alder+ Doug fir
10
1000 1100
900
800
700
Photosynthesis (kg C)
600
500
Doug fir
60m Alder + Doug fir
90m Alder + Doug fir
60m Alder
+ Doug fir
Study Site
Impacts to Timing and Magnitude of Basin wide water patterns
Daily [10 yr] Average PSN
Additional
Water Use
Occurs Late
Summer
Doug fir
08/00
NPP - Allocated to leaves, stems and
roots; which impact photosynthetic
capacity and respiration costs
Species Changes
60m Alder
+ Doug fir
02/00
!20
Our long-term objective is to model multiple, interacting processes
that influence forest eco-hydrologic responses to warming at the
watershed scale. In this project , we focus on modeling interactions
between species change, climate warming and hydrologic
response.
A highly likely species change scenario is a shift from dominant
conifer species (Douglas fir, Western hemlock) to alder in riparian
areas following a fire – here we consider how this change might
interact and compare with changes in vegetation water use due to
warming
07/99
Date
m - decay of saturated hydraulic conductivity with depth
K - saturated hydraulic conductivity at the surface (units of meters per day)
psia – soil air entry pressure (units of meters of water)
po – soil pore size index
Precipitation lapse with elevation rate – variation from PRISM-derived lapse rate
RHESSys’ Vertical hydrological processing
Gross PSN - f(light, nutrient
availability, conductance),
and leaf area
Modeled Flow
Observed Flow
01/99
RHESSys’ Carbon & Nitrogen processing
10!yr Average of Total Annual NPP
Calibration of the watershed applies a basin-wide
scaling of initial soil drainage parameter values
utilizing a Monte-Carlo based approach. Streamflow
performance is based on metrics of Nash-Sutcliffe
efficiency for monthly and log-transformed flow and
total annual streamflow error.
As with all hydrologic models, calibration is needed to
define flow rates through critical zone soil and
permeable bedrock layers. We calibrate four
subsurface flow parameters:
Observed and Modeled Streamflow
We use the Regional Hydrologic Simulation System
(RHESSys) to simulate forest growth and net
photosynthesis. RHESSys is a GIS based, terrestrial
eco-hydrologic modeling framework designed to
simulate interactions among carbon, water, and nutrient
fluxes at the watershed scale. RHESSys models the
temporal and spatial variability of ecosystem processes
and interactions at a daily time step over multiple years
by combining a set of physically based process models
and a methodology for partitioning and parameterizing
the landscape.
!30
Introduction
Increased drought stress is a widely cited impact of a warming
climate on forest health. Warmer, drier conditions have been
linked to changes in forest productivity, biogeochemical
cycling, water use and vulnerability to disturbances such as fire
and insects. Changes in forest structure and function in turn
may have important hydrologic implications. Reductions in
summer streamflow often associated with earlier snowmelt
may be modified by concurrent changes in vegetation water
use. Understanding and modeling forest vulnerability to climate
variability requires considering how controls on vegetation
water use vary in space and time. Factors such as elevation,
tree size and species are often a key focus. This study
compares how variation in tree species composition and
uniform warming scenarios of 2 oC and 4 oC effect basin wide
transpiration, net primary productivity and streamflow.
Doug fir
10
Elizabeth Garcia1, Christina Tague2, Janet Choate2
1Department
Oct Nov Dec Jan
Mar Apr
Jun
Jul
Aug Sep
For all vegetation types, warmer climate resulted in
a greater amount of net annual transpiration. There
are declines in later summer ET – showing
evidence of water stress. These however are
balanced by increases in spring and winter ET