INCREASED WINTER-­‐SEASON SNOWPACK ABLATION FOLLOWING SEVERE FOREST DISTURBANCE: IMPLICATIONS FOR NEGATIVE FEEDBACKS ON WATER AVALIABILITY 1,2
2
2
3
Adrian Harpold , Paul Brooks , Joel Biederman , and David Gochis 1Ins$tute for Ar$c and Alpine Research, University of Colorado, Boulder, CO, 2 University of Arizona, Tucson, AZ, 3Na$onal Center for Atmospheric Research, Boulder, CO Background and Field Sites
•  Severe forest disturbance due to fire, insects, and drought is increasing in the Western US •  We summarize the results of two studies inves?ga?ng winter-­‐season snowpack processes in the Rocky Mountain (Fig. 1) at three field sites: Chimney Park (tree morality from Mountain Pine Beetle (MPB) is >60%), Niwot Ridge (healthy control site), and Valles Caldera (100% tree mortality at Post-­‐burn site, 0% at Unburned site). Healthy
Unburned
MPB-effected
Post-burn
Competing Hypotheses for Snowpack Processes Following Disturbance
1. Reduced interception and sublimation from the canopy results in greater inputs to the snowpack (a)
2. Increased winter-season ablation from the snowpack surface results in reduced peak accumulation
Snow depth (m)
0.3
Results and Discussion
New Snow Accumulation Increases Following Disturbance
Fig. 2 •  Healthy forests intercept roughly 20-40% of incoming
snowfall (Fig. 2 following MPB and 3 following fire).
•  Interception by grey phase trees is near zero (Fig. 2).
•  A single storm produces significantly more new snow in a
post-burn area (Fig. 3b) than a healthy forest (Fig. 3a).
0.2
Mean = 0.143 m
0.2
x
x
a
a,b
0.1
0.1
x
b,c
a,b
Fig. 3 0.0
0.0
Open
Snow depth (m)
Fig. 1(b) 0.3
x
Open
Sparse Medium Dense
(a)
Mean = 0.512 m
0.9
b
x
x
y
c
0.3
0.3
0.0
0.0
Open
Sparse Medium Dense
y
Sparse Medium Dense
Fig. 4 140
(b)
Mean = 0.456 m
0.6
b
Sparse Medium Dense
0.9
a
0.6
Open
Changes in Peak Snow Distribution Following Disturbance
Fig. 5 a.
110
80
50
0.4
New snow accumulation
3/11 and 3/12/2012
b.
110
110
80
-80
-60
-40
-20
0
20
Position on Transect [m]
40
Declines in Peak Snow Following Disturbance
60
80
100
0.1
Fig. 6 None Sparse Medium Dense
•  There is no statistical difference in the SWE to P ratio (SWE:P), or
the amount of SWE at max accumulation relative to total
precipitation, in green, red, and grey MPB-effected areas (Fig. 6).
•  The SWE:P ratio declined relative to pre-burn snowpacks and
suggested 10% greater losses than an Unburned area (Fig. 7).
0.75
a.
0.0
α
4/1/2010
ω
0.6
0.65
τ
3/10/2012
δ
mean = 0.63
n = 923
0.6
d.
ω, η
(b)
mean = 0.63 0.75
n = 1199
θ
θ
θ
η
µ
0.55
0.0
Sparse Medium Dense
UnburnNone
2010
Pre-burn
2010
Sparse Medium Dense
Canopy Density
to
a.
δ2 H „
a
a
c
0.3
Snow Temperature (C)
-6
-4
-2
-130
-140
-150
-150
-160
-160
-170
-170
-21
-20
-19
-18
-23
-22
-21
-20
-19
Unburn
2010
60
Conclusions and Implications
0
(b)
80
Pre-burn Unburn Post-burn
2010
2012
2012
60
40
40
20
20
Fig. 9 0
0.1
0.2
0.3
0.4
3
-4-4
-2-2
00
(a)
(c)
80
80
0.0
100
100
60
60
40
40
40
40
20
20
20
20
-6
100
0.1
0.1
0.2
0.2
0.3
0.3
3
Snow density (kg/m3 )
Snow density (kg/m )
Snow Temperature (C)
New Snow
-190
80
0.4
0.4
-4
-2
Snow
temperature
0.2
0.3
-6
-6
-4
-4
-2
-2
0
0
(b)
(d)
0
0
0.0
0.0
0
0.1
0.1
0.2
0.2
0.3
0.3
-4
Rounds
-6
-2
0
Small Facets
(c)
80
60
60
40
40
20
20
0
0.4
0.4
3
Snow density (kg/m3 )
Snow density (kg/m )
Snow Temperature (C)
100
Crust layers
0.4
3
Snow density (kg/m )
Snow Temperature (C)
Snow Temperature (C)
60
60
0.0
0.0
0.1
80
80
00
δ18O „
-2
(a)
80
-180
-18
-4
100
-6-6
100
100
-140
Snowfall
Snowpack 6.2δ2H + 25.2
LMWL 8.1δ2H + 16.5
-6
0
Snow density (kg/m )
Snow Temperature (C)
Snow Temperature (C)
b.
Snowfall
Snowpack
LMWL 8.0δ2H + 10
b
0.6
0
-130
-22
Fig. 7 0.0
Fig. 8 -23
0.3
100
0.0
-190
c
Snow Temperature (C)
0.65
0.3
Evidence of Increased Snowpack Sublimation Following Disturbance
-180
b
0.9
0.65
0.55
None
Unburn 2010
Pre-burn 2010
0.6
(b)
0.9
η
0.3
a
0.0
0.55
Snow depth (m)
SWE : P
Snow depth (m)
ε
0.55
c.
a
mean = 0.64 0.75
n = 746
π
ε
1.2
0.75
0.9
Unburn 2010
Pre-burn 2010
(a)
0.0
•  Isotopic enrichment of the snowpack relative to the local
meteoric water line, suggests kinetic fractionation due
increased sublimation following MPB disturbance (Fig. 8b).
•  The large facets in the Post-burn site (Fig. 9b) indicate
strong temperature and vapor pressure gradients
b.
γ
1.2
0.9
mean = 0.62
n = 1766
β
0.65
None Sparse Medium Dense
SWE:P (m/m)
-100
0.2
North
South
50
1.2
Snow depth (m)
c.
Snow depth (cm)
140
(a)
0.3
Snow depth
depth (cm)
Snow
(cm)
50
none
sparse
medium
dense
Snow depth (m)
80
Snow depth (cm)
140
Depth [cm]
•  Snow surveys made at peak accumulation show a
strong dependence on forest structure in healthy areas,
where larger depths are associated with canopy gaps.
•  Spatial variability in snowdepths decline following loss
of forest canopy by MPB (Fig. 4) and fire (Fig. 5).
Mean = 0.094 m
(d)
Large Facets
0
•  Despite a decrease in snow intercep?on following severe forest disturbance, there was liCle change (aEer MPB-­‐
effects) or a small decline in peak snowpacks (aEer fire). •  Increased sublima?on is likely driving compensa?ng winter vapor losses due to increases in turbulence and/or radia?on following the loss of the forest canopy. •  Snowpack energy balance modeling is needed to help quan?fy the importance of these compensa?ng effects. 0.0
0.1
0.2
0.3
3
Snow density (kg/m )
Please look for our papers submiCed to Ecohydrology: Biederman et al. (2012) and Harpold et al. (2012) Thanks to support from NSF for the Jemez Cri?cal Zone Observatory, NSF Pine Beetle Funding (Brook and Gochis), and NSF post-­‐doctoral funding (Harpold) 0.4
0.0
0.1
0.2
0.3
3
Snow density (kg/m )
Snow temperature
Rounds
New Snow
Small Facets
Crust layers
Large Facets
0.4