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Engineering Urban Green Spaces for
Evapotranspiration: Vegetation and Climate
1 Department of Civil, Architectural and Environmental Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104
2 Center for Climate Systems Research/NASA Goddard Institute for Space Studies, Columbia University, 2880 Broadway, New York, NY 10025
Kimberly DiGiovanni1, Stephanie Miller1
Franco Montalto1 and Stuart Gaffin2
PA Stormwater Management Symposium 2013
Session 2b: Stormwater Control Measures -The Importance of Vegetation
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Practical Motivation & Goal
• Engineer for ET
• ET is a significant flux that expands the capacity for GI facilities to capture stormwater and provide other ecosystem services
• NPDES permit holders seek to quantify ET volumes from different kinds of GI facilities (Philadelphia Water Department 2012; NYCDEP 2013)
2
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Factors Impacting ET
• Meteorology
• Vegetation
• Media Moisture
3
Image adapted from Brouwer and Heibloem (1986)
Meteorology
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Objective
• Quantify ET from in-situ urban green spaces and in laboratory studies
• Evaluate the role of vegetation and climate in influencing ET
4
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Definitions
• Evapotranspiration (ET)
• Potential Evapotranspiration (PET)
• Reference Evapotranspiration (RET)
• Actual Evapotranspiration (AET)
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Overview of presentation
• Field Studies – Monitoring Sites
– Methods
– Results
• Laboratory Studies – Methods
– Results
• Implications of Findings and Future Work
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MONITORING AND DATA ACQUISITION
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Google Maps and Bing9
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Sites1. UP-Alley Pond (UP-AP)
2. Bio-Colfax (Bio-C)
3. GR-Columbia (GR-C)
4. GR- Fieldston (GR-F)
5. Air-John F. Kennedy (Air-JFK)
6. Air-La Guardia (Air-LG)
7. Bio-Nashville (Bio-N)
8. GR-Queens Botanical Garden (GR-QBG)
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Monitored Sites
• Bioretention (Bio)
• Green Roofs (GR)
• Urban Parks (UP)
• Airports (Air)*
11
*Data sets acquired from NRCC
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Bio-C
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GR-F
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UP-AP
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Weighing Lysimeters
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Weighing Lysimeters
16
GR-F
Green Roof
Sedum
species
Bio-N
Bioretention Area
(receives precipitation
and stormwater)
Juncus effusus
Bio-C
Bioretention Area
(right, receives
precipitation)
Juncus effusus
UP-AP
Urban Park (left)
Mixed natives
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DETERMINATION OF EVAPOTRANSPIRATION
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ASCE Standardized Reference Evapotranspiration Equation (short grass reference surface – well watered)
𝐸𝑇𝑠𝑧 =0.408 𝛥 𝑅𝑛 − 𝐺 + 𝛾
𝐶𝑛𝑇 + 273 𝑢2 𝑒𝑠 − 𝑒𝑎
𝛥 + 𝛾 1 + 𝐶𝑑𝑢2
ASCE-EWRI (2005)
18
Rn – net radiationG – soil heat flux Δ – slope of the saturation vapor pressure deficit curveγ – phsychrometric constant
Cn – numerator constantu2 – wind speed at 2 meters height es – saturated vapor pressure ea – actual vapor pressure Cd – denominator constant
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Water Balance: Weighing Lysimeter
𝐸𝑇 =
𝑖=1
𝑥
𝑓𝑚𝑖 −𝑚𝑖+1𝜌𝐴
𝑖
• 𝑓 is a conversion factor
• 𝑚 is the mass of the lysimeter
• 𝐴 is the surface area of the lysimeter
• ρ is the density of water
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RESULTS
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Jul-2011 Sep-2011 Oct-2011 Dec-2011 Jan-2012 Mar-2012 May-2012 Jun-20120
1
2
3
4
5
6
7
8
9
10E
va
potr
ansp
iratio
n (
mm
/da
y)
21
Ref
eren
ce E
vap
otr
ansp
irat
ion
(m
m/d
ay)
Bio-C
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22
327 non-consecutive days over one year period
Bio-C GR-C GR-F Air-JFK Air-LG GR-QBG
Ref
eren
ce E
vap
otr
ansp
irat
ion
(m
m/d
ay)
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0
200
400
600
800
1000
1200
1 92 183 274 365
Cu
mu
lati
ve R
efer
ence
Ev
apo
tran
spir
atio
n (
mm
)
Day of Year
RET from Bio-C
RET from GR-C
RET from GR-F
RET from Air-JFK
RET from Air-LG
RET from GR-QBG
23
327 non-consecutive days over one year period
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0
200
400
600
800
1000
1200
1 92 183 274 365
Cu
mu
lati
ve R
efer
ence
Ev
apo
tran
spir
atio
n (
mm
)
Day of Year
RET from Bio-C
RET from GR-C
RET from GR-F
RET from Air-JFK
RET from Air-LG
RET from GR-QBG
24
40%
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0
200
400
600
800
1000
1200
1 92 183 274 365
Cu
mu
lati
ve R
efer
ence
Ev
apo
tran
spir
atio
n (
mm
)
Day of Year
RET from Bio-C
RET from GR-C
RET from GR-F
RET from Air-JFK
RET from Air-LG
RET from GR-QBG
Bio-C ≈ GR-F
25
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Bio-C GR-F
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27
Act
ual
Eva
po
tran
spir
atio
n (
mm
/day
)
Act
ual
Eva
po
tran
spir
atio
n (
mm
/day
)
Bio-C GR-F
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28
0
50
100
150
200
250
300
350
400
1 92 183 274 365
Cu
mu
lati
veA
ctu
al
Evap
otr
ansp
irat
ion
(m
m)
Day of Year
AET from Bio-C
AET from GR-F>25%
193 non-consecutive days
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LABORATORY STUDIES
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Objectives
30
• Quantify daily evapotranspiration (ET) rates from four species of vegetation commonly used in green infrastructure (GI) under uniform conditions
• Evaluate differences in seasonal ET trends for the four species
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Plant Species
31
• Species chosen based on their popularity in GI installations in NYC
• NYC 2012 Interagency Bioswale Planting List consulted• Top 4 most frequent species
chosen for experiment
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• 4 Carex lurida
• 4 Liriope muscari
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• 4 Asclepias incarnata
• 4 Echinacea purpurea
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Methods
33
• Use lysimeters to record daily weight changes of plants
• Replenished plant water supply every 4 days• Weighed plants before watering
and after gravitational drainage ceased
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Water Balance: Weighing Lysimeter
𝐸𝑇 =
𝑖=1
𝑥
𝑓𝑚𝑖 −𝑚𝑖+1𝜌𝐴
𝑖
• 𝑓 is a conversion factor
• 𝑚 is the mass of the lysimeter
• 𝐴 is the surface area of the lysimeter
• ρ is the density of water
34
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0
0.5
1
1.5
2
2.5
3
27-Jul 28-Jul 29-Jul 30-Jul 31-Jul 1-Aug 2-Aug
AET
(m
m/d
ay)
Date
Daily ET Values (8/19-8/29)
Carex Liriope Asclepias EchinaceaWatered 7/26
Watered 7/30
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Results
Species Replicate 1 Replicate 2 Replicate 3 Replicate 4Avg
Cumulative ET
Carex 151.13 149.68 108.96 122.24 133.75
Liriope 124.30 110.20 110.40 106.89 112.95
Asclepias 102.39 105.96 139.40 107.69 105.35
Echinacea 159.67 105.28* 152.17 123.83 145.22
36
Results shown as depth in mm*This replicate began to senesce two weeks before the end of the experiment so it was not used in calculating average seasonal ET
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Results
• Non-parametric statistics
• Kruskal-Wallis Median Test
• p value of .05 or below considered significant
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0
20
40
60
80
100
120
140
160
27-Jun 4-Jul 11-Jul 18-Jul 25-Jul 1-Aug 8-Aug 15-Aug 22-Aug 29-Aug 5-Sep
Cu
mu
lati
ve A
ET (
de
pth
in m
m)
Date
Seasonal Evapotranspiration
Carex Liriope Asclepias Echinacea
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Preliminary Conclusions
40
• There are significant differences between the four species
• Intra-species variability is very high forCarex and Echinacea• Further analysis is needed to explain
differences within species
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Future Work
41
• Repeat experiment for 2014 growing season
• Determine relative influence of climatic factors, hours since irrigation, and species have on daily ET• So far only looked at seasonal totals
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FINDINGS AND ONGOING WORK
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Preliminary Conclusions
• Urban micrometeorological conditions dictate evaporative capacity from urban green spaces
– RET rates as determined at a daily time-step are statistically significantly differences across sites in an urban area
– Differences in cumulative RET up to 40 percent on an annual basis are observed between sites
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Ongoing Related Work
• Micrometeorological variation across cities is an important factor in consideration of evapotranspiration
• Actual ET is influenced by other factors
– Vegetation Type
– Media Moisture Conditions
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Vegetation Coefficients
45
𝑘𝑐 𝑅𝐸𝑇 = 𝐸𝑇𝑐𝑟𝑜𝑝
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Ratio of Low Elevation AET to High Elevation AET (Furmanville Bioretention Area)
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Future Objectives
• Engineer for ET
• Urban micrometeorological conditions dictate evaporative capacity from urban green spaces
• Vegetation selection controls ET
• Runoff routing dictates ability to fulfill ET capacity
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1 92 184 275 3660
0.5
1
1.5
2
Day of Year
Actu
alE
T:R
efe
renceE
T
Kc
AET:RET
1: 1 𝑅𝑎𝑡𝑖𝑜
𝐾𝑐𝑎𝑣𝑔 = 0.59
𝑊𝑖𝑛𝑡𝑒𝑟
𝑆𝑝𝑟𝑖𝑛𝑔𝑆𝑢𝑚𝑚𝑒𝑟
𝐹𝑎𝑙𝑙
𝐾𝑐𝑎𝑣𝑔 = 0.83𝐾𝑐𝑎𝑣𝑔 = 0.95
𝐾𝑐𝑎𝑣𝑔 = 0.98
𝐸𝑇𝑐𝑟𝑜𝑝 = 𝑘𝑐 𝑅𝐸𝑇
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0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9
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Evapotranspiration by Method 1.1 (mm/day)
Evapotr
anspiration b
y M
eth
od 4
.1 w
ith M
eth
od A
Non-w
ate
r Lim
ited C
onditio
ns (
mm
/day)
1: 1 𝑅𝑎𝑡𝑖𝑜
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Sanford and Selnick (2013)
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Bioretention Cells Cross-Section from EPA (2013)