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Evapotranspiration in a lotic wetland ecosystem
G. Milton Ward
Department of Biological Sciences and
Center for Freshwater Studies
University of Alabama
Tuscaloosa, Alabama, USA
Storage Surface
Outflow
Groundwater Flow
Evapotranspiration
Precipitation
Water Budget
Juncus effusus
Nymphaea odorata
Beaver Dam
Talladega Wetlandin October
Alnusserrulata
Talladega Wetland hydrologic installation
TWE Precipitation and Pan Evaporation
0200400600800
1000120014001600
93 94 95 96 97 98 99 00 01 02 03
Years
mm
of
Pre
cip
ita
tio
n
or
Ev
ap
ora
tio
n
Precipitation
Evaporation
Storage = -3301
22%
Surface
Outflow
Groundwater Flow
286 21%
Evapotranspiration (mm)
879 64 %
1367
Precipitation
Preliminary Water Budget for Talladega Wetland
Evapotranspiration-
The E-T component of this budget was a spatially integrated average calculated from a modified Penman-Monteith equation.
Spatial Mean = 75% of Pan estimate
Because of the inherent uncertainty of such calculations, and the need to refine our ET estimate, we installed an eddy covariance system in the wetland.
Measurement of E-T using eddy covariance techniques
Net Radiometer
3-D Anemometer
Krypton Hygrometer
Surface Energy Budget
Rn= LE + H ± G
Campbell Scientific, Inc.
Equipment is deployed on a tower that rises above the canopy
Equipment is deployed on a tower that rises above the canopy
•10 m tower in a stand of alder, Alnus serrulata
•Surface energy budget data collected 10x per second and averaged over 30 min
•ET (mm) calculated from effluxes of latent energy
•Daily ET was calculated as the sum of daytime (Rn>0) 30 min ET
•When necessary, data were corrected for closure of energy balance using the Bowen Ratio
Surface energy budget for 25 Jul 03
-1000
100200
300400500
600700
800900
Time of Day
W/m
2
Rn
LE
H
G
ET = 3.38 mm H2O
Surface Energy Budget Closure Is Needed !!!Advection can be a source of error
Daily Variation ranged between 0.46 to 4.4 mm/d in 2003 and 0.40 to 6.0 mm/d in 2002. The slightly higher rate in 2002 was attributed to a somewhat warmer and less humid growing season.
0.01.53.04.56.0
A M J J A S O
ET (m
m)
0.01.53.04.56.0
A M J J A S OMonth
ET
(m
m)
2002
2003
0
1
2
3
4
5
6
April May June July August September October
Da
ily
Da
yti
me
E-T
(m
m/d
)
0
5
10
15
20
25
30
Me
an
Da
ily
Air
Te
mp
(C
)
2002_ET 2003_ET 2002_Temp 2003_Temp
Monthly E-T fluxes from Talladega Wetland
Can ET be indexed to PET?Our longer term measurement of evaporation is that of potential ET, using a class-A evaporation pan. A comparison of actual ET versus PET in 2002 revealed that pan evaporation measurements tended to capture only 84% of AET as measured by Eddy Covariance.
y = 0.8379x + 1.1661
R2 = 0.7337
0
1
2
3
4
5
6
0 1 2 3 4 5 6Daily Pan Evaporation (mm)
Da
ily E
T (
mm
)
Habitat State ET (mm/d) Citation Method
Wetland FL 4.16 Jacobs et al ‘02 Eddy Covariance
Wetland FL 0.5 (Feb)
10.0 (Sep)
Dolan et al ’84 Water Table
Vegetated Lake
FL 0.2 (Feb)
5.0 (Jul)
Sumner ’96 Eddy Covariance
Wetland AL 3.81 (May-Oct ‘02)
2.38 (Apr-Oct ’03)
This Study Eddy Covariance
Riparian-
Salt-cedar
NM 5.4 (Flooded site)
3.4 (Non-Flooded)
Cleverly et al ‘02
Eddy Covariance
E- T at Talladega Wetland in comparison with similar habitats in the SE US
Where are those beaver when you really need them?
Beaver pond exposed!!!!
Research support by funding from NSF EPSCoR and NSF IGERT programs
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