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