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Evapotranspiration Monitoringat a Wetland Site in the New
Jersey Pinelands
Robert NicholsonDavid SumnerRick Edwards
U.S. Geological Survey
Kirkwood-Cohansey ProjectA hydrologic/ecological investigation in the
New Jersey Pinelands
New Jersey Pinelands CommissionU.S. Geological SurveyRutgers UniversityU.S. Fish and Wildlife ServiceNew Jersey Department of Environmental Protection
Why MonitorEvapotranspiration?
Applications of ET Monitoring Data
• Water budget evaluations
• Quantifying seasonal water availability– Water used by vegetation types– Water available for aquifer recharge
• Drought management– Tracking and anticipating conditions
Land Surface Water Budget
PRECIPITATION(P)
EVAPOTRANSPIRATION(ET)
AQUIFER RECHARGE (R)
STORM RUNOFF(RO)
P = ET + RO + R
8.0
55.0
37.0
DIRECT RUNOFF
EVAPOTRANSPIRATION
RECHARGE
Surface water budget: Precipitation=100 cm/yr
Water Budget Example
26.0
2.04.05.0
DISCHARGE TOWETLANDS ANDSTREAMSNET LEAKAGE TODEEPER AQUIFERS
NET LATERAL FLOWTO ADJACENTWATERSHEDSSUPPLY WELLS
Ground-Water Budget: Recharge = 37 cm/yr
Methods of ET Estimation– Empirical (Thornthwaite - rough estimate for a region)
– Physically-Based Models (Penman-Monteith, Priestly-Taylor)
– Chamber methods• Surround all or part of individual plant; measure flux; extrapolate to larger area
– Micrometeorological methods• Profile method• Bowen ratio – energy budget• Eddy correlation
methods allow:• high temporal resolution• spatial averaging
Net radiation Rn
Latent heat flux λE
Sensible heat flux H
Storage of energy S incanopy, soil, air column
Available energy drivingevapotranspiration
Bowen ratio method
Rn – S = λE + H
H γ ΔtemperatureΔvapor pressureλE
=B =
λE =Rn – S
1 + B
Eddy correlation method
ρv = vapor density
w = vertical wind speed
…. and similarly for H
ET = ρvw = cov (ρv ,w)
Use as “direct” ET measurementor use results in an energybalance variant method
EvapotranspirationObjectives
Subseasonal ET estimates
Relations between ET and• Soil Moisture• Depth to water• Radiation
ET MonitoringSite
McDonalds Branch Tower SiteWetlands Vegetation
Predominantly
pitch pine lowlands
Atlantic white cedar
TowerSite
ETSource
Footprint
80-ft FluxTower at
McDonaldsBranch Site
ET Instrumentation ArrayAnemometer
(Wind speed anddirection)
Static dissipater
Net radiometers (2)
Temperature and relativehumidity probe
Eddy Correlationsetup
Pyranometer(incoming solar
radiation)
3-D sonicanemometer
Kryptonhygrometer
Eddy correlation
instrumentation
Year 1 ET Monitoring Results
Daily Energy Fluxes, 11/9/04-12/20/05
-100
-50
0
50
100
150
200
250
300
N D J F M A M J J A S O N D J
Month
Ener
gy F
lux
(w/m
2)
Net RadiationLatent Heat Flux
Year 1 ET Monitoring ResultsMonthly Energy Fluxes, 12/04-11/05
020406080
100120140160180200
D J F M A M J J A S O N
Month
Ener
gy F
lux
(w/m
2)
Net RadiationLatent Heat Flux
1 w/m2 LE ~ 1.07 mm ET (depends on temperature)
Total ET for 12-month period was 81.4 cm, or about 32 “
Total precipitation was 120.8 cm, or about 48”
Relation Between Weekly LE and Rn Measurements
-25
0
25
50
75
100
125
150
175
200
225
250
-25 0 25 50 75 100 125 150 175 200 225 250
Average Latent Heat Flux (LE) , w/m2
Ave
rage
Net
Rad
iatio
n (R
n),
w/m
2R2 = 0.85
1:1
Year 1 ET Monitoring Results
Hydrologic Conditions at ET Tower Site , 11/9/04-12/20/05
0
20
40
60
80
100
120
140
N D J F M A M J J A S O N D
Date
Precipitation (mm)Soil Moisture (% vol)Water level (cm)
Year 1 ET Monitoring ResultsWeekly Measurements Showing Lower LE
during dry period
-0.20.00.20.40.60.81.01.21.41.6
N D J F M A M J J A S O N D J
Month
Soil Moisture (fraction)LE/RnPrecipitation (dm)
dryperiod
LE = Latent Heat Flux Rn = Net Radiation
ET Results in Water BudgetWEEKLY WATER FLUXES
-20.0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
N D J F M A M J J A S O N D J F
Month
Wat
er F
lux
(mill
imet
ers)
EXCESS WATER (Storm Runoff + Recharge)
ET
PRECIPITATION
GROUND-WATER-DERIVED ET
SUMMARY
Evapotranspiration…• is a large part of the water budget
• can be measured directly
Quantified on a daily basis
Spatially averaged
Related to other conditions
• monitoring can provide an improvementin quantitative analysis of water availabilityand drought conditions