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SOIL & WATER MANGEMENT RESEARCH
United States
Department of
Agriculture
Agricultural
Research
Service
Time Series Evapotranspiration
Mapping Using Landsat-TM
and MODIS Data
George Paul
Prasanna Gowda
Xiangming Xiao
Jeff Basara
Jean Steiner
SOIL & WATER MANAGEMENT RESEARCH Page 2
Project Objective
Developing time series evapotranspiration (ET)
maps using Landsat-TM and Terra-MODIS
satellite data
SOIL & WATER MANAGEMENT RESEARCH Page 3
Landsat-based ET Maps
Two selected locations
High resolution ET maps (30 m spatial
resolution)
Two source model (Norman et al., 1995)
Period: 2001 – 2010
Data used – 16-day coverage
Products:
Surface temperature
Evaporation, transpiration and evapotranspiration
(daily time-step)
SOIL & WATER MANAGEMENT RESEARCH Page 4
Landsat-Based ET maps
Intensive field experiment locations
Landsat scenes used
SOIL & WATER MANAGEMENT RESEARCH Page 5
MODIS-based ET Maps
Study area
SOIL & WATER MANAGEMENT RESEARCH Page 6
MODIS-based ET Maps
Medium resolution ET maps (250 m)
Surface Energy Balance System (Su, 2002)
Period: 2001 – 2010
Data used: Daily coverage
Products:
Surface temperature (daily time series)
Evapotranspiration (daily time series)
SOIL & WATER MANAGEMENT RESEARCH Page 7
Energy Balance Equation
Page 7
ET is calculated as a residual of the energy balance
equation
Rn (Radiation
from sun and
sky)
ET = Rn - G - H
G (Heat to ground)
ETH (Heat to air)
Basic Truth:
ET consumes energy
SOIL & WATER MANAGEMENT RESEARCH Page 8
Soil Heat Flux (G)
Bastiaanssen (1995)
Tasumi et al. (2003)
SOIL & WATER MANAGEMENT RESEARCH Page 9
Net Radiation (Rn)
SOIL & WATER MANAGEMENT RESEARCH Page 10
Sensible Heat Flux (H)
H = (ρ × cp × dT) / rah
ku
z
zln
r*
1
2
ah
dT = the near surface temperature difference (K)
rah –Aerodynamic resistance to heat transport
HrahdT
z1
z2
U* - Friction velocity
k – von karmon constant (0.41)
SOIL & WATER MANAGEMENT RESEARCH Page 11
SEBS
Rn and G are calculated as in the SEBAL
Estimation of the evaporative fraction based on energy balance at limiting cases – Wet & Dry limit for each pixel
00 dryndry HGRLE 0GRH ndry
wetnwet HGRLE 0 wetnwet LEGRH 0
At Dry-limit, the latent heat is assumed to be zero due to
the limitation of soil moisture.
At Wet-limit, evaporation takes place at potential rate
SOIL & WATER MANAGEMENT RESEARCH Page 12
SEBS
ie
satpne
rr
eecGRrE
.).(
).(.).(.
Menenti, 1984
1
.
).(.)(
ew
satp
n
wet
r
eecGR
H
Su (2002)
SOIL & WATER MANAGEMENT RESEARCH Page 13
WetLELE r.
SEBS
Su (2002)
SOIL & WATER MANAGEMENT RESEARCH Page 14
Two Source Model
SOIL & WATER MANAGEMENT RESEARCH Page 15
TSM (Two Source Model)
SOIL & WATER MANAGEMENT RESEARCH
Rns = cRn exp(-β . LAI)
or
Rns = Rn - Rnc
Page 16
Two Source Model
SOIL & WATER MANAGEMENT RESEARCH Page 17
TSM - Energy Balance
SOIL & WATER MANAGEMENT RESEARCH Page 18
Instantaneous ET to Daily ET
Evaporative Fraction Method
SOIL & WATER MANAGEMENT RESEARCH Page 19
Time Series Daily ET
Calculation of daily ET (2001-2010)
Image
Acq
uis
itio
n D
ate
Image
Acq
uis
itio
n D
ate
Image
Acq
uis
itio
n D
ate
Image
Acq
uis
itio
n D
ate
EF
1 =
Da
ily
ET
/ E
To
EF
2 =
Da
ily
ET
/ E
To
EF
3 =
Da
ily
ET
/ E
To
EF
4 =
Da
ily
ET
/ E
To
ET = ETo x EF3 ET = ETo x EF4ET = ETo x EF2ET = ETo x EF1
SOIL & WATER MANAGEMENT RESEARCH Page 20
ET0 Calculation
SOIL & WATER MANAGEMENT RESEARCH Page 21
Landsat ET – Accuracy Assessment
El Reno Control Site (2005)
SOIL & WATER MANAGEMENT RESEARCH Page 22
Landsat ET – Accuracy Assessment
El Reno Control Site (2005)
SOIL & WATER MANAGEMENT RESEARCH Page 23
Landsat ET – Accuracy Assessment
El Reno Burnt Site (2005)
SOIL & WATER MANAGEMENT RESEARCH Page 24
Landsat ET – Accuracy Assessment
El Reno Burnt Site (2005)
SOIL & WATER MANAGEMENT RESEARCH Page 25
Landsat ET – Accuracy Assessment
El Reno Control Site (2006)
SOIL & WATER MANAGEMENT RESEARCH Page 26
Landsat ET – Accuracy Assessment
El Reno Control Site (2006)
SOIL & WATER MANAGEMENT RESEARCH Page 27
Landsat ET – Accuracy Assessment
El Reno Burnt Site (2006)
SOIL & WATER MANAGEMENT RESEARCH Page 28
Landsat ET – Accuracy Assessment
El Reno Burnt Site (2006)
y = 0.50x + 0.91R² = 0.35
0
1.5
3
4.5
6
0 1.5 3 4.5 6
Esti
mate
d E
T (
mm
d-1
)
Observed ET (mm d-1)
SOIL & WATER MANAGEMENT RESEARCH Page 29
Performance Statistics
N Mean
(mm d-
1)
Mean
(mm d-
1)
MBE
(mm d-1)
% MBE
(%)
RMSE
(mm d-
1)
%
RMSE
(%)
MAE
(mm h-
1)
MAPD
(%)
NSE R2
ElReno
Control 20052.2 1.8 -0.43 -19.4 0.87 38.9 0.65 29.5 0.71 0.79
ElReno Burn
20052.0 1.8 -0.15 -7.7 0.80 39.8 0.62 30.7 0.73 0.77
ElReno
Control 20061.8 1.8 -0.03 -0.5 1.1 62.8 0.92 51.9 0.38 0.42
ElReno Burn
20061.6 1.7 0.07 6.2 1.1 68.6 0.90 55.6 0.28 0.35
SOIL & WATER MANAGEMENT RESEARCH Page 30
Actual E, T and ET
Julian Day
ET
(m
m/d
)
SOIL & WATER MANAGEMENT RESEARCH
Actual ET vs Potential ET
Page 31
Julian Day
ET
(m
m/d
)
SOIL & WATER MANAGEMENT RESEARCH Page 32
Summary
Development of Landsat-TM based time series
ET database is complete
Available Products
Surface temperature maps
E, T, and ET maps (2001-2010)
Accuracy of ET maps depended upon
availability of cloud-free Landsat images in a
given year
MODIS-based ET maps will be available in
December 2014
