1. Remote Sensing of Evapotranspiration with MODIS
Daniel Siegel

2. What is MODIS? Moderate-Resolution Imaging Spectroradiometer
Launched in 1999 aboard the EOS AM (Terra); EOS PM (Aqua) followed in 2002
Monitors 36 spectral
bands between 0.4 m
and 14.4 m
Images entire Earth
every 1-2 days at 1 km
resolution

3. Why use MODIS?
ASTER and Landsat have 60 m resolution but available once a month at best
Geostationary satellites capture data with 15 min frequency but 5 km resolution

4. Relevent MODIS Products
MOD11 - Surface temperature and emissivity
MOD43 - Albedo
MOD15 - Leaf Area Index (LAI)
MOD13 - NDVI
Mod07 - Atmospheric stability; temperature and vapor pressure at 20 vertical levels
MOD03 - Lattitude, longitude, ground elevation, solar zenith angle, satellite zenith angle and azimuth angle

5. NDVI First measured by the original Landsat in 1972 Measurement of a
pixel’s “greenness”

6. Accessing MODIS Data
Level 1 and Atmosphere Archive and Distribution System (LAADS)
Warehouse Inventory Search Tool (WIST) submits orders via EOS ClearingHouse (ECHO)
HDF can interface with C, Fortran, Perl, MATLAB, IDL or Mathmatica

7. WIST

8. Surface Energy Balance System (Su 2002)
RnGo E
RnRd + Ld - s
Go Rd + Ld - s
Go = Rn[c + (1-fc)(s - c)]
s
c
= Measured by MODIS
fc = percentage
of ground covered
by vegetation
= Variables

9. Calculating H
= cannot be measured remotely

10. z0m and z0h Can vary by several orders of magnitude
Using LAI and wind speed, z0m can be calculated as a
function of canopy height following Massman (1997)
Zoh = zom/exp(kB-1)
Wind speed

11. Limiting Cases Hdry = Rn - Go
Constraining the result between these values
decreases the uncertainty considerably

12. Summary: Local Variables
Rd - Measured with a radiation sensor
Ld - Stephen-Boltzman equation using air temp
Wind speed and canopy height must be measured on site

13. Results

15. Triangle Method (Jiang and Islam 2001)
NDVI, soil moisture)

17. Results
Original Priestly-Taylor Eq
Triangle Method

18. Complementary Model (Venturini & Islam 2007)
From Priestly-Taylor
ET + ETpot = 2Etwet (Bouchet 1963)
From Penman
Uses temp profile as surrogate
for humidity deficit
EF = ET / (Rn-G)

20. Benefits of Isolating EF
Rn is a large source of error because of atmospheric interference and cloud cover
Generally constant during daytime
Useful for mapping drought conditions

21. Results

22. Future Research
Removing cloud-contaminaed pixels biases results, ignores diffuse radiation
Nocturnal transpiration
3°K error in in Ts causes 75% error in H