convictive evidence observed in com field is that, NDVI almost until VWC greater than 5 Kg/m“. This is quick consistent with
saturates when VWC is 3-4Kg/m 2 , but NDWI don’t saturate the result of T.J.Jackson 3 [4] and Chen [5] .
Fig.l The comparison of VWC estimation by NDVI and NDWI. Top-Left: NDVI used for soybean; Top-Right: NDWI used for
soybean; Bottom-Left: NDVI used for com; Bottom-Right: NDWI used for com.
3. VEGETATION TRAN SMITTI VIT Y
In the presence of a vegetation layer, the direct backscattering
contribution of the ground surface is / 2 (0)a ( '(0) , where
flB) is the two-way transmissivity of the vegetation layer,
/ 2 (^) = exp(-2rsec^) (1)
And t is its optical thickness. For a certain vegetation type and
frequency, optical depth is usually directly related to the
vegetation water content (Kg/m 2 ). Jackson and 0’Neill [6]
reported the following linear relationship between r and VWC :
T = b- VWC (2)
A series of field campaigns have been reported in the literature
which focussed on the determination of b for various canopies
at different frequencies. Adriaan A. Van de Griend et.al [r|
collected all data available from literature and set up functions
between b and wavelength A for different kind of canopies.
For soybean field, the relation is :
log 10 b = 0.274 - 0.842 log 10 A (3)
For com field, the function is:
(4)
Combining with the VWC from last section, transmissivity of
the vegetation layer can be obtained.
4. SURFACE BACKSCATTERING EXTRACTION
When vegetation layer is present, a solution via the radiative
transfer approach can simplify to three: a term representing the
diffuse scattering contribution by the vegetation volume, a
second term representing the diffuse surface-volume interaction
contribution, and a third representing the direct surface
backscattering contribution, including two-way attenuation.
Equivalently,
= a°,(0)+y\8)<jI(0)+a°„(8) (5)
Where the subscripts t, v, s and sv denote total, volume, surface,
and surface-volume contribution, respectively. Because cr° (Û)
is observed in experiment, to get cr°(#)> er 0 (#) and cr° sv (0)
must be removed from cr, 0 (#) after f(g) has been obtained
from optical resource (Section 3, from NDWI).
Volume Contribution
The volume term has a complex scattering mechanism for it’s
influenced not only by incidence angle, wavelength, and
polarization etc. configuration parameters, but also by
properties of vegetation, such as construction, direction
preference and VWC etc. Nowadays, it has been realized that
log 10 b = -0.374 - 0.409 log 10 A
