heavily vegetated areas. The EVI is related to the optical measures of vegetation, a direct measure of photosynthetic potential resulting from composite chlorophyll,
leaf area, canopy cover, and structure. It is developed to optimize the vegetation signal with improved sensitivity in high biomass regions and improved vegetation
monitoring through a de-coupling of the canopy background signal and a reduction in atmosphere influences. The equation takes the form,
EVI = G ρ
NIR
– ρ
Red
ρ
NIR
+ C
1
ρ
Red
– C
2
ρ
Blue
+ L
Where, EVI = Enhanced Vegetation Index
G = Gain factor =2.5
ρ
NIR
= Near Infrared Reflectance ρ
Red
= Red Reflectance ρ
Blue
= Blue Reflectance C
1
= Atmosphere Resistance Red Correction Coefficients =1 C
2
= Atmosphere Resistance Blue Correction Coefficients =6.0 L
= Canopy Background Brightness Correction Factor =1
The input reflectance to the EVI equation may be atmospherically- corrected or partially atmosphere corrected for Rayleigh scattering and ozone
absorption. C1 and C2 are the coefficients of the aerosol resistance term, which uses the blue band to correct for aerosol influences in the red band. The canopy
background adjustment factor, L, addresses non-linear, differential NIR and red radiant transfer through a canopy and renders the EVI insensitive to most canopy
backgrounds, with snow backgrounds as the exception.
3.4.4 Estimation of Net Primary Production NPP
This estimation was based on the value of EVI. The methods for measurement of primary production vary depend on the focus that we would like
to calculate, the gross or net production, terrestrial or aquatic system. The approach for estimation of NPP was conducted using relationship of monthly
production of plant biomass Potter et al. 2009 which is estimated as a product of time varying surface solar irradiance and EVI from the MODIS satellite, with a
constant light utilization efficiency term that is modified by time varying stress scalar terms for temperature and moisture effects. As documented in Potter 1999,
the monthly NPP flux, defined as net fixation of CO
2
by vegetation, is computed in NASA–CASA on the basis of light-use efficiency Monteith 1972. Monthly
production of plant biomass is estimated as a product of time-varying surface solar irradiance Sr Kistler et al. 2001, and EVI from the MODIS satellite
Huete et al., 2002, and a constant light utilization efficiency term emax that is modified by time-varying stress scalar terms for temperature T and moisture W
effects. The equation to estimate the NPP is defined below.
NPP = Sr EVI emax T
scalar
W
scalar
Where, NPP = Net Primary Production gC m
-2
year
-1
Sr = Solar irradiance W m
-1
EVI = Enhanced Vegetation Index from MODIS
emax = Constant Light Utilization Efficiency Term T
scalar
= Optimal temperature for plant production W
scalar
= Monthly water deficit
The emax term is set uniformly at 0.39 gC MJ
−1
PAR, a value that derives from calibration of predicted annual NPP to previous field estimates Potter et al.
1993. T
scalar
is computed with reference to derivation of optimal temperatures T
opt
for plant production. T
opt
setting varied by latitude and longitude, ranging from near 0°C in the Arctic to the middle thirties in low-latitude deserts. W
scalar
is estimated from monthly water deficits, based on a comparison of moisture supply
precipitation and stored soil water to potential evapotranspiration PET.
The PAR values are actually restricted to just a portion of electromagnetic spectrum from 0.4 to 0.7 micrometers m which is comparable to the range of
light of human eye can see. Therefore, this value was assumed to be approximately 0.5 of the incoming solar radiation Rasib et al. 2008 and it was
used for this research.
T
scalar
is estimated using the equation developed for the terrestrial ecosystem model Raich et al. 1991. The equation for T
scalar
is:
T
scalar
= [ T – T
max
T – T
min
] [ T – T
max
T – T
min
– T – T
opt 2
]
where T is the observed temperature
o
C and T
min
, T
max
, and T
opt
are minimum, maximum, and optimal temperature for photosynthesis with a value of 20
o
C, 40
o
C, and 30
o
C respectively.
W
scalar
is the effect of water deficit on plant photosynthesis, and is estimated as a function of rainfall, run off, groundwater reserves and potential
evapotranspiration. The equation for W
scalar
is defined below:
W
scalar
= 0.5 + [ 0.5 EET PET ]
where EET and PET are estimated and potential evapotranspiration, where W
scalar
ranged between 0.5 dry to 1 wet. Therefore, the function in W
scalar
can be described as below:
PPT PET =
EET = PET PPT PET
= EET = PPT
where PPT is the total precipitation. Water run-off and groundwater reserves are ignored and where PET is calculated based on Priestley and Taylor 1972 with
the equation:
PET = α [ Δ Δ + γ ] R
n
– G
where R
n
is the net-radiation MJ m
-2
month
-1
, G is the heat flux at ground level assumed to be 0,
γ is the constant psychometric with a value of about 66 Pa K
-1
. and
α are the latent heat of evaporation and the empirical factor of with values 2.5 MJ Kg-1 and 1.26 respectively.
Δ is calculated using the following mathematical equation:
Δ = 2504 exp [17.27 T T + 237.2] T + 237.3
2
where Δ is the slope vapour pressure curve KPa
o
C
-1
and T the air temperature
o
C.
3.4.5 Validation with the ground check
