Intercept Photosynthetically Active Radiation Estimated By Spectral
Jin Zhonghui, Zhang Hongming, Wang Jiasheng, Huang Xinshou
Beijing Agricultural University, Beijing, China
Abstract :
This paper introduce the theoretical formulas and the experimental formulas between the leaf-area index and intercepted photosynthetically active radiation PAR by wheat. Intercepted photosynthetically active radiation are calculated by measuring the leaf-area index of wheat. Then, to find out separately various spectral vegetation indices and the linear regression formulas between various spectral vegetation indexes and intercepted photosynthetically active radiation by wheat. It came to the conclusion that it is the most reliable method to estimate intercepted photosynthetically active radiation by using the spectral vegetation index PVI and the linear regression formula PAR = 78.6 PVI + 55.2.
1. Theoretical Fundamentals and Methods
It is often necessary to estimate intercept photosynthetically active radiation PAR by crop at inspecting crop growth and forecasting crop yield models. Having used the guantum sensor, Hipps et al measured intercept PAR by wheat canopy at different growth stages and measured the leaf-area index LAI of wheat at the same time. They got the function relation between PAR and LAI of wheat canopy before and after heading.
Before heading PAR = 93.5 1.0 – exp (-0.9 LAI) (1)
After heading PAR = 93.5 1.0 – 0.2 exp (-0.95 LAI) (2)
Monsi et al represents the function relation between interception of light rays P and LAI of plant canopy, as
P = 1 – e-k’ LAI (3)
Where k’, the angular shape coefficient, depends exclusively on the leaf angle distribution. In this paper, if the leaves are horizontal k’ is given as
K’ = 1 (4)
For uniform canopies with spherical leaf angle distribution k’ is given by
K’ = 0.5 / cos h (5)
Where c is the sun zenith angle. Because the scattering is low in the PAR region, the intercept radiation and the radiant absorption by leaves have a similar value. The P in (3) may be taken as the absorbed photosynthic radiation by leaves. (3) can be rewritten as
PAR = 1 – e-k’ LAI (6)
Mang researches has fund the correlation between spectral vegetation indices and biological variable of crop (for example leaf-area index, yield and so on). The most often using spectral vegetation indices are given as
Ratio vegetation index RVI =
dhdg (7)
Normalized difference vegetation index ND =
dh -
dg /
dh +
dg (8)
Perpendicular vegetation index PVI = 0.939Jn – 0.344Jr + 0.09 (9)
Where
dhand
dg area separately the spectral reflectance of the plant canopy at near-infrared and red wave bands. In this paper,
dhand
dg are taken as the spectral reflectance of TM4 (0.76 – 0.90
mm) and TM3 (0.63 – 0.69
mm) wave bands.
From (1) to (6) are utilized to find out intercept PAR by wheat through measuring the leaf-area index of what canopy, then we find out
various spectral vegetation indices by measuring spectral reflectance of wheat canopy, finally we find our separately the linear regression formula between various spectral vegetation indices and PAR. It will be seen from above cumulative results that which spectral vegetation index will be used to estimate intercept PAR by wheat canopy. It will be the most reliable.
2. Experimental methods
Eight experimental plots of winter sheet are at test field of Beijing Agriculture University. Each plot was planted in north-south and sowed in 29.9.1989. The spectral reflectance measurements were made repeatedly eight item in the growth stages of wheat. The measurement are randomly carried out on nine sample points of each plot. The mean value of the spectral reflectance are taken as calculative values. The spectral reflectance measurements were made with a 15° filed – of – view eight channels radiometer made in BARNES company in America. The radiometer was hand-held at a height of 1.0 m or 1.5 m above the height of wheat and the sun was at one elevation of 35. We used the grey plate made in AN HUI optic machine institute as standard in measuring spectral reflectance. The spectral reflectance of two wave bands (TM3 and TM4) only are used in this paper calculation, though we have got the spectral reflectance of eight wave bands. We took twenty wheat plants from each test plot to measure area of each leaf. Then wheat plants from each test plot to measure are of each leaf. Then the mean leaf-area of each plant times the gross plants of each mu. Finally, the gross leaf-area of each mu are divided by the area of each mu gives the leaf-area index LAI.
3. Experiment date and calculative Results
The spectral reflectance of two wave bands (TM3 and TM4) and LAI values at different growth stages of wheat are listed in table 1.
PAR values calculated from LAI values are listed in table 2, where A stands for PAR values calculated from (1) or (2), B.C.D stand for PAR values calculated from (5) and (6) when c are separately taken as 20°, 35°, 50°, E stands for values calculated from (6) when k’ is taken as 1.
The spectral vegetation indices at different growth are listed in table 3.
The linear regression formulas between PAR and x (spectral vegetation index) and their correlation coefficients and residual standard errors are listed in table 4.
