Spectral response of rice plant canopy – A research report by National Institute of Hydrology

The study was conducted with the objective of establishing the relationship between spectral reflectance of rice canopy and crop growth. Significant relationship was developed between spectral indices and growth period. The coefficient of correlation between crop growth period and spectral reflectance ranged from 0.41 to 0.97 for the plant growth stages and canopy reflectance in single band and ratio spectra.

The inherently congenial climatic environment for traditional rice culture in the eastern region is relatively less congenial for modern rice culture involving the use of high yielding varieties and high levels of fertilizers and pesticides.

The study took rice cover density variation as the basis for choosing two fields for radiometric and agronomic measurements during the period August to November 1997. A hand held radiometer (model 100 BX) was used to measure spectral reflectance with four bands corresponding to band 4, 5, 6 and 7 of the multi-spectral scanner on board LANDSAT 1-5. The radiometer with 2 pi steridian diffuser cap was set upward to measure direct (sun) and diffuse (sky) irradiance with a 15^o field of view of the canopy with a zenith angle of 57.5^o.

The study presents the relations among spectral reflectance and crop growth stages of rice plants grown on irrigated light textured soil in a semi arid region. There was a linear relation between spectral reflectance and rice plant height (r=0.97), for the band 1 (0.45 – 0.52 µm) reflectance values.

On the other hand, in bands 2 (0.52 – 0.60 µm) and 3 (0.63 – 0.69 µm), reflectance values decreased until 70 days after planting (DAP) and then increased during the reproductive phase of the crop. This suggests that band 2 is affected by the greenness of the plant. The near infrared band 4 (0.76-0.90 µm) showed maximum reflectance at 59 DAP (panicle initiation stage) and a decline in reflectance thereafter through maturity.

Future sensors having capability to monitor chlorophyll such as LISS III onboard IRS-ID would be extremely useful for the regional assessment of crop yields.

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