Total reflected solar radiation calculated from multi-band sensor data
Jackson, R.D.
Reflected solar radiation is a significant term in the net radiation equation and is the one most strongly affected by surface conditions. The evaluation of net radiation over a heterogeneous area requires a detailed knowledge of the areal distribution of the reflected solar radiation. Remote sensing offers a means to obtain this areal distribution, provided that the total reflected solar spectrum can be estimated from discrete band multispectral radiometric data. A radiative transfer model was used to calculate the irradiance at the earth's surface for a number of atmospheric scattering and adsorption conditions. Response functions of two multiband radiometers were used to obtain the partial spectrum/total spectrum (P/T) ratio for each radiometer at each atmospheric condition. It was found that the P/T ratio was essentially independent of atmospheric scattering and only mildly dependent on water vapor absorption. Reflectance spectral distributions for 14 different surface conditions (bare soil to full green canopy) were used along with the irradiance data to determine the P/T ratio for reflected solar radiation. Multispectral data, with the appropriate P/T ratio, were used to calculate the total incoming radiation and the total reflected radiation from a wheat canopy. The calculated data differed from wide band pyranometer data by about 5%. It was concluded that both total incoming and reflected solar radiation can be evaluated from multispectral radiometric data. This development is a step towards regional net radiation maps, and possibly regional evapotranspiration maps.
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