Payero, JO, Neale, CMU, Wright, JL (2006). Near-noon albedo values of alfalfa and tall fescue grass derived from multispectral data. INTERNATIONAL JOURNAL OF REMOTE SENSING, 27(3), 569-586.
A remote sensing approach was applied to estimate near-noon values of shortwave albedo (alpha), the fraction of solar radiation reflected by a surface, for alfalfa and tall fescue grass at Kimberly, Idaho. The approach was based on the (P/T) ratio, which is the ratio of the partial radiation ( P) sensed by a multi-band radiometer and the total incident radiation ( T) in a given wavelength range. It was found that instead of being constant, as previously suggested, the upward component of the (P/T) ratio under clear-sky conditions [(P/T)(u)] followed a logistic growth function of solar altitude angle (Lambda z) for both crops (r(2) = 50.84). The downward component [(P/T)(d)], on the other hand, linearly increased with Lz (r(2) = 50.83). By applying the (P/T) ratio methodology, using variable ratios, it was found that the diurnal pattern of clear-sky a for both crops followed a decreasing function of Lambda z (r(2) = 50.80). Near-noon a values for alfalfa estimated using remote sensing were linearly related to plant canopy height (h) (r(2) = 50.92), but not to lambda z. For grass, on the other hand, the near-noon a values obtained by remote sensing were not correlated with either h or Lambda(z). The near-noon alpha values for alfalfa obtained with remote sensing deviated considerably from those estimated using an empirical function of day of the year (DOY). For alfalfa, the near-noon net radiation (R-n) values calculated using alpha values derived by remote sensing were better correlated to measured R-n values than those obtained using a estimated as a function of DOY. For grass, the alpha values derived from remote sensing did not significantly improve the accuracy of the calculated near-noon Rn compared with using alpha values estimated as a function of Lambda(z).