Near‐noon albedo values of alfalfa and tall fescue grass derived from multispectral data

Abstract

A remote sensing approach was applied to estimate near‐noon values of shortwave albedo (α), 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 (Λz) for both crops (r 2 = 0.84). The downward component [(P/T)d], on the other hand, linearly increased with Λz (r 2 = 0.83). By applying the (P/T) ratio methodology, using variable ratios, it was found that the diurnal pattern of clear‐sky α for both crops followed a decreasing function of Λz (r 2 = 0.80). Near‐noon α values for alfalfa estimated using remote sensing were linearly related to plant canopy height (h) (r 2 = 0.92), but not to Λz. For grass, on the other hand, the near‐noon α values obtained by remote sensing were not correlated with either h or Λz. The near‐noon α 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 α values derived by remote sensing were better correlated to measured R n values than those obtained using α estimated as a function of DOY. For grass, the α values derived from remote sensing did not significantly improve the accuracy of the calculated near‐noon R n compared with using α values estimated as a function of Λz.