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 (Λ_z) for both crops (r ² = 0.84). The downward component [(P/T)_d], on the other hand, linearly increased with Λ_z (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 Λ_z (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 Λ_z. For grass, on the other hand, the near‐noon α values obtained by remote sensing were not correlated with either h or Λ_z. 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 Λ_z.

Acknowledgments

This project was supported by the Utah Agricultural Experiment Station, The University of Nebraska Agricultural Research Division, the Remote Sensing Services Laboratory at Utah State University, The Instituto Superior de Agricultura, The Organization of American States, The Latin American Scholarship Program for American Universities, and USDA‐ARS. A contribution of the University of Nebraska Agricultural Research Division, Lincoln, NE 68583, Journal Series No. 14958.