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 (a), 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 (? z ) for both crops ( r 2 = 0.84). The downward component [(P/T) d ], on the other hand, linearly increased with ? 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 a for both crops followed a decreasing function of ? z ( r 2 = 0.80). Near-noon a values for alfalfa estimated using remote sensing were linearly related to plant canopy height ( h ) ( r 2 = 0.92), but not to ? z . For grass, on the other hand, the near-noon a values obtained by remote sensing were not correlated with either h or ? z . The near-noon a 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 a 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 a values derived from remote sensing did not significantly improve the accuracy of the calculated near-noon R n compared with using a values estimated as a function of ? z .
Document Type: Research Article
Affiliations: 1: West Central Research and Extension Center, 461 West University Drive, North Platte, Nebraska 69101, USA 2: Biological and Irrigation Engineering Department, Utah State University, Logan, Utah 84322‐4105, USA 3: USDA Agricultural Research Service, 3793 N 3600 E, Kimberly, Idaho 83341, USA
Publication date: 2006-02-10