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Matthias, Allan D.; Post, Donald F.; Accioly, Luciano; Fimbres, Adan; Sano, Edson E.; Batchily, A. Karim

Technical Articles

Soil albedo (α) is the ratio of reflected (K↑) to incoming (K↓) hemispherical radiation fluxes (wavelength, λ ≈ 0.3 to 3 μm) measured by inverted and upright pyranometers positioned directly over the soil. The research on soil albedo described in this paper was part of a larger soil classification study relating albedo with soil color and spectral reflectances for 26 samples of soil collected throughout the United States. Specifically, this paper describes (i) a method for measuring the albedos of small homogeneous areas of soil placed on square trays surrounded by either black painted plywood or field soil and (ii) experiments performed to evaluate the method using data from a subset of the 26 soils studied. Measurement of albedo of a small area of soil surrounded by a dissimilar surface may be confounded by light reflected to the inverted pyranometer from the surrounding surface and by light reflected from the soil shaded by the pyranometer. To determine the albedo of the small soil area, the fractional amount of reflected radiation received by the inverted pyranometer from each component area of the heterogeneous surface within the 180° field-of-view of the pyranometer must be modeled using view factor relationships taken from the literature. Measurements of α were made at two heights (0.18 m and 0.25 m) over five smoothed soil surfaces (dry and wet) placed on square trays with areas of 1.4 m2 and 13 m2. Two measurement heights and two soil areas were used to provide a wide range of view factors and surface conditions for evaluating the method. Black painted plywood surrounded the 1.4 m2 areas, and field soil surrounded the 13 m2 areas. Albedos of these surrounding areas were known. Results indicate good agreement between soil albedos (α1.4 and α13) measured at the same height over the two areas. Linear regression analysis showed that α1.4 = 0.951 α13 + 0.011 (r 2 = 0.989, n = 20) based on K↑ data measured at the two heights above the two areas of the soils. These results indicate that the albedo of a small homogeneous area within a heterogeneous surface can be measured to a reasonable level of accuracy if the fractional amount of radiation reflected from each surface component is known.

Department of Soil, Water and Environmental Science, Shantz Bldg. #38, Rm. 429, 1200 E. South Campus Dr., The University of Arizona, Tucson AZ 85721-0038. Dr. Matthias is corresponding author. E-mail:

Received July 31, 1998; accepted Dec. 21, 1998.

© 1999 Lippincott Williams & Wilkins, Inc.