Measurement of albedos for small areas of soil

Soil albedo (alpha) is the ratio of reflected (K up arrow) to incoming (K d own arrow) hemispherical radiation fluxes (wavelength, lambda approximate t o 0.3 to 3 mu m) measured by inverted and upright pyranometers positioned d irectly over the soil. The research on soil albedo described in this paper was part of a larger soil classification study relating albedo with soil co lor and spectral reflectances for 26 samples of soil collected throughout t he United States. Specifically, this paper describes (i) a method for measu ring the albedos of small homogeneous areas of soil placed on square trays surrounded by either black painted plywood or field soil and (ii) experimen ts performed to evaluate the method using data from a subset of the 26 soil s studied. Measurement of albedo of a small area of soil surrounded by a di ssimilar surface may be confounded by light reflected to the inverted pyran ometer from the surrounding surface and by light reflected from the soil sh aded by the pyranometer. To determine the albedo of the small soil area, th e fractional amount of reflected radiation received by the inverted pyranom eter from each component area of the heterogeneous surface within the 180 d egrees field-of-view of the pyranometer must be modeled using view factor r elationships taken from the literature. Measurements of alpha were made at two heights (0.18 m and 0.25 m) over five smoothed soil surfaces (dry and w et) placed on square trays with areas of 1.4 m(2) and 13 m(2). Two measurem ent heights and two soil areas were used to provide a wide range of view fa ctors and surface conditions for evaluating the method. Black painted plywo od surrounded the 1.4 m2 areas, and field soil surrounded the 13 m2 areas. Albedos of these surrounding areas were known. Results indicate good agreem ent between soil albedos (alpha(1.4) and alpha(13)) measured at the same he ight over the two areas. Linear regression analysis showed that alpha(1.4) = 0.951 alpha(13) + 0.011 (r(2) = 0.989, n = 20) based on K up arrow data m easured 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 fractiona l amount of radiation reflected from each surface component is known.