Surface Roughness Effects on Soil Albedo

much has been learned in recent years about the factors affecting reflection of sunlight by soils, the influence Knowledge of how surface roughness influences the reflectance of of w on albedo of rough soil surfaces is still poorly sunlight from cultivated soils is useful in various applications, such as estimating albedo values used as inputs to soil temperature models understood (Cresswell et al., 1993). Light scattering theand erosion models. The albedos of two soils were studied for dry ory generally predicts that the albedos of rough soil and wet surfaces with four different roughness conditions, changed surfaces diminish with increasing w (Ogilvy, 1991; Cierfrom a reference smooth soil surface. The soils were the Gila fine sandy niewski and Verbrugghe, 1997). The non-Lambertian loam [coarse-loamy, mixed (calcareous), thermic Typic Torrifluvent] behavior of soil surfaces has been observed for various (Ap horizon), 10YR 6.0/3.2 dry and 10YR 4.1/3.3 wet, and the Pima soils, including dry and wet Avondale loam (fine-loamy, clay loam [fine-silty, mixed (calcareous), thermic Typic Torrifluvent] mixed, superactive, calcareous, hyperthermic Typic Tor(Ap horizon), 10YR 5.2/2.3 dry and 10YR 3.3/2.3 wet. Albedo mearifluvent) (Idso et al., 1975), dry and wet Sacramento surements were made during selected mornings with clear skies in clay (very-fine, smectitic, thermic Cumulic Vertic En1995 and 1996. The mean albedos of reference smooth surfaces (,2 doaquoll), and wet Yolo loam (fine-silty, mixed, nonmm sieved soil) were 0.279 and 0.155 for dry and wet Gila soil and 0.221 and 0.114 for dry and wet Pima soil. Four tillage conditions acid, thermic Typic Xerorthent) (Coulson and Reynwere studied: rough plow, disk, disk–disk, and seedbed. Tillage direcolds, 1971). For especially rough surfaces a may increase tion was north–south in 1995 and east–west in 1996. The goodness of fit slightly as w increases. Coulson and Reynolds (1971) of linear relationships determined between mean albedo and surface reported a positive relationship between a and w for roughness, measured with a roughness meter and reported as the root dry Yolo loam, which contrasted markedly with the mean square deviation, were relatively high; however, the slopes for inverse relationship they observed for wet Yolo loam. the regression equations were different for the two soils and the two Coulson and Reynolds (1971) suggested that light trapmoisture conditions. The different slopes indicate that the sensitivity ping by gaps between soil particles recur more readily of albedo to surface roughness was highest for the most reflective at high sun angles, which minimizes albedo at high sun surface (dry Gila soil) and lowest for the least reflective surface (wet angles. Cresswell et al. (1993) also observed that the a Pima soil). The albedos were on average 27, 18, 10, and 8% lower for dry and wet rough-plow, disk, disk–disk, and seedbed treatments, of rough-tilled Templeton silt loam (fine-silty, isotic, respectively, as compared with the albedo of the reference smooth isomesic Andic Dystrudept) increased slightly as w insoil. These reduction percentages can be used as a general guide to creased from 41 to 76.58. They suggested that a is more estimate the albedos of tilled soils similar to the Gila and Pima soils likely to increase with increasing w for relatively rough studied here. tilled soils where the light-trapping mechanism of the surface may function nearly equally well under both low and high w. Cresswell et al. (1993) pointed out that S albedo (a) is the ratio between reflected (K↑) the sun angle effect is complex and appears to be someand incoming (K↓) radiation within the short wave what specific to a particular soil and surface condition. (l ≈ 0.3–3.0 mm) portion of the solar spectrum (Coulson The variation of albedo with roughness and w may and Reynolds, 1971; Oke, 1987). Albedo is an important also be influenced by the relative amounts of diffuse input parameter for modeling various environmental radiation and direct beam radiation incident upon the processes, such as the soil surface energy balance (e.g., surface (Cresswell et al., 1993). Scattering of incident Bristow, 1987) and soil erosion (e.g., Laflen et al., 1991). light by air molecules, clouds and aerosols varies with Albedo helps regulate soil temperature, which in turn w and affects the relative contributions of direct and influences soil biophysical processes, such as seed germidiffuse components in K↓ (Rosenberg et al., 1983). At nation and microbial activity. Soil albedo can be altered very low w the diffuse contribution is relatively large, to change soil temperature (e.g., Stanhill, 1965; Creswell which increases a. et al., 1993), and a common way is by changing the The degree of roughness of a surface depends intrinsiroughness of the surface by tillage (Potter et al., 1987). cally on w and the wavelength (l) of the scattered radiaThis is often done in vegetable production areas of the tion (Ogilvy, 1991). Intuitively, we expect that a plowed southwestern United States where soil temperature field should appear relatively rough, with mostly diffuse modification early in the season can help optimize plantscattering of light when w is large at mid day. However, ing and harvesting dates. that same plowed field may appear smooth with a preIn this study we addressed the question of how albeponderance of specular reflection at small w early in the dos of dry and wet soils are affected by the roughness morning or late in the afternoon. Whether a soil surface of the surface as a function of the sun angle (w). Though is smooth or rough may be defined in terms of the Rayleigh criterion (Ra) (Ogilvy, 1991) Dep. of Soil, Water and Environmental Science, Shantz Building 38, Room 429, P.O. Box 210038, 1200 E. South Campus Drive, The Univ. Ra 5 ks sin w [1] of Arizona, Tucson AZ 85721-0038. Received 15 July 1998. *Corresponding author ([email protected]). Abbreviations: CV, coefficient of variation; rmsd, root mean square deviation. Published in Soil Sci. Soc. Am. J. 64:1035–1041 (2000).