Surface roughness effects on soil albedo

Knowledge of how surface roughness influences the reflectance of sunlight from cultivated soils is useful in various applications, such as estimating albedo values used as inputs to soil temperature models and erosion models. The albedos of two soils were studied for dry and wet surfaces with four different roughness conditions, changed from a reference smooth soil surface. The soils were the Gila fine sandy loam [coarse-loamy, mixed (calcareous), thermic Typic Torrifluvent] (Ap horizon), 10YR 6.0/3.2 dry and 10YR 4.1/3.3 wet, and the Pima clay Loam [fine-silty, mixed (calcareous), thermic Typic Torrinuvent] (Ap horizon), 10YR 5.2/2.3 dry and 10YR 3.3/2.3 wet. Albedo measurements were made during selected mornings with clear skies in 1995 and 1996. The mean albedos of reference smooth surfaces (<2 mm 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 were studied: rough plow, disk, disk-disk, and seedbed. Tillage direction was north-south in 1995 and east-west in 1996. The goodness of fit of linear relationships determined between mean albedo and surface roughness, measured with a roughness meter and reported as the root mean square deviation, were relatively high; however, the slopes for the regression equations were different for the two soils and the two moisture conditions. The different slopes indicate that the sensitivity of albedo to surface roughness was highest for the most reflective surface (dry Gila soil) and lowest for the least reflective surface (wet 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, respectively, as compared with the albedo of the reference smooth soil. These reduction percentages can be used as a general guide to estimate the albedos of tilled soils similar to the Gila and Pima soils studied here.