This paper quantitatively explores, in terms of roughness indices, the effect of soil surface irregularities on the diurnal variation of the broadband blue-sky albedo of a large range of soil properties. Field studies were carried out on cultivated and uncultivated soil surfaces in Poland and Israel that vary in roughness and brightness. It was found that these irregularities, formed by different agricultural equipment and modified by rain or sprinkler irrigation, can be quantified by two roughness indices. Soil roughness not only affects the overall level of the diurnal variation of the albedo, but also affects the intensity of the diurnal increase from the solar zenith angle (µ ) at the local noon to about 75 80 . The roughness indices are variables that precisely determine only the albedo at the local solar noon of soils with the same color value. If the contents of soil organic carbon (SOC) and calcium carbonate are treated as the dominant variables, combined with one of the indices, these three variables together would significantly describe the albedo at the local solar noon of all soil surfaces. The soils, with their high irregularities, showed almost no rising values of albedo at a µ lower than 75 , while the smooth soil surfaces exhibited a gradual increase of the albedo at these angles. It is concluded that the roughness indices provide sufficient means to accurately describe the diurnal variation of the albedo of a wide range of surfaces, disregarding other soil properties.Index Terms-Diurnal albedo variation, field measurements, soil albedo, soils in Poland and Israel, soil surface roughness.

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Abstract:The albedo of bare soil depends on its organic matter, iron oxide, carbonate contents, and reflectance geometry, features considered stable over time, and also depends on salinity, moisture and roughness, which change dynamically due to agricultural practices. This paper deals with the quantitative estimation of the amount of shortwave radiation that could be reflected by air-dried bare soils in clear-sky conditions within arable lands in Israel throughout the year, assuming that they were shaped by a plough, a disk harrow, or a smoothing harrow. An area of bare soils was extracted from Landsat 8 images, within the contours of arable lands. The radiation reflected from the bare soils was calculated by equations predicting variations in their half-diurnal albedo as the solar zenith angle function. Accordingly, laboratory reflectance data of Israeli soil samples were used. The results clearly showed annual variation in the amount of short-wave radiation reflected from all bare soils within arable lands. The minimum radiation occurred in the winter, between the 1st and 70th day of the year (DOY), and the maximum was identified in the summer between 200th and 250th DOY. This could reach about 3-5 PJ/day and 16-23 PJ/day, respectively.

Smoothing a rough, deeply plowed soil increases its albedo, which determines a lower amount of shortwave radiation absorbed by its surface layer. That surface emits less longwave radiation, leading to a reduction in its temperature, which in turn can affect the climate, influencing the energy transfer between soil, vegetation, and the atmosphere. This paper presents a multistage procedure for estimating the annual dynamics of shortwave radiation reflected from bare soils as a consequence of smoothing the previously plowed and disk-harrowed fields in Poland. This procedure takes into account the spatial diversity of soil units and their properties within bare soil surfaces (extracted from Landsat 8 images), analyzed using digital maps of land use and soils as well as soil datasets stored in soil databases. One minimum and two peaks were found in the annual distribution of the radiation amount reflected from the soils only when smoothing the data. Expressing this reflected radiation as a fraction of the daily energy reaching the studied areas with clear-skies, it was predicted that those spring and summer peaks can reach about 2.2%–2.3% and 1%, respectively, of the incident shortwave radiation for soils that had been plowed and disk harrowed.