Influence of Erosion on Multitemporal Reflectivity of the Bare Soil Surface—The Effect of Polynomial Ladder

The reflectivity of the bare soil surface (BSS) is influenced by soil type, moisture, salinity, tillage, erosion, and other factors. To investigate the direct impact of erosion on the spectral characteristics of the BSS (SCBSS), a study site in the forest-steppe zone (Mtsensk district, Oryol Oblast, Russia), unaffected by salinity, carbonates, gypsum, and other factors, was selected. To suppress the influence of moisture and tillage, a multitemporal soil line (MSL) construction method was selected, which averages the influence of these factors, using the effect of big data. It was possible to reduce the influence of various factors on the SCBSS to two: zonal soil types and the extent of soil degradation from erosion (erosion degree). Soil types and erosion degree were determined by a ground survey with excavation of 488 pits/soil profiles. It was found that the relationship of soil types on the SCBSS has the form of a second-degree polynomial with a determination coefficient of R2 = 0.95. Spectral reflectance decreases across the zonal series of soils: sod-podzolic, light gray forest, gray forest, dark gray forest, podzolized chernozem, leached chernozem, typical chernozem, and meadow-chernozem soils. The influence of erosion leads to a linear increase in reflectance for each soil type in the following erosion degree series: non-eroded, slightly eroded, moderately eroded, and strongly eroded. Superimposing two functional relationships yields a distribution in the form of a polynomial ladder. This distribution maintains the general trend of a polynomial decrease in soil reflectance across the zonal series with stepwise deviations at the erosion degree. The polynomial ladder allows us to demonstrate how the erosion degree can change the spectral characteristics of one soil type to those of another.